{"id":327,"date":"2022-02-11T18:06:15","date_gmt":"2022-02-11T18:06:15","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/astronomy\/?post_type=chapter&#038;p=327"},"modified":"2022-04-28T16:13:58","modified_gmt":"2022-04-28T16:13:58","slug":"7-1-overview-of-our-planetary-system","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/astronomy\/chapter\/7-1-overview-of-our-planetary-system\/","title":{"raw":"7.1 Overview of Our Planetary System","rendered":"7.1 Overview of Our Planetary System"},"content":{"raw":"<div class=\"textbox textbox--learning-objectives\"><header class=\"textbox__header\">\r\n<h3 class=\"textbox__title\">Learning Objectives<\/h3>\r\n<\/header>\r\n<div class=\"textbox__content\">\r\n<p id=\"fs-id1168584045918\" class=\" \">By the end of this section, you will be able to:<\/p>\r\n\r\n<ul id=\"fs-id1170326074186\">\r\n \t<li>Describe how the objects in our solar system are identified, explored, and characterized<\/li>\r\n \t<li>Describe the types of small bodies in our solar system, their locations, and how they formed<\/li>\r\n \t<li>Model the solar system with distances from everyday life to better comprehend distances in space<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n<p id=\"fs-id1170326318339\" class=\" has-noteref\">The solar system<sup id=\"footnote-ref1\" data-type=\"footnote-number\"><a role=\"doc-noteref\" href=\"https:\/\/openstax.org\/books\/astronomy\/pages\/7-1-overview-of-our-planetary-system#fs-id1170326145109\" data-type=\"footnote-link\">1<\/a><\/sup>\u00a0consists of the Sun and many smaller objects: the planets, their moons and rings, and such \u201cdebris\u201d as asteroids, comets, and dust. Decades of observation and spacecraft exploration have revealed that most of these objects formed together with the Sun about 4.5 billion years ago. They represent clumps of material that condensed from an enormous cloud of gas and dust. The central part of this cloud became the Sun, and a small fraction of the material in the outer parts eventually formed the other objects.<\/p>\r\n<p id=\"fs-id1170326328411\" class=\" \">During the past 50 years, we have learned more about the solar system than anyone imagined before the space age. In addition to gathering information with powerful new telescopes, we have sent spacecraft directly to many members of the planetary system. (Planetary astronomy is the only branch of our science in which we can, at least vicariously, travel to the objects we want to study.) With evocative names such as\u00a0<em data-effect=\"italics\">Voyager<\/em>,\u00a0<em data-effect=\"italics\">Pioneer<\/em>,\u00a0<em data-effect=\"italics\">Curiosity<\/em>, and\u00a0<em data-effect=\"italics\">Pathfinder<\/em>, our robot explorers have flown past, orbited, or landed on every planet, returning images and data that have dazzled both astronomers and the public. In the process, we have also investigated two dwarf planets, hundreds of fascinating moons, four ring systems, a dozen asteroids, and several comets (smaller members of our solar system that we will discuss later).<\/p>\r\n<p id=\"fs-id1170326078025\" class=\" \">Our probes have penetrated the atmosphere of Jupiter and landed on the surfaces of Venus, Mars, our\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term312\" class=\"no-emphasis\" data-type=\"term\">Moon<\/span>, Saturn\u2019s moon Titan, the asteroids Eros and Itokawa, and the Comet Churyumov-Gerasimenko (usually referred to as 67P). Humans have set foot on the Moon and returned samples of its surface soil for laboratory analysis (<a class=\"autogenerated-content\" href=\"https:\/\/openstax.org\/books\/astronomy\/pages\/7-1-overview-of-our-planetary-system#OSC_Astro_07_01_Astronaut\">Figure 7.2<\/a>). We have even discovered other places in our solar system that might be able to support some kind of life.<\/p>\r\n\r\n<div id=\"OSC_Astro_07_01_Astronaut\" class=\"os-figure\">\r\n<figure data-id=\"OSC_Astro_07_01_Astronaut\">\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"975\"]<img id=\"2\" src=\"https:\/\/openstax.org\/apps\/archive\/20210823.155019\/resources\/9ff17cb19720f321d73e68a333f23529ec323c25\" alt=\"Photograph of Astronauts on the Moon. At center is the landing module, and to the right is the Lunar rover used by the Astronauts to travel large distances from the landing site. At left an Astronaut salutes the American flag placed near the lander. Scattered throughout the foreground are footprints in the Lunar soil.\" width=\"975\" height=\"450\" data-media-type=\"image\/jpeg\" \/> <strong>Figure\u00a07.2<\/strong>\u00a0Astronauts on the Moon.\u00a0The lunar lander and surface rover from the Apollo 15 mission are seen in this view of the one place beyond Earth that has been explored directly by humans. (credit: modification of work by David R. Scott, NASA)[\/caption]\r\n\r\n<div class=\"textbox textbox--exercises\"><header class=\"textbox__header\">\r\n<h3 class=\"textbox__title\">Link to Learning<\/h3>\r\n<\/header>\r\n<div class=\"textbox__content\">\r\n\r\nView this gallery of\u00a0<a href=\"https:\/\/openstax.org\/l\/30projapolloarc\" target=\"_blank\" rel=\"noopener nofollow noreferrer\">NASA images<\/a>\u00a0that trace the history of the Apollo mission.\r\n\r\n<\/div>\r\n<\/div>\r\n<h3 data-type=\"title\">An Inventory<\/h3>\r\n<p id=\"fs-id1170326086893\" class=\" \">The Sun, a star that is brighter than about 80% of the stars in the Galaxy, is by far the most massive member of the solar system, as shown in\u00a0Table 7.1. It is an enormous ball about 1.4 million kilometers in diameter, with surface layers of incandescent gas and an interior temperature of millions of degrees. The Sun will be discussed in later chapters as our first, and best-studied, example of a star.<\/p>\r\n\r\n<div class=\"os-table os-top-titled-container\">\r\n<table id=\"fs-id1170326274321\" class=\"grid landscape aligncenter\" summary=\"Table 7.1 \"><caption>Table 7.1 Mass of Members of the Solar System<\/caption>\r\n<thead>\r\n<tr valign=\"top\">\r\n<th scope=\"col\" data-valign=\"top\" data-align=\"center\">Object<\/th>\r\n<th scope=\"col\" data-valign=\"top\" data-align=\"center\">Percentage of Total Mass of Solar System<\/th>\r\n<\/tr>\r\n<\/thead>\r\n<tbody>\r\n<tr valign=\"top\">\r\n<td data-valign=\"top\" data-align=\"left\">Sun<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">99.80<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td data-valign=\"top\" data-align=\"left\">Jupiter<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">0.10<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td data-valign=\"top\" data-align=\"left\">Comets<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">0.0005\u20130.03 (estimate)<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td data-valign=\"top\" data-align=\"left\">All other planets and dwarf planets<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">0.04<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td data-valign=\"top\" data-align=\"left\">Moons and rings<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">0.00005<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td data-valign=\"top\" data-align=\"left\">Asteroids<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">0.000002 (estimate)<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td data-valign=\"top\" data-align=\"left\">Cosmic dust<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">0.0000001 (estimate)<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<div class=\"os-caption-container\">Table 7.1<span style=\"text-align: initial;font-size: 1em\">\u00a0also shows that most of the material of the planets is actually concentrated in the largest one,\u00a0<\/span><span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term313\" class=\"no-emphasis\" style=\"text-align: initial;font-size: 1em\" data-type=\"term\">Jupiter<\/span><span style=\"text-align: initial;font-size: 1em\">, which is more massive than all the rest of the planets combined. Astronomers were able to determine the masses of the planets centuries ago using Kepler\u2019s laws of planetary motion and Newton\u2019s law of gravity to measure the planets\u2019 gravitational effects on one another or on moons that orbit them (see\u00a0<\/span>Orbits and Gravity<span style=\"text-align: initial;font-size: 1em\">). Today, we make even more precise measurements of their masses by tracking their gravitational effects on the motion of spacecraft that pass near them.<\/span><\/div>\r\n<\/div>\r\n<p id=\"fs-id1170326093715\" class=\" \">Beside Earth, five other planets were known to the ancients\u2014Mercury, Venus, Mars, Jupiter, and Saturn\u2014and two were discovered after the invention of the telescope: Uranus and Neptune. The eight planets all revolve in the same direction around the Sun. They orbit in approximately the same plane, like cars traveling on concentric tracks on a giant, flat racecourse. Each planet stays in its own \u201ctraffic lane,\u201d following a nearly circular orbit about the Sun and obeying the \u201ctraffic\u201d laws discovered by Galileo, Kepler, and Newton. Besides these planets, we have also been discovering smaller worlds beyond Neptune that are called\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term314\" class=\"no-emphasis\" data-type=\"term\">trans-Neptunian object<\/span>s or TNOs (see\u00a0Figure 7.3). The first to be found, in 1930, was\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term315\" class=\"no-emphasis\" data-type=\"term\">Pluto<\/span>, but others have been discovered during the twenty-first century. One of them,\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term316\" class=\"no-emphasis\" data-type=\"term\">Eris<\/span>, is about the same size as Pluto and has at least one moon (Pluto has five known moons.) The largest TNOs are also classed as\u00a0<em data-effect=\"italics\">dwarf planets,<\/em>\u00a0as is the largest asteroid,\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term317\" class=\"no-emphasis\" data-type=\"term\">Ceres<\/span>. (Dwarf planets will be discussed further in the chapter on\u00a0Rings, Moons, and Pluto). To date, more than 2600 of these TNOs have been discovered.<\/p>\r\n\r\n<div id=\"OSC_Astro_07_01_Orbits\" class=\"os-figure\">\r\n<figure data-id=\"OSC_Astro_07_01_Orbits\">\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"975\"]<img id=\"4\" src=\"https:\/\/openstax.org\/apps\/archive\/20210823.155019\/resources\/66fc832ba8fa89f299d718228e06c7cb0ad85924\" alt=\"Diagram of the Orbits of the Planets and the five known Dwarf-Planets. The orbits of each object is shown as a blue ellipse. All eight major planets and the asteroids orbit the Sun in roughly the same plane, but the orbits of the outer dwarf planets do not. The objects plotted in the diagram moving outward from the Sun are Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, Makemake, and Eris.\" width=\"975\" height=\"582\" data-media-type=\"image\/jpeg\" \/> <strong>Figure\u00a07.3<\/strong>\u00a0Orbits of the Planets.\u00a0All eight major planets orbit the Sun in roughly the same plane. The five currently known dwarf planets are also shown:\u00a0Eris,\u00a0Haumea,\u00a0Pluto,\u00a0Ceres, and\u00a0Makemake. Note that Pluto\u2019s orbit is not in the plane of the planets.[\/caption]<\/figure>\r\n<\/div>\r\n<p id=\"fs-id1170326428987\" class=\" \">Each of the planets and dwarf planets also rotates (spins) about an axis running through it, and in most cases the direction of rotation is the same as the direction of revolution about the Sun. The exceptions are\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term323\" class=\"no-emphasis\" data-type=\"term\">Venus<\/span>, which rotates backward very slowly (that is, in a retrograde direction), and Uranus and\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term324\" class=\"no-emphasis\" data-type=\"term\">Pluto<\/span>, which also have strange rotations, each spinning about an axis tipped nearly on its side. We do not yet know the spin orientations of Eris, Haumea, and Makemake.<\/p>\r\n<p id=\"fs-id1170326129783\" class=\" \">The four planets closest to the Sun (Mercury through Mars) are called the inner or\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term325\" data-type=\"term\">terrestrial planets<\/span>. Often, the\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term326\" class=\"no-emphasis\" data-type=\"term\">Moon<\/span>\u00a0is also discussed as a part of this group, bringing the total of terrestrial objects to five. (We generally call Earth\u2019s satellite \u201cthe Moon,\u201d with a capital M, and the other satellites \u201cmoons,\u201d with lowercase m\u2019s.) The terrestrial planets are relatively small worlds, composed primarily of rock and metal. All of them have solid surfaces that bear the records of their geological history in the forms of craters, mountains, and volcanoes (Figure 7.4).<\/p>\r\n\r\n<div id=\"OSC_Astro_07_01_Mercury\" class=\"os-figure\">\r\n<figure data-id=\"OSC_Astro_07_01_Mercury\">\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"975\"]<img id=\"6\" src=\"https:\/\/openstax.org\/apps\/archive\/20210823.155019\/resources\/927d5307819f2c7d45e7b8f1c722a948938991d4\" alt=\"Image of the surface of Mercury taken from Mariner 10. Large craters, with many overlapping one upon the other, cover the surface of this 400 km wide scene.\" width=\"975\" height=\"390\" data-media-type=\"image\/jpeg\" \/> <strong>Figure\u00a07.4<\/strong>\u00a0Surface of Mercury.\u00a0The pockmarked face of the terrestrial world of\u00a0Mercury\u00a0is more typical of the inner planets than the watery surface of Earth. This black-and-white image, taken with the Mariner 10 spacecraft, shows a region more than 400 kilometers wide. (credit: modification of work by NASA\/John Hopkins University Applied Physics Laboratory\/Carnegie Institution of Washington)[\/caption]<\/figure>\r\n<\/div>\r\n<p id=\"fs-id1170326113812\" class=\" \">The next four planets (Jupiter through Neptune) are much larger and are composed primarily of lighter ices, liquids, and gases. We call these four the jovian planets (after \u201cJove,\u201d another name for Jupiter in mythology) or\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term328\" data-type=\"term\">giant planets<\/span>\u2014a name they richly deserve (Figure 7.5). About 1,300 Earths could fit inside Jupiter, for example. These planets do not have solid surfaces on which future explorers might land. They are more like vast, spherical oceans with much smaller, dense cores.<\/p>\r\n\r\n<div id=\"OSC_Astro_07_01_Giant\" class=\"os-figure\">\r\n<figure data-id=\"OSC_Astro_07_01_Giant\">\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"975\"]<img id=\"8\" src=\"https:\/\/openstax.org\/apps\/archive\/20210823.155019\/resources\/011f181406075bfc7cae1139d664a4a7b2e4adc1\" alt=\"Diagram of the Four Giant Planets Shown to Scale. Arranged from left to right are Jupiter, Saturn, Uranus, and Neptune. Also shown to scale at lower center is the Earth.\" width=\"975\" height=\"345\" data-media-type=\"image\/jpeg\" \/> <strong>Figure 7.5<\/strong> The Four Giant Planets. This montage shows the four giant planets: Jupiter, Saturn, Uranus, and Neptune. Below them, Earth is shown to scale. (credit: modification of work by NASA, Solar System Exploration)[\/caption]<\/figure>\r\n<\/div>\r\n<p id=\"fs-id1170326091199\" class=\" \">Near the outer edge of the system lies\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term333\" class=\"no-emphasis\" data-type=\"term\">Pluto<\/span>, which was the first of the distant icy worlds to be discovered beyond Neptune (Pluto was visited by a spacecraft, the NASA New Horizons mission, in 2015 [see\u00a0Figure 7.6]).\u00a0Table 7.2\u00a0summarizes some of the main facts about the planets.<\/p>\r\n\r\n<div id=\"OSC_Astro_07_01_PlutoNH\" class=\"os-figure\">\r\n<figure data-id=\"OSC_Astro_07_01_PlutoNH\">\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"975\"]<img id=\"10\" src=\"https:\/\/openstax.org\/apps\/archive\/20210823.155019\/resources\/cc6c0afa476e672d5eaab316b3c6fdee534b683c\" alt=\"Image of a portion of the surface of Pluto. In this photograph from New Horizons, the smooth, white Sputnik plains are seen covering most of the upper right of the image. Rugged, heavily cratered terrain covers the lower center and upper left.\" width=\"975\" height=\"327\" data-media-type=\"image\/jpeg\" \/> <strong>Figure\u00a07.6\u00a0<\/strong>Pluto Close-up.\u00a0This intriguing image from the New Horizons spacecraft, taken when it flew by the dwarf planet in July 2015, shows some of its complex surface features. The rounded white area is called the Sputnik Plain, after humanity\u2019s first spacecraft. (credit: modification of work by NASA\/Johns Hopkins University Applied Physics Laboratory\/Southwest Research Institute)[\/caption]<\/figure>\r\n<div class=\"os-caption-container\"><\/div>\r\n<\/div>\r\n<div class=\"os-table os-top-titled-container\">\r\n<table id=\"fs-id1170326125105\" class=\"grid landscape aligncenter\" style=\"height: 152px\" summary=\"Table 7.2 \"><caption>Table 7.2 The Planets<\/caption>\r\n<thead>\r\n<tr style=\"height: 32px\" valign=\"top\">\r\n<th style=\"height: 32px;width: 66.8594px\" scope=\"col\" data-valign=\"top\" data-align=\"center\">Name<\/th>\r\n<th style=\"height: 32px;width: 146.781px\" scope=\"col\" data-valign=\"top\" data-align=\"center\">Distance from Sun<span data-type=\"newline\">\r\n<\/span>(AU)<sup id=\"footnote-ref2\" data-type=\"footnote-number\"><a role=\"doc-noteref\" href=\"https:\/\/openstax.org\/books\/astronomy\/pages\/7-1-overview-of-our-planetary-system#fs-id1170326168588\" data-type=\"footnote-link\">2<\/a><\/sup><\/th>\r\n<th style=\"height: 32px;width: 141.344px\" scope=\"col\" data-valign=\"top\" data-align=\"center\">Revolution Period<span data-type=\"newline\">\r\n<\/span>(y)<\/th>\r\n<th style=\"height: 32px;width: 75.1094px\" scope=\"col\" data-valign=\"top\" data-align=\"center\">Diameter<span data-type=\"newline\">\r\n<\/span>(km)<\/th>\r\n<th style=\"height: 32px;width: 65.625px\" scope=\"col\" data-valign=\"top\" data-align=\"center\">Mass<span data-type=\"newline\">\r\n<\/span>[latex]{10^{23}}[\/latex]\u00a0kg)<\/th>\r\n<th style=\"height: 32px;width: 69.1562px\" scope=\"col\" data-valign=\"top\" data-align=\"center\">Density<span data-type=\"newline\">\r\n<\/span>[latex]{\\rm{g\/c}}{{\\rm{m}}^3}[\/latex]<sup id=\"footnote-ref3\" data-type=\"footnote-number\"><a role=\"doc-noteref\" href=\"https:\/\/openstax.org\/books\/astronomy\/pages\/7-1-overview-of-our-planetary-system#fs-id1170326176843\" data-type=\"footnote-link\">3<\/a><\/sup><\/th>\r\n<\/tr>\r\n<\/thead>\r\n<tbody>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 67.3594px\" data-valign=\"top\" data-align=\"left\"><span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term334\" class=\"no-emphasis\" data-type=\"term\">Mercury<\/span><\/td>\r\n<td style=\"height: 15px;width: 147.781px\" data-valign=\"top\" data-align=\"left\">0.39<\/td>\r\n<td style=\"height: 15px;width: 142.344px\" data-valign=\"top\" data-align=\"left\">0.24<\/td>\r\n<td style=\"height: 15px;width: 76.1094px\" data-valign=\"top\" data-align=\"left\">4,878<\/td>\r\n<td style=\"height: 15px;width: 66.625px\" data-valign=\"top\" data-align=\"left\">3.3<\/td>\r\n<td style=\"height: 15px;width: 69.6562px\" data-valign=\"top\" data-align=\"left\">5.4<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 67.3594px\" data-valign=\"top\" data-align=\"left\"><span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term335\" class=\"no-emphasis\" data-type=\"term\">Venus<\/span><\/td>\r\n<td style=\"height: 15px;width: 147.781px\" data-valign=\"top\" data-align=\"left\">0.72<\/td>\r\n<td style=\"height: 15px;width: 142.344px\" data-valign=\"top\" data-align=\"left\">0.62<\/td>\r\n<td style=\"height: 15px;width: 76.1094px\" data-valign=\"top\" data-align=\"left\">12,120<\/td>\r\n<td style=\"height: 15px;width: 66.625px\" data-valign=\"top\" data-align=\"left\">48.7<\/td>\r\n<td style=\"height: 15px;width: 69.6562px\" data-valign=\"top\" data-align=\"left\">5.2<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 67.3594px\" data-valign=\"top\" data-align=\"left\"><span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term336\" class=\"no-emphasis\" data-type=\"term\">Earth<\/span><\/td>\r\n<td style=\"height: 15px;width: 147.781px\" data-valign=\"top\" data-align=\"left\">1.00<\/td>\r\n<td style=\"height: 15px;width: 142.344px\" data-valign=\"top\" data-align=\"left\">1.00<\/td>\r\n<td style=\"height: 15px;width: 76.1094px\" data-valign=\"top\" data-align=\"left\">12,756<\/td>\r\n<td style=\"height: 15px;width: 66.625px\" data-valign=\"top\" data-align=\"left\">59.8<\/td>\r\n<td style=\"height: 15px;width: 69.6562px\" data-valign=\"top\" data-align=\"left\">5.5<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 67.3594px\" data-valign=\"top\" data-align=\"left\"><span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term337\" class=\"no-emphasis\" data-type=\"term\">Mars<\/span><\/td>\r\n<td style=\"height: 15px;width: 147.781px\" data-valign=\"top\" data-align=\"left\">1.52<\/td>\r\n<td style=\"height: 15px;width: 142.344px\" data-valign=\"top\" data-align=\"left\">1.88<\/td>\r\n<td style=\"height: 15px;width: 76.1094px\" data-valign=\"top\" data-align=\"left\">6,787<\/td>\r\n<td style=\"height: 15px;width: 66.625px\" data-valign=\"top\" data-align=\"left\">6.4<\/td>\r\n<td style=\"height: 15px;width: 69.6562px\" data-valign=\"top\" data-align=\"left\">3.9<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 67.3594px\" data-valign=\"top\" data-align=\"left\"><span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term338\" class=\"no-emphasis\" data-type=\"term\">Jupiter<\/span><\/td>\r\n<td style=\"height: 15px;width: 147.781px\" data-valign=\"top\" data-align=\"left\">5.20<\/td>\r\n<td style=\"height: 15px;width: 142.344px\" data-valign=\"top\" data-align=\"left\">11.86<\/td>\r\n<td style=\"height: 15px;width: 76.1094px\" data-valign=\"top\" data-align=\"left\">142,984<\/td>\r\n<td style=\"height: 15px;width: 66.625px\" data-valign=\"top\" data-align=\"left\">18,991<\/td>\r\n<td style=\"height: 15px;width: 69.6562px\" data-valign=\"top\" data-align=\"left\">1.3<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 67.3594px\" data-valign=\"top\" data-align=\"left\"><span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term339\" class=\"no-emphasis\" data-type=\"term\">Saturn<\/span><\/td>\r\n<td style=\"height: 15px;width: 147.781px\" data-valign=\"top\" data-align=\"left\">9.54<\/td>\r\n<td style=\"height: 15px;width: 142.344px\" data-valign=\"top\" data-align=\"left\">29.46<\/td>\r\n<td style=\"height: 15px;width: 76.1094px\" data-valign=\"top\" data-align=\"left\">120,536<\/td>\r\n<td style=\"height: 15px;width: 66.625px\" data-valign=\"top\" data-align=\"left\">5686<\/td>\r\n<td style=\"height: 15px;width: 69.6562px\" data-valign=\"top\" data-align=\"left\">0.7<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 67.3594px\" data-valign=\"top\" data-align=\"left\"><span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term340\" class=\"no-emphasis\" data-type=\"term\">Uranus<\/span><\/td>\r\n<td style=\"height: 15px;width: 147.781px\" data-valign=\"top\" data-align=\"left\">19.18<\/td>\r\n<td style=\"height: 15px;width: 142.344px\" data-valign=\"top\" data-align=\"left\">84.07<\/td>\r\n<td style=\"height: 15px;width: 76.1094px\" data-valign=\"top\" data-align=\"left\">51,118<\/td>\r\n<td style=\"height: 15px;width: 66.625px\" data-valign=\"top\" data-align=\"left\">866<\/td>\r\n<td style=\"height: 15px;width: 69.6562px\" data-valign=\"top\" data-align=\"left\">1.3<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 67.3594px\" data-valign=\"top\" data-align=\"left\"><span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term341\" class=\"no-emphasis\" data-type=\"term\">Neptune<\/span><\/td>\r\n<td style=\"height: 15px;width: 147.781px\" data-valign=\"top\" data-align=\"left\">30.06<\/td>\r\n<td style=\"height: 15px;width: 142.344px\" data-valign=\"top\" data-align=\"left\">164.82<\/td>\r\n<td style=\"height: 15px;width: 76.1094px\" data-valign=\"top\" data-align=\"left\">49,660<\/td>\r\n<td style=\"height: 15px;width: 66.625px\" data-valign=\"top\" data-align=\"left\">1030<\/td>\r\n<td style=\"height: 15px;width: 69.6562px\" data-valign=\"top\" data-align=\"left\">1.6<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<div class=\"os-caption-container\">\r\n<h3 data-type=\"title\">Smaller Members of the Solar System<\/h3>\r\n<p id=\"fs-id1170326497885\" class=\" \">Most of the planets are accompanied by one or more moons; only Mercury and Venus move through space alone. There are more than 210 known moons orbiting planets and dwarf planets (see\u00a0Appendix G\u00a0for a listing of the larger ones), and undoubtedly many other small ones remain undiscovered. The largest of the moons are as big as small planets and just as interesting. In addition to our Moon, they include the four largest moons of Jupiter (called the Galilean moons, after their discoverer) and the largest moons of Saturn and Neptune (confusingly named Titan and Triton).<\/p>\r\n<p id=\"fs-id1170326145312\" class=\" \">Each of the giant planets also has rings made up of countless small bodies ranging in size from mountains to mere grains of dust, all in orbit about the equator of the planet. The bright rings of\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term343\" class=\"no-emphasis\" data-type=\"term\">Saturn<\/span>\u00a0are, by far, the easiest to see. They are among the most beautiful sights in the solar system (Figure 7.7). But, all four ring systems are interesting to scientists because of their complicated forms, influenced by the pull of the moons that also orbit these giant planets.<\/p>\r\n\r\n<div id=\"OSC_Astro_07_01_Voyager1\" class=\"os-figure\">\r\n<figure data-id=\"OSC_Astro_07_01_Voyager1\">\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"975\"]<img id=\"13\" src=\"https:\/\/openstax.org\/apps\/archive\/20210823.155019\/resources\/b8cd2e7a4c3ab26415acf3954d95cbe1682414a1\" alt=\"Image of Saturn and its Rings. Taken almost directly over one of Saturn\u2019s poles, the rings are seen nearly face-on, completely encircling the planet. Sunlight arrives from lower left as the rings cast a thin shadow on Saturn\u2019s cloud tops, while Saturn itself casts a shadow on the rings at upper right.\" width=\"975\" height=\"340\" data-media-type=\"image\/jpeg\" \/> Figure\u00a07.7\u00a0Saturn and Its Rings.\u00a0This 2007 Cassini image shows\u00a0Saturn\u00a0and its complex system of rings, taken from a distance of about 1.2 million kilometers. This natural-color image is a composite of 36 images taken over the course of 2.5 hours. (credit: modification of work by NASA\/JPL\/Space Science Institute)[\/caption]<\/figure>\r\n<\/div>\r\n<p id=\"fs-id1170326093329\" class=\" \">The solar system has many other less-conspicuous members. Another group is the\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term345\" data-type=\"term\">asteroids<\/span>, rocky bodies that orbit the Sun like miniature planets, mostly in the space between Mars and Jupiter (although some do cross the orbits of planets like Earth\u2014see\u00a0Figure 7.8). Most asteroids are remnants of the initial population of the solar system that existed before the planets themselves formed. Some of the smallest moons of the planets, such as the moons of Mars, are very likely captured asteroids.<\/p>\r\n\r\n<div id=\"OSC_Astro_07_01_Asteroid\" class=\"os-figure\">\r\n<figure data-id=\"OSC_Astro_07_01_Asteroid\">\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"975\"]<img id=\"15\" src=\"https:\/\/openstax.org\/apps\/archive\/20210823.155019\/resources\/b7978cb6097d52a591f540060354508927fb14ee\" alt=\"Image of the Asteroid Eros. Like nearly all asteroids, Eros is not spherical but very irregular in shape, in this case similar to a potato. The surface is pock-marked with many craters.\" width=\"975\" height=\"281\" data-media-type=\"image\/jpeg\" \/> <strong>Figure\u00a07.8<\/strong>\u00a0Asteroid Eros.\u00a0This small Earth-crossing asteroid image was taken by the NEAR-Shoemaker spacecraft from an altitude of about 100 kilometers. This view of the heavily cratered surface is about 10 kilometers wide. The spacecraft orbited Eros for a year before landing gently on its surface. (credit: modification of work by NASA\/JHUAPL)[\/caption]<\/figure>\r\n<\/div>\r\n<p id=\"fs-id1170326449205\" class=\" \">Another class of small bodies is composed mostly of ice, made of frozen gases such as water, carbon dioxide, and carbon monoxide; these objects are called\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term346\" data-type=\"term\">comets<\/span>\u00a0(see\u00a0Figure 7.9). Comets also are remnants from the formation of the solar system, but they were formed and continue (with rare exceptions) to orbit the Sun in distant, cooler regions\u2014stored in a sort of cosmic deep freeze. This is also the realm of the larger icy worlds, called dwarf planets.<\/p>\r\n\r\n<div id=\"OSC_Astro_07_01_Comet\" class=\"os-figure\">\r\n<figure data-id=\"OSC_Astro_07_01_Comet\">\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"975\"]<img id=\"17\" src=\"https:\/\/openstax.org\/apps\/archive\/20210823.155019\/resources\/a3d16ced5e8f6825a9bd9080ce3e33d90723c76b\" alt=\"Image of Comet Churyumov-Gerasimenko (67P). Two lobes of this irregularly shaped object are illuminated by sunlight coming from the upper left. Bright streaks of material are seen radiating away from the sunlit surfaces of the comet. These streaks are not seen coming from the shaded portions.\" width=\"975\" height=\"410\" data-media-type=\"image\/jpeg\" \/> <strong>Figure\u00a07.9<\/strong>\u00a0Comet Churyumov-Gerasimenko (67P).\u00a0This image shows Comet Churyumov-Gerasimenko, also known as 67P, near its closest approach to the Sun in 2015, as seen from the\u00a0Rosetta\u00a0spacecraft. Note the jets of gas escaping from the solid surface. (credit: modification of work by ESA\/Rosetta\/NAVACAM,\u00a0CC BY-SA IGO 3.0)[\/caption]<\/figure>\r\n<\/div>\r\n<p id=\"fs-id1170326096149\" class=\" \">Finally, there are countless grains of broken rock, which we call cosmic dust, scattered throughout the solar system. When these particles enter Earth\u2019s atmosphere (as millions do each day) they burn up, producing a brief flash of light in the night sky known as a\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term347\" data-type=\"term\">meteor<\/span>\u00a0(meteors are often referred to as shooting stars). Occasionally, some larger chunk of rocky or metallic material survives its passage through the atmosphere and lands on Earth. Any piece that strikes the ground is known as a\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term348\" data-type=\"term\">meteorite<\/span>. (You can see meteorites on display in many natural history museums and can sometimes even purchase pieces of them from gem and mineral dealers.)<\/p>\r\n\r\n<div class=\"textbox textbox--examples\"><header class=\"textbox__header\">\r\n<h3 class=\"textbox__title\">Voyagers in Astronomy<\/h3>\r\n<\/header>\r\n<div class=\"textbox__content\">\r\n<h4 id=\"18\" class=\"os-subtitle\" data-type=\"title\"><span class=\"os-subtitle-label\">Carl Sagan: Solar System Advocate<\/span><\/h4>\r\n<p id=\"fs-id1170326086114\" class=\" \">The best-known astronomer in the world during the 1970s and 1980s, Carl\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term349\" class=\"no-emphasis\" data-type=\"term\">Sagan<\/span>\u00a0devoted most of his professional career to studying the planets and considerable energy to raising public awareness of what we can learn from exploring the solar system (see\u00a0Figure 7.10). Born in Brooklyn, New York, in 1934, Sagan became interested in astronomy as a youngster; he also credits science fiction stories for sustaining his fascination with what\u2019s out in the universe.<\/p>\r\n\r\n<div id=\"OSC_Astro_07_04_Sagan\" class=\"os-figure\">\r\n<figure data-id=\"OSC_Astro_07_04_Sagan\">\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"732\"]<img id=\"20\" src=\"https:\/\/openstax.org\/apps\/archive\/20210823.155019\/resources\/e7453e5380c0b28d8eaa0eff31474de7e375f8f3\" alt=\"Left image: photograph of Carl Sagan. Right image: Photograph of Neil deGrasse Tyson.\" width=\"732\" height=\"429\" data-media-type=\"image\/jpeg\" \/> <strong>Figure\u00a07.10\u00a0<\/strong>Carl Sagan (1934\u20131996) and Neil deGrasse Tyson.\u00a0Sagan was Tyson\u2019s inspiration to become a scientist. (credit \u201cSagan\u201d: modification of work by NASA, JPL; credit \u201cTyson\u201d: modification of work by Bruce F. Press)[\/caption]<\/figure>\r\n<div class=\"os-caption-container\"><\/div>\r\n<\/div>\r\n<p id=\"fs-id1170326095818\" class=\" \">In the early 1960s, when many scientists still thought Venus might turn out to be a hospitable place, Sagan calculated that the thick atmosphere of Venus could act like a giant greenhouse, keeping the heat in and raising the temperature enormously. He showed that the seasonal changes astronomers had seen on Mars were caused, not by vegetation, but by wind-blown dust. He was a member of the scientific teams for many of the robotic missions that explored the solar system and was instrumental in getting NASA to put a message-bearing plaque aboard the Pioneer spacecraft, as well as audio-video records on the Voyager spacecraft\u2014all of them destined to leave our solar system entirely and send these little bits of Earth technology out among the stars.<\/p>\r\n<p id=\"fs-id1170326112426\" class=\" \">To encourage public interest and public support of planetary exploration, Sagan helped found The Planetary Society, now the largest space-interest organization in the world. He was a tireless and eloquent advocate of the need to study the solar system close-up and the value of learning about other worlds in order to take better care of our own.<\/p>\r\n<p id=\"fs-id1170326129355\" class=\" \">Sagan simulated conditions on early Earth to demonstrate how some of life\u2019s fundamental building blocks might have formed from the \u201cprimordial soup\u201d of natural compounds on our planet. In addition, he and his colleagues developed computer models showing the consequences of nuclear war for Earth would be even more devastating than anyone had thought (this is now called the nuclear winter hypothesis) and demonstrating some of the serious consequences of continued pollution of our atmosphere.<\/p>\r\n<p id=\"fs-id1170326113790\" class=\" \">Sagan was perhaps best known, however, as a brilliant popularizer of astronomy and the author of many books on science, including the best-selling\u00a0<em data-effect=\"italics\">Cosmos<\/em>, and several evocative tributes to solar system exploration such as\u00a0<em data-effect=\"italics\">The Cosmic Connection<\/em>\u00a0and\u00a0<em data-effect=\"italics\">Pale Blue Dot<\/em>. His book\u00a0<em data-effect=\"italics\">The Demon Haunted World<\/em>, completed just before his death in 1996, is perhaps the best antidote to fuzzy thinking about pseudo-science and irrationality in print today. An intriguing science fiction novel he wrote, titled\u00a0<em data-effect=\"italics\">Contact<\/em>, which became a successful film as well, is still recommended by many science instructors as a scenario for making contact with life elsewhere that is much more reasonable than most science fiction.<\/p>\r\n<p id=\"fs-id1170326471153\" class=\" \">Sagan was a master, too, of the television medium. His 13-part public television series,\u00a0<em data-effect=\"italics\">Cosmos<\/em>, was seen by an estimated 500 million people in 60 countries and has become one of the most-watched series in the history of public broadcasting. A few astronomers scoffed at a scientist who spent so much time in the public eye, but it is probably fair to say that Sagan\u2019s enthusiasm and skill as an explainer won more friends for the science of astronomy than anyone or anything else in the second half of the twentieth century.<\/p>\r\n<p id=\"fs-id1170326169519\" class=\" \">In the two decades since Sagan\u2019s death, no other scientist has achieved the same level of public recognition. Perhaps closest is the director of the Hayden Planetarium, Neil deGrasse\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term350\" class=\"no-emphasis\" data-type=\"term\">Tyson<\/span>, who followed in Sagan\u2019s footsteps by making an updated version of the\u00a0<em data-effect=\"italics\">Cosmos<\/em>\u00a0program in 2014. Tyson is quick to point out that Sagan was his inspiration to become a scientist, telling how Sagan invited him to visit for a day at Cornell when he was a high school student looking for a career. However, the media environment has fragmented a great deal since Sagan\u2019s time. It is interesting to speculate whether Sagan could have adapted his communication style to the world of cable television, Twitter, Facebook, and podcasts.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n<div class=\"textbox textbox--exercises\"><header class=\"textbox__header\">\r\n<h3 class=\"textbox__title\">Link to Learning<\/h3>\r\n<\/header>\r\n<div class=\"textbox__content\">\r\n\r\nTwo imaginative videos provide a tour of the solar system objects we have been discussing. Shane Gellert\u2019s\u00a0<a href=\"https:\/\/openstax.org\/l\/30needsomespace\" target=\"_blank\" rel=\"noopener nofollow noreferrer\">I Need Some Space<\/a>\u00a0uses NASA photography and models to show the various worlds with which we share our system. In the more science fiction-oriented\u00a0<a href=\"https:\/\/openstax.org\/l\/30wanderers\" target=\"_blank\" rel=\"noopener nofollow noreferrer\">Wanderers<\/a>\u00a0video, we see some of the planets and moons as tourist destinations for future explorers, with commentary taken from recordings by Carl Sagan.\r\n\r\n<\/div>\r\n<\/div>\r\n<h3 data-type=\"title\">A Scale Model of the Solar System<\/h3>\r\n<p id=\"fs-id1170326093910\" class=\" \">Astronomy often deals with dimensions and distances that far exceed our ordinary experience. What does 1.4 billion kilometers\u2014the distance from the Sun to Saturn\u2014really mean to anyone? It can be helpful to visualize such large systems in terms of a scale model.<\/p>\r\n<p id=\"fs-id1170326386156\" class=\" \">In our imaginations, let us build a scale model of the solar system, adopting a scale factor of 1 billion ([latex]{10^9}[\/latex])\u2014that is, reducing the actual solar system by dividing every dimension by a factor of [latex]{10^9}[\/latex]. Earth, then, has a diameter of 1.3 centimeters, about the size of a grape. The Moon is a pea orbiting this at a distance of 40 centimeters, or a little more than a foot away. The Earth-Moon system fits into a standard backpack.<\/p>\r\n<p id=\"fs-id1170326042510\" class=\" \">In this model, the Sun is nearly 1.5 meters in diameter, about the average height of an adult, and our Earth is at a distance of 150 meters\u2014about one city block\u2014from the Sun. Jupiter is five blocks away from the Sun, and its diameter is 15 centimeters, about the size of a very large grapefruit. Saturn is 10 blocks from the Sun; Uranus, 20 blocks; and Neptune, 30 blocks. Pluto, with a distance that varies quite a bit during its 249-year orbit, is currently just beyond 30 blocks and getting farther with time. Most of the moons of the outer solar system are the sizes of various kinds of seeds orbiting the grapefruit, oranges, and lemons that represent the outer planets.<\/p>\r\n<p id=\"fs-id1170326128525\" class=\" \">In our scale model, a human is reduced to the dimensions of a single atom, and cars and spacecraft to the size of molecules. Sending the Voyager spacecraft to Neptune involves navigating a single molecule from the Earth\u2013grape toward a lemon 5 kilometers away with an accuracy equivalent to the width of a thread in a spider\u2019s web.<\/p>\r\n<p id=\"fs-id1170326087667\" class=\" \">If that model represents the solar system, where would the nearest stars be? If we keep the same scale, the closest stars would be tens of thousands of kilometers away. If you built this scale model in the city where you live, you would have to place the representations of these stars on the other side of Earth or beyond.<\/p>\r\n<p id=\"fs-id1170326286048\" class=\" \">By the way, model solar systems like the one we just presented have been built in cities throughout the world. In Sweden, for example, Stockholm\u2019s huge Globe Arena has become a model for the Sun, and Pluto is represented by a 12-centimeter sculpture in the small town of Delsbo, 300 kilometers away. Another model solar system is in Washington on the Mall between the White House and Congress (perhaps proving they are worlds apart?).<\/p>\r\n\r\n<div class=\"textbox textbox--exercises\"><header class=\"textbox__header\">\r\n<h3 class=\"textbox__title\">Link to Learning<\/h3>\r\n<\/header>\r\n<div class=\"textbox__content\">\r\n\r\nThis\u00a0<a href=\"https:\/\/openstax.org\/l\/30modsolsys\" target=\"_blank\" rel=\"noopener nofollow noreferrer\">model of the solar system<\/a>\u00a0shows all orbits and sizes to scale, and it lets you fly between the planets at an enhanced speed.\r\n\r\n<\/div>\r\n<\/div>\r\n<div class=\"textbox textbox--examples\"><header class=\"textbox__header\">\r\n<h3 class=\"textbox__title\">Making Connections<\/h3>\r\n<\/header>\r\n<div class=\"textbox__content\">\r\n<h4 id=\"21\" class=\"os-subtitle\" data-type=\"title\"><span class=\"os-subtitle-label\">Names in the Solar System<\/span><\/h4>\r\n<p id=\"fs-id1170326097579\" class=\" \">We humans just don\u2019t feel comfortable until something has a name. Types of butterflies, new elements, and the mountains of Venus all need names for us to feel we are acquainted with them. How do we give names to objects and features in the solar system?<\/p>\r\n<p id=\"fs-id1170326454356\" class=\" \">Planets and moons are named after gods and heroes in Greek and Roman mythology (with a few exceptions among the moons of Uranus, which have names drawn from English literature). When William Herschel, a German immigrant to England, first discovered the planet we now call Uranus, he wanted to name it Georgium Sidus (George\u2019s star) after King George III of his adopted country. This caused such an outcry among astronomers in other nations, however, that the classic tradition was upheld\u2014and has been maintained ever since. Luckily, there were a lot of minor gods in the ancient pantheon, so plenty of names are left for the many small moons we are discovering around the giant planets. (Appendix G\u00a0lists the larger moons). More recently, the names of dwarf planets and their moons have been drawn from the mythology of other cultures besides Greek and Roman.<\/p>\r\n<p id=\"fs-id1170326478164\" class=\" \">Comets are often named after their discoverers (offering an extra incentive to comet hunters). Asteroids are named by their discoverers after just about anyone or anything they want. Recently, asteroid names have been used to recognize people who have made significant contributions to astronomy, including the three senior authors of this book.<\/p>\r\n<p id=\"fs-id1170326146431\" class=\" \">That was pretty much all the naming that was needed while our study of the solar system was confined to Earth. But now, our spacecraft have surveyed and photographed many worlds in great detail, and each world has a host of features that also need names. To make sure that naming things in space remains multinational, rational, and somewhat dignified, astronomers have given the responsibility of approving names to a special committee of the\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term351\" class=\"no-emphasis\" data-type=\"term\">International Astronomical Union<\/span>\u00a0(IAU), the body that includes scientists from every country that does astronomy.<\/p>\r\n<p id=\"fs-id1170326066472\" class=\" \">This IAU committee has developed a set of rules for naming features on other worlds. For example, craters on Venus are named for women who have made significant contributions to human knowledge and welfare. Volcanic features on Jupiter\u2019s moon Io, which is in a constant state of volcanic activity, are named after gods of fire and thunder from the mythologies of many cultures. Craters on Mercury commemorate famous novelists, playwrights, artists, and composers. On Saturn\u2019s moon Tethys, all the features are named after characters and places in Homer\u2019s great epic poem,\u00a0<em data-effect=\"italics\">The Odyssey<\/em>. As we explore further, it may well turn out that more places in the solar system need names than Earth history can provide. Perhaps by then, explorers and settlers on these worlds will be ready to develop their own names for the places they may (if but for a while) call home.<\/p>\r\n<p id=\"fs-id1170326096369\" class=\" \">You may be surprised to know that the meaning of the word\u00a0<em data-effect=\"italics\">planet<\/em>\u00a0has recently become controversial because we have discovered many other planetary systems that don\u2019t look very much like our own. Even within our solar system, the planets differ greatly in size and chemical properties. The biggest dispute concerns Pluto, which is much smaller than the other eight major planets. The category of dwarf planet was invented to include Pluto and similar icy objects beyond Neptune. But is a dwarf planet also a planet? Logically, it should be, but even this simple issue of grammar has been the subject of heated debate among both astronomers and the general public.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n<h3 data-type=\"footnote-refs-title\">Footnotes<\/h3>\r\n<ul data-list-type=\"bulleted\" data-bullet-style=\"none\">\r\n \t<li id=\"fs-id1170326145109\" data-type=\"footnote-ref\"><a role=\"doc-backlink\" href=\"https:\/\/openstax.org\/books\/astronomy\/pages\/7-1-overview-of-our-planetary-system#footnote-ref1\">1<\/a> <span data-type=\"footnote-ref-content\">The generic term for a group of planets and other bodies circling a star is\u00a0<em data-effect=\"italics\">planetary system<\/em>. Ours is called the\u00a0<em data-effect=\"italics\">solar system<\/em>\u00a0because our Sun is sometimes called\u00a0<em data-effect=\"italics\">Sol<\/em>. Strictly speaking, then, there is only one solar system; planets orbiting other stars are in planetary systems.<\/span><\/li>\r\n \t<li id=\"fs-id1170326168588\" data-type=\"footnote-ref\"><a role=\"doc-backlink\" href=\"https:\/\/openstax.org\/books\/astronomy\/pages\/7-1-overview-of-our-planetary-system#footnote-ref2\">2<\/a> <span data-type=\"footnote-ref-content\">An AU (or astronomical unit) is the distance from Earth to the Sun.<\/span><\/li>\r\n \t<li id=\"fs-id1170326176843\" data-type=\"footnote-ref\"><a role=\"doc-backlink\" href=\"https:\/\/openstax.org\/books\/astronomy\/pages\/7-1-overview-of-our-planetary-system#footnote-ref3\">3<\/a> <span data-type=\"footnote-ref-content\">We give densities in units where the density of water is 1 g\/cm<sup>3<\/sup>. To get densities in units of kg\/m<sup>3<\/sup>, multiply the given value by 1000.<\/span><\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div><\/figure>\r\n<\/div>\r\n<div class=\"textbox\">This book was adapted from the following: Fraknoi, A., Morrison, D., &amp; Wolff, S. C. (2016). 7.1 Overview of Our Planetary System. In <i>Astronomy<\/i>. OpenStax. https:\/\/openstax.org\/books\/astronomy\/pages\/7-1-overview-of-our-planetary-system under a <a href=\"http:\/\/creativecommons.org\/licenses\/by\/4.0\/\" target=\"_blank\" rel=\"noopener noreferrer\">Creative Commons Attribution License 4.0<\/a><\/div>\r\n<div>Access the entire book for free at <a href=\"https:\/\/openstax.org\/books\/astronomy\/pages\/1-introduction\">https:\/\/openstax.org\/books\/astronomy\/pages\/1-introduction<\/a><\/div>","rendered":"<div class=\"textbox textbox--learning-objectives\">\n<header class=\"textbox__header\">\n<h3 class=\"textbox__title\">Learning Objectives<\/h3>\n<\/header>\n<div class=\"textbox__content\">\n<p id=\"fs-id1168584045918\" class=\"\">By the end of this section, you will be able to:<\/p>\n<ul id=\"fs-id1170326074186\">\n<li>Describe how the objects in our solar system are identified, explored, and characterized<\/li>\n<li>Describe the types of small bodies in our solar system, their locations, and how they formed<\/li>\n<li>Model the solar system with distances from everyday life to better comprehend distances in space<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<p id=\"fs-id1170326318339\" class=\"has-noteref\">The solar system<sup id=\"footnote-ref1\" data-type=\"footnote-number\"><a role=\"doc-noteref\" href=\"https:\/\/openstax.org\/books\/astronomy\/pages\/7-1-overview-of-our-planetary-system#fs-id1170326145109\" data-type=\"footnote-link\">1<\/a><\/sup>\u00a0consists of the Sun and many smaller objects: the planets, their moons and rings, and such \u201cdebris\u201d as asteroids, comets, and dust. Decades of observation and spacecraft exploration have revealed that most of these objects formed together with the Sun about 4.5 billion years ago. They represent clumps of material that condensed from an enormous cloud of gas and dust. The central part of this cloud became the Sun, and a small fraction of the material in the outer parts eventually formed the other objects.<\/p>\n<p id=\"fs-id1170326328411\" class=\"\">During the past 50 years, we have learned more about the solar system than anyone imagined before the space age. In addition to gathering information with powerful new telescopes, we have sent spacecraft directly to many members of the planetary system. (Planetary astronomy is the only branch of our science in which we can, at least vicariously, travel to the objects we want to study.) With evocative names such as\u00a0<em data-effect=\"italics\">Voyager<\/em>,\u00a0<em data-effect=\"italics\">Pioneer<\/em>,\u00a0<em data-effect=\"italics\">Curiosity<\/em>, and\u00a0<em data-effect=\"italics\">Pathfinder<\/em>, our robot explorers have flown past, orbited, or landed on every planet, returning images and data that have dazzled both astronomers and the public. In the process, we have also investigated two dwarf planets, hundreds of fascinating moons, four ring systems, a dozen asteroids, and several comets (smaller members of our solar system that we will discuss later).<\/p>\n<p id=\"fs-id1170326078025\" class=\"\">Our probes have penetrated the atmosphere of Jupiter and landed on the surfaces of Venus, Mars, our\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term312\" class=\"no-emphasis\" data-type=\"term\">Moon<\/span>, Saturn\u2019s moon Titan, the asteroids Eros and Itokawa, and the Comet Churyumov-Gerasimenko (usually referred to as 67P). Humans have set foot on the Moon and returned samples of its surface soil for laboratory analysis (<a class=\"autogenerated-content\" href=\"https:\/\/openstax.org\/books\/astronomy\/pages\/7-1-overview-of-our-planetary-system#OSC_Astro_07_01_Astronaut\">Figure 7.2<\/a>). We have even discovered other places in our solar system that might be able to support some kind of life.<\/p>\n<div id=\"OSC_Astro_07_01_Astronaut\" class=\"os-figure\">\n<figure data-id=\"OSC_Astro_07_01_Astronaut\">\n<figure style=\"width: 975px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" id=\"2\" src=\"https:\/\/openstax.org\/apps\/archive\/20210823.155019\/resources\/9ff17cb19720f321d73e68a333f23529ec323c25\" alt=\"Photograph of Astronauts on the Moon. At center is the landing module, and to the right is the Lunar rover used by the Astronauts to travel large distances from the landing site. At left an Astronaut salutes the American flag placed near the lander. Scattered throughout the foreground are footprints in the Lunar soil.\" width=\"975\" height=\"450\" data-media-type=\"image\/jpeg\" \/><figcaption class=\"wp-caption-text\"><strong>Figure\u00a07.2<\/strong>\u00a0Astronauts on the Moon.\u00a0The lunar lander and surface rover from the Apollo 15 mission are seen in this view of the one place beyond Earth that has been explored directly by humans. (credit: modification of work by David R. Scott, NASA)<\/figcaption><\/figure>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h3 class=\"textbox__title\">Link to Learning<\/h3>\n<\/header>\n<div class=\"textbox__content\">\n<p>View this gallery of\u00a0<a href=\"https:\/\/openstax.org\/l\/30projapolloarc\" target=\"_blank\" rel=\"noopener nofollow noreferrer\">NASA images<\/a>\u00a0that trace the history of the Apollo mission.<\/p>\n<\/div>\n<\/div>\n<h3 data-type=\"title\">An Inventory<\/h3>\n<p id=\"fs-id1170326086893\" class=\"\">The Sun, a star that is brighter than about 80% of the stars in the Galaxy, is by far the most massive member of the solar system, as shown in\u00a0Table 7.1. It is an enormous ball about 1.4 million kilometers in diameter, with surface layers of incandescent gas and an interior temperature of millions of degrees. The Sun will be discussed in later chapters as our first, and best-studied, example of a star.<\/p>\n<div class=\"os-table os-top-titled-container\">\n<table id=\"fs-id1170326274321\" class=\"grid landscape aligncenter\" summary=\"Table 7.1\">\n<caption>Table 7.1 Mass of Members of the Solar System<\/caption>\n<thead>\n<tr valign=\"top\">\n<th scope=\"col\" data-valign=\"top\" data-align=\"center\">Object<\/th>\n<th scope=\"col\" data-valign=\"top\" data-align=\"center\">Percentage of Total Mass of Solar System<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr valign=\"top\">\n<td data-valign=\"top\" data-align=\"left\">Sun<\/td>\n<td data-valign=\"top\" data-align=\"left\">99.80<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td data-valign=\"top\" data-align=\"left\">Jupiter<\/td>\n<td data-valign=\"top\" data-align=\"left\">0.10<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td data-valign=\"top\" data-align=\"left\">Comets<\/td>\n<td data-valign=\"top\" data-align=\"left\">0.0005\u20130.03 (estimate)<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td data-valign=\"top\" data-align=\"left\">All other planets and dwarf planets<\/td>\n<td data-valign=\"top\" data-align=\"left\">0.04<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td data-valign=\"top\" data-align=\"left\">Moons and rings<\/td>\n<td data-valign=\"top\" data-align=\"left\">0.00005<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td data-valign=\"top\" data-align=\"left\">Asteroids<\/td>\n<td data-valign=\"top\" data-align=\"left\">0.000002 (estimate)<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td data-valign=\"top\" data-align=\"left\">Cosmic dust<\/td>\n<td data-valign=\"top\" data-align=\"left\">0.0000001 (estimate)<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<div class=\"os-caption-container\">Table 7.1<span style=\"text-align: initial;font-size: 1em\">\u00a0also shows that most of the material of the planets is actually concentrated in the largest one,\u00a0<\/span><span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term313\" class=\"no-emphasis\" style=\"text-align: initial;font-size: 1em\" data-type=\"term\">Jupiter<\/span><span style=\"text-align: initial;font-size: 1em\">, which is more massive than all the rest of the planets combined. Astronomers were able to determine the masses of the planets centuries ago using Kepler\u2019s laws of planetary motion and Newton\u2019s law of gravity to measure the planets\u2019 gravitational effects on one another or on moons that orbit them (see\u00a0<\/span>Orbits and Gravity<span style=\"text-align: initial;font-size: 1em\">). Today, we make even more precise measurements of their masses by tracking their gravitational effects on the motion of spacecraft that pass near them.<\/span><\/div>\n<\/div>\n<p id=\"fs-id1170326093715\" class=\"\">Beside Earth, five other planets were known to the ancients\u2014Mercury, Venus, Mars, Jupiter, and Saturn\u2014and two were discovered after the invention of the telescope: Uranus and Neptune. The eight planets all revolve in the same direction around the Sun. They orbit in approximately the same plane, like cars traveling on concentric tracks on a giant, flat racecourse. Each planet stays in its own \u201ctraffic lane,\u201d following a nearly circular orbit about the Sun and obeying the \u201ctraffic\u201d laws discovered by Galileo, Kepler, and Newton. Besides these planets, we have also been discovering smaller worlds beyond Neptune that are called\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term314\" class=\"no-emphasis\" data-type=\"term\">trans-Neptunian object<\/span>s or TNOs (see\u00a0Figure 7.3). The first to be found, in 1930, was\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term315\" class=\"no-emphasis\" data-type=\"term\">Pluto<\/span>, but others have been discovered during the twenty-first century. One of them,\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term316\" class=\"no-emphasis\" data-type=\"term\">Eris<\/span>, is about the same size as Pluto and has at least one moon (Pluto has five known moons.) The largest TNOs are also classed as\u00a0<em data-effect=\"italics\">dwarf planets,<\/em>\u00a0as is the largest asteroid,\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term317\" class=\"no-emphasis\" data-type=\"term\">Ceres<\/span>. (Dwarf planets will be discussed further in the chapter on\u00a0Rings, Moons, and Pluto). To date, more than 2600 of these TNOs have been discovered.<\/p>\n<div id=\"OSC_Astro_07_01_Orbits\" class=\"os-figure\">\n<figure data-id=\"OSC_Astro_07_01_Orbits\">\n<figure style=\"width: 975px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" id=\"4\" src=\"https:\/\/openstax.org\/apps\/archive\/20210823.155019\/resources\/66fc832ba8fa89f299d718228e06c7cb0ad85924\" alt=\"Diagram of the Orbits of the Planets and the five known Dwarf-Planets. The orbits of each object is shown as a blue ellipse. All eight major planets and the asteroids orbit the Sun in roughly the same plane, but the orbits of the outer dwarf planets do not. The objects plotted in the diagram moving outward from the Sun are Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, Makemake, and Eris.\" width=\"975\" height=\"582\" data-media-type=\"image\/jpeg\" \/><figcaption class=\"wp-caption-text\"><strong>Figure\u00a07.3<\/strong>\u00a0Orbits of the Planets.\u00a0All eight major planets orbit the Sun in roughly the same plane. The five currently known dwarf planets are also shown:\u00a0Eris,\u00a0Haumea,\u00a0Pluto,\u00a0Ceres, and\u00a0Makemake. Note that Pluto\u2019s orbit is not in the plane of the planets.<\/figcaption><\/figure>\n<\/figure>\n<\/div>\n<p id=\"fs-id1170326428987\" class=\"\">Each of the planets and dwarf planets also rotates (spins) about an axis running through it, and in most cases the direction of rotation is the same as the direction of revolution about the Sun. The exceptions are\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term323\" class=\"no-emphasis\" data-type=\"term\">Venus<\/span>, which rotates backward very slowly (that is, in a retrograde direction), and Uranus and\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term324\" class=\"no-emphasis\" data-type=\"term\">Pluto<\/span>, which also have strange rotations, each spinning about an axis tipped nearly on its side. We do not yet know the spin orientations of Eris, Haumea, and Makemake.<\/p>\n<p id=\"fs-id1170326129783\" class=\"\">The four planets closest to the Sun (Mercury through Mars) are called the inner or\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term325\" data-type=\"term\">terrestrial planets<\/span>. Often, the\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term326\" class=\"no-emphasis\" data-type=\"term\">Moon<\/span>\u00a0is also discussed as a part of this group, bringing the total of terrestrial objects to five. (We generally call Earth\u2019s satellite \u201cthe Moon,\u201d with a capital M, and the other satellites \u201cmoons,\u201d with lowercase m\u2019s.) The terrestrial planets are relatively small worlds, composed primarily of rock and metal. All of them have solid surfaces that bear the records of their geological history in the forms of craters, mountains, and volcanoes (Figure 7.4).<\/p>\n<div id=\"OSC_Astro_07_01_Mercury\" class=\"os-figure\">\n<figure data-id=\"OSC_Astro_07_01_Mercury\">\n<figure style=\"width: 975px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" id=\"6\" src=\"https:\/\/openstax.org\/apps\/archive\/20210823.155019\/resources\/927d5307819f2c7d45e7b8f1c722a948938991d4\" alt=\"Image of the surface of Mercury taken from Mariner 10. Large craters, with many overlapping one upon the other, cover the surface of this 400 km wide scene.\" width=\"975\" height=\"390\" data-media-type=\"image\/jpeg\" \/><figcaption class=\"wp-caption-text\"><strong>Figure\u00a07.4<\/strong>\u00a0Surface of Mercury.\u00a0The pockmarked face of the terrestrial world of\u00a0Mercury\u00a0is more typical of the inner planets than the watery surface of Earth. This black-and-white image, taken with the Mariner 10 spacecraft, shows a region more than 400 kilometers wide. (credit: modification of work by NASA\/John Hopkins University Applied Physics Laboratory\/Carnegie Institution of Washington)<\/figcaption><\/figure>\n<\/figure>\n<\/div>\n<p id=\"fs-id1170326113812\" class=\"\">The next four planets (Jupiter through Neptune) are much larger and are composed primarily of lighter ices, liquids, and gases. We call these four the jovian planets (after \u201cJove,\u201d another name for Jupiter in mythology) or\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term328\" data-type=\"term\">giant planets<\/span>\u2014a name they richly deserve (Figure 7.5). About 1,300 Earths could fit inside Jupiter, for example. These planets do not have solid surfaces on which future explorers might land. They are more like vast, spherical oceans with much smaller, dense cores.<\/p>\n<div id=\"OSC_Astro_07_01_Giant\" class=\"os-figure\">\n<figure data-id=\"OSC_Astro_07_01_Giant\">\n<figure style=\"width: 975px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" id=\"8\" src=\"https:\/\/openstax.org\/apps\/archive\/20210823.155019\/resources\/011f181406075bfc7cae1139d664a4a7b2e4adc1\" alt=\"Diagram of the Four Giant Planets Shown to Scale. Arranged from left to right are Jupiter, Saturn, Uranus, and Neptune. Also shown to scale at lower center is the Earth.\" width=\"975\" height=\"345\" data-media-type=\"image\/jpeg\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 7.5<\/strong> The Four Giant Planets. This montage shows the four giant planets: Jupiter, Saturn, Uranus, and Neptune. Below them, Earth is shown to scale. (credit: modification of work by NASA, Solar System Exploration)<\/figcaption><\/figure>\n<\/figure>\n<\/div>\n<p id=\"fs-id1170326091199\" class=\"\">Near the outer edge of the system lies\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term333\" class=\"no-emphasis\" data-type=\"term\">Pluto<\/span>, which was the first of the distant icy worlds to be discovered beyond Neptune (Pluto was visited by a spacecraft, the NASA New Horizons mission, in 2015 [see\u00a0Figure 7.6]).\u00a0Table 7.2\u00a0summarizes some of the main facts about the planets.<\/p>\n<div id=\"OSC_Astro_07_01_PlutoNH\" class=\"os-figure\">\n<figure data-id=\"OSC_Astro_07_01_PlutoNH\">\n<figure style=\"width: 975px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" id=\"10\" src=\"https:\/\/openstax.org\/apps\/archive\/20210823.155019\/resources\/cc6c0afa476e672d5eaab316b3c6fdee534b683c\" alt=\"Image of a portion of the surface of Pluto. In this photograph from New Horizons, the smooth, white Sputnik plains are seen covering most of the upper right of the image. Rugged, heavily cratered terrain covers the lower center and upper left.\" width=\"975\" height=\"327\" data-media-type=\"image\/jpeg\" \/><figcaption class=\"wp-caption-text\"><strong>Figure\u00a07.6\u00a0<\/strong>Pluto Close-up.\u00a0This intriguing image from the New Horizons spacecraft, taken when it flew by the dwarf planet in July 2015, shows some of its complex surface features. The rounded white area is called the Sputnik Plain, after humanity\u2019s first spacecraft. (credit: modification of work by NASA\/Johns Hopkins University Applied Physics Laboratory\/Southwest Research Institute)<\/figcaption><\/figure>\n<\/figure>\n<div class=\"os-caption-container\"><\/div>\n<\/div>\n<div class=\"os-table os-top-titled-container\">\n<table id=\"fs-id1170326125105\" class=\"grid landscape aligncenter\" style=\"height: 152px\" summary=\"Table 7.2\">\n<caption>Table 7.2 The Planets<\/caption>\n<thead>\n<tr style=\"height: 32px\" valign=\"top\">\n<th style=\"height: 32px;width: 66.8594px\" scope=\"col\" data-valign=\"top\" data-align=\"center\">Name<\/th>\n<th style=\"height: 32px;width: 146.781px\" scope=\"col\" data-valign=\"top\" data-align=\"center\">Distance from Sun<span data-type=\"newline\"><br \/>\n<\/span>(AU)<sup id=\"footnote-ref2\" data-type=\"footnote-number\"><a role=\"doc-noteref\" href=\"https:\/\/openstax.org\/books\/astronomy\/pages\/7-1-overview-of-our-planetary-system#fs-id1170326168588\" data-type=\"footnote-link\">2<\/a><\/sup><\/th>\n<th style=\"height: 32px;width: 141.344px\" scope=\"col\" data-valign=\"top\" data-align=\"center\">Revolution Period<span data-type=\"newline\"><br \/>\n<\/span>(y)<\/th>\n<th style=\"height: 32px;width: 75.1094px\" scope=\"col\" data-valign=\"top\" data-align=\"center\">Diameter<span data-type=\"newline\"><br \/>\n<\/span>(km)<\/th>\n<th style=\"height: 32px;width: 65.625px\" scope=\"col\" data-valign=\"top\" data-align=\"center\">Mass<span data-type=\"newline\"><br \/>\n<\/span>[latex]{10^{23}}[\/latex]\u00a0kg)<\/th>\n<th style=\"height: 32px;width: 69.1562px\" scope=\"col\" data-valign=\"top\" data-align=\"center\">Density<span data-type=\"newline\"><br \/>\n<\/span>[latex]{\\rm{g\/c}}{{\\rm{m}}^3}[\/latex]<sup id=\"footnote-ref3\" data-type=\"footnote-number\"><a role=\"doc-noteref\" href=\"https:\/\/openstax.org\/books\/astronomy\/pages\/7-1-overview-of-our-planetary-system#fs-id1170326176843\" data-type=\"footnote-link\">3<\/a><\/sup><\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 67.3594px\" data-valign=\"top\" data-align=\"left\"><span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term334\" class=\"no-emphasis\" data-type=\"term\">Mercury<\/span><\/td>\n<td style=\"height: 15px;width: 147.781px\" data-valign=\"top\" data-align=\"left\">0.39<\/td>\n<td style=\"height: 15px;width: 142.344px\" data-valign=\"top\" data-align=\"left\">0.24<\/td>\n<td style=\"height: 15px;width: 76.1094px\" data-valign=\"top\" data-align=\"left\">4,878<\/td>\n<td style=\"height: 15px;width: 66.625px\" data-valign=\"top\" data-align=\"left\">3.3<\/td>\n<td style=\"height: 15px;width: 69.6562px\" data-valign=\"top\" data-align=\"left\">5.4<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 67.3594px\" data-valign=\"top\" data-align=\"left\"><span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term335\" class=\"no-emphasis\" data-type=\"term\">Venus<\/span><\/td>\n<td style=\"height: 15px;width: 147.781px\" data-valign=\"top\" data-align=\"left\">0.72<\/td>\n<td style=\"height: 15px;width: 142.344px\" data-valign=\"top\" data-align=\"left\">0.62<\/td>\n<td style=\"height: 15px;width: 76.1094px\" data-valign=\"top\" data-align=\"left\">12,120<\/td>\n<td style=\"height: 15px;width: 66.625px\" data-valign=\"top\" data-align=\"left\">48.7<\/td>\n<td style=\"height: 15px;width: 69.6562px\" data-valign=\"top\" data-align=\"left\">5.2<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 67.3594px\" data-valign=\"top\" data-align=\"left\"><span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term336\" class=\"no-emphasis\" data-type=\"term\">Earth<\/span><\/td>\n<td style=\"height: 15px;width: 147.781px\" data-valign=\"top\" data-align=\"left\">1.00<\/td>\n<td style=\"height: 15px;width: 142.344px\" data-valign=\"top\" data-align=\"left\">1.00<\/td>\n<td style=\"height: 15px;width: 76.1094px\" data-valign=\"top\" data-align=\"left\">12,756<\/td>\n<td style=\"height: 15px;width: 66.625px\" data-valign=\"top\" data-align=\"left\">59.8<\/td>\n<td style=\"height: 15px;width: 69.6562px\" data-valign=\"top\" data-align=\"left\">5.5<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 67.3594px\" data-valign=\"top\" data-align=\"left\"><span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term337\" class=\"no-emphasis\" data-type=\"term\">Mars<\/span><\/td>\n<td style=\"height: 15px;width: 147.781px\" data-valign=\"top\" data-align=\"left\">1.52<\/td>\n<td style=\"height: 15px;width: 142.344px\" data-valign=\"top\" data-align=\"left\">1.88<\/td>\n<td style=\"height: 15px;width: 76.1094px\" data-valign=\"top\" data-align=\"left\">6,787<\/td>\n<td style=\"height: 15px;width: 66.625px\" data-valign=\"top\" data-align=\"left\">6.4<\/td>\n<td style=\"height: 15px;width: 69.6562px\" data-valign=\"top\" data-align=\"left\">3.9<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 67.3594px\" data-valign=\"top\" data-align=\"left\"><span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term338\" class=\"no-emphasis\" data-type=\"term\">Jupiter<\/span><\/td>\n<td style=\"height: 15px;width: 147.781px\" data-valign=\"top\" data-align=\"left\">5.20<\/td>\n<td style=\"height: 15px;width: 142.344px\" data-valign=\"top\" data-align=\"left\">11.86<\/td>\n<td style=\"height: 15px;width: 76.1094px\" data-valign=\"top\" data-align=\"left\">142,984<\/td>\n<td style=\"height: 15px;width: 66.625px\" data-valign=\"top\" data-align=\"left\">18,991<\/td>\n<td style=\"height: 15px;width: 69.6562px\" data-valign=\"top\" data-align=\"left\">1.3<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 67.3594px\" data-valign=\"top\" data-align=\"left\"><span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term339\" class=\"no-emphasis\" data-type=\"term\">Saturn<\/span><\/td>\n<td style=\"height: 15px;width: 147.781px\" data-valign=\"top\" data-align=\"left\">9.54<\/td>\n<td style=\"height: 15px;width: 142.344px\" data-valign=\"top\" data-align=\"left\">29.46<\/td>\n<td style=\"height: 15px;width: 76.1094px\" data-valign=\"top\" data-align=\"left\">120,536<\/td>\n<td style=\"height: 15px;width: 66.625px\" data-valign=\"top\" data-align=\"left\">5686<\/td>\n<td style=\"height: 15px;width: 69.6562px\" data-valign=\"top\" data-align=\"left\">0.7<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 67.3594px\" data-valign=\"top\" data-align=\"left\"><span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term340\" class=\"no-emphasis\" data-type=\"term\">Uranus<\/span><\/td>\n<td style=\"height: 15px;width: 147.781px\" data-valign=\"top\" data-align=\"left\">19.18<\/td>\n<td style=\"height: 15px;width: 142.344px\" data-valign=\"top\" data-align=\"left\">84.07<\/td>\n<td style=\"height: 15px;width: 76.1094px\" data-valign=\"top\" data-align=\"left\">51,118<\/td>\n<td style=\"height: 15px;width: 66.625px\" data-valign=\"top\" data-align=\"left\">866<\/td>\n<td style=\"height: 15px;width: 69.6562px\" data-valign=\"top\" data-align=\"left\">1.3<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 67.3594px\" data-valign=\"top\" data-align=\"left\"><span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term341\" class=\"no-emphasis\" data-type=\"term\">Neptune<\/span><\/td>\n<td style=\"height: 15px;width: 147.781px\" data-valign=\"top\" data-align=\"left\">30.06<\/td>\n<td style=\"height: 15px;width: 142.344px\" data-valign=\"top\" data-align=\"left\">164.82<\/td>\n<td style=\"height: 15px;width: 76.1094px\" data-valign=\"top\" data-align=\"left\">49,660<\/td>\n<td style=\"height: 15px;width: 66.625px\" data-valign=\"top\" data-align=\"left\">1030<\/td>\n<td style=\"height: 15px;width: 69.6562px\" data-valign=\"top\" data-align=\"left\">1.6<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<div class=\"os-caption-container\">\n<h3 data-type=\"title\">Smaller Members of the Solar System<\/h3>\n<p id=\"fs-id1170326497885\" class=\"\">Most of the planets are accompanied by one or more moons; only Mercury and Venus move through space alone. There are more than 210 known moons orbiting planets and dwarf planets (see\u00a0Appendix G\u00a0for a listing of the larger ones), and undoubtedly many other small ones remain undiscovered. The largest of the moons are as big as small planets and just as interesting. In addition to our Moon, they include the four largest moons of Jupiter (called the Galilean moons, after their discoverer) and the largest moons of Saturn and Neptune (confusingly named Titan and Triton).<\/p>\n<p id=\"fs-id1170326145312\" class=\"\">Each of the giant planets also has rings made up of countless small bodies ranging in size from mountains to mere grains of dust, all in orbit about the equator of the planet. The bright rings of\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term343\" class=\"no-emphasis\" data-type=\"term\">Saturn<\/span>\u00a0are, by far, the easiest to see. They are among the most beautiful sights in the solar system (Figure 7.7). But, all four ring systems are interesting to scientists because of their complicated forms, influenced by the pull of the moons that also orbit these giant planets.<\/p>\n<div id=\"OSC_Astro_07_01_Voyager1\" class=\"os-figure\">\n<figure data-id=\"OSC_Astro_07_01_Voyager1\">\n<figure style=\"width: 975px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" id=\"13\" src=\"https:\/\/openstax.org\/apps\/archive\/20210823.155019\/resources\/b8cd2e7a4c3ab26415acf3954d95cbe1682414a1\" alt=\"Image of Saturn and its Rings. Taken almost directly over one of Saturn\u2019s poles, the rings are seen nearly face-on, completely encircling the planet. Sunlight arrives from lower left as the rings cast a thin shadow on Saturn\u2019s cloud tops, while Saturn itself casts a shadow on the rings at upper right.\" width=\"975\" height=\"340\" data-media-type=\"image\/jpeg\" \/><figcaption class=\"wp-caption-text\">Figure\u00a07.7\u00a0Saturn and Its Rings.\u00a0This 2007 Cassini image shows\u00a0Saturn\u00a0and its complex system of rings, taken from a distance of about 1.2 million kilometers. This natural-color image is a composite of 36 images taken over the course of 2.5 hours. (credit: modification of work by NASA\/JPL\/Space Science Institute)<\/figcaption><\/figure>\n<\/figure>\n<\/div>\n<p id=\"fs-id1170326093329\" class=\"\">The solar system has many other less-conspicuous members. Another group is the\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term345\" data-type=\"term\">asteroids<\/span>, rocky bodies that orbit the Sun like miniature planets, mostly in the space between Mars and Jupiter (although some do cross the orbits of planets like Earth\u2014see\u00a0Figure 7.8). Most asteroids are remnants of the initial population of the solar system that existed before the planets themselves formed. Some of the smallest moons of the planets, such as the moons of Mars, are very likely captured asteroids.<\/p>\n<div id=\"OSC_Astro_07_01_Asteroid\" class=\"os-figure\">\n<figure data-id=\"OSC_Astro_07_01_Asteroid\">\n<figure style=\"width: 975px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" id=\"15\" src=\"https:\/\/openstax.org\/apps\/archive\/20210823.155019\/resources\/b7978cb6097d52a591f540060354508927fb14ee\" alt=\"Image of the Asteroid Eros. Like nearly all asteroids, Eros is not spherical but very irregular in shape, in this case similar to a potato. The surface is pock-marked with many craters.\" width=\"975\" height=\"281\" data-media-type=\"image\/jpeg\" \/><figcaption class=\"wp-caption-text\"><strong>Figure\u00a07.8<\/strong>\u00a0Asteroid Eros.\u00a0This small Earth-crossing asteroid image was taken by the NEAR-Shoemaker spacecraft from an altitude of about 100 kilometers. This view of the heavily cratered surface is about 10 kilometers wide. The spacecraft orbited Eros for a year before landing gently on its surface. (credit: modification of work by NASA\/JHUAPL)<\/figcaption><\/figure>\n<\/figure>\n<\/div>\n<p id=\"fs-id1170326449205\" class=\"\">Another class of small bodies is composed mostly of ice, made of frozen gases such as water, carbon dioxide, and carbon monoxide; these objects are called\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term346\" data-type=\"term\">comets<\/span>\u00a0(see\u00a0Figure 7.9). Comets also are remnants from the formation of the solar system, but they were formed and continue (with rare exceptions) to orbit the Sun in distant, cooler regions\u2014stored in a sort of cosmic deep freeze. This is also the realm of the larger icy worlds, called dwarf planets.<\/p>\n<div id=\"OSC_Astro_07_01_Comet\" class=\"os-figure\">\n<figure data-id=\"OSC_Astro_07_01_Comet\">\n<figure style=\"width: 975px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" id=\"17\" src=\"https:\/\/openstax.org\/apps\/archive\/20210823.155019\/resources\/a3d16ced5e8f6825a9bd9080ce3e33d90723c76b\" alt=\"Image of Comet Churyumov-Gerasimenko (67P). Two lobes of this irregularly shaped object are illuminated by sunlight coming from the upper left. Bright streaks of material are seen radiating away from the sunlit surfaces of the comet. These streaks are not seen coming from the shaded portions.\" width=\"975\" height=\"410\" data-media-type=\"image\/jpeg\" \/><figcaption class=\"wp-caption-text\"><strong>Figure\u00a07.9<\/strong>\u00a0Comet Churyumov-Gerasimenko (67P).\u00a0This image shows Comet Churyumov-Gerasimenko, also known as 67P, near its closest approach to the Sun in 2015, as seen from the\u00a0Rosetta\u00a0spacecraft. Note the jets of gas escaping from the solid surface. (credit: modification of work by ESA\/Rosetta\/NAVACAM,\u00a0CC BY-SA IGO 3.0)<\/figcaption><\/figure>\n<\/figure>\n<\/div>\n<p id=\"fs-id1170326096149\" class=\"\">Finally, there are countless grains of broken rock, which we call cosmic dust, scattered throughout the solar system. When these particles enter Earth\u2019s atmosphere (as millions do each day) they burn up, producing a brief flash of light in the night sky known as a\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term347\" data-type=\"term\">meteor<\/span>\u00a0(meteors are often referred to as shooting stars). Occasionally, some larger chunk of rocky or metallic material survives its passage through the atmosphere and lands on Earth. Any piece that strikes the ground is known as a\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term348\" data-type=\"term\">meteorite<\/span>. (You can see meteorites on display in many natural history museums and can sometimes even purchase pieces of them from gem and mineral dealers.)<\/p>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h3 class=\"textbox__title\">Voyagers in Astronomy<\/h3>\n<\/header>\n<div class=\"textbox__content\">\n<h4 id=\"18\" class=\"os-subtitle\" data-type=\"title\"><span class=\"os-subtitle-label\">Carl Sagan: Solar System Advocate<\/span><\/h4>\n<p id=\"fs-id1170326086114\" class=\"\">The best-known astronomer in the world during the 1970s and 1980s, Carl\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term349\" class=\"no-emphasis\" data-type=\"term\">Sagan<\/span>\u00a0devoted most of his professional career to studying the planets and considerable energy to raising public awareness of what we can learn from exploring the solar system (see\u00a0Figure 7.10). Born in Brooklyn, New York, in 1934, Sagan became interested in astronomy as a youngster; he also credits science fiction stories for sustaining his fascination with what\u2019s out in the universe.<\/p>\n<div id=\"OSC_Astro_07_04_Sagan\" class=\"os-figure\">\n<figure data-id=\"OSC_Astro_07_04_Sagan\">\n<figure style=\"width: 732px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" id=\"20\" src=\"https:\/\/openstax.org\/apps\/archive\/20210823.155019\/resources\/e7453e5380c0b28d8eaa0eff31474de7e375f8f3\" alt=\"Left image: photograph of Carl Sagan. Right image: Photograph of Neil deGrasse Tyson.\" width=\"732\" height=\"429\" data-media-type=\"image\/jpeg\" \/><figcaption class=\"wp-caption-text\"><strong>Figure\u00a07.10\u00a0<\/strong>Carl Sagan (1934\u20131996) and Neil deGrasse Tyson.\u00a0Sagan was Tyson\u2019s inspiration to become a scientist. (credit \u201cSagan\u201d: modification of work by NASA, JPL; credit \u201cTyson\u201d: modification of work by Bruce F. Press)<\/figcaption><\/figure>\n<\/figure>\n<div class=\"os-caption-container\"><\/div>\n<\/div>\n<p id=\"fs-id1170326095818\" class=\"\">In the early 1960s, when many scientists still thought Venus might turn out to be a hospitable place, Sagan calculated that the thick atmosphere of Venus could act like a giant greenhouse, keeping the heat in and raising the temperature enormously. He showed that the seasonal changes astronomers had seen on Mars were caused, not by vegetation, but by wind-blown dust. He was a member of the scientific teams for many of the robotic missions that explored the solar system and was instrumental in getting NASA to put a message-bearing plaque aboard the Pioneer spacecraft, as well as audio-video records on the Voyager spacecraft\u2014all of them destined to leave our solar system entirely and send these little bits of Earth technology out among the stars.<\/p>\n<p id=\"fs-id1170326112426\" class=\"\">To encourage public interest and public support of planetary exploration, Sagan helped found The Planetary Society, now the largest space-interest organization in the world. He was a tireless and eloquent advocate of the need to study the solar system close-up and the value of learning about other worlds in order to take better care of our own.<\/p>\n<p id=\"fs-id1170326129355\" class=\"\">Sagan simulated conditions on early Earth to demonstrate how some of life\u2019s fundamental building blocks might have formed from the \u201cprimordial soup\u201d of natural compounds on our planet. In addition, he and his colleagues developed computer models showing the consequences of nuclear war for Earth would be even more devastating than anyone had thought (this is now called the nuclear winter hypothesis) and demonstrating some of the serious consequences of continued pollution of our atmosphere.<\/p>\n<p id=\"fs-id1170326113790\" class=\"\">Sagan was perhaps best known, however, as a brilliant popularizer of astronomy and the author of many books on science, including the best-selling\u00a0<em data-effect=\"italics\">Cosmos<\/em>, and several evocative tributes to solar system exploration such as\u00a0<em data-effect=\"italics\">The Cosmic Connection<\/em>\u00a0and\u00a0<em data-effect=\"italics\">Pale Blue Dot<\/em>. His book\u00a0<em data-effect=\"italics\">The Demon Haunted World<\/em>, completed just before his death in 1996, is perhaps the best antidote to fuzzy thinking about pseudo-science and irrationality in print today. An intriguing science fiction novel he wrote, titled\u00a0<em data-effect=\"italics\">Contact<\/em>, which became a successful film as well, is still recommended by many science instructors as a scenario for making contact with life elsewhere that is much more reasonable than most science fiction.<\/p>\n<p id=\"fs-id1170326471153\" class=\"\">Sagan was a master, too, of the television medium. His 13-part public television series,\u00a0<em data-effect=\"italics\">Cosmos<\/em>, was seen by an estimated 500 million people in 60 countries and has become one of the most-watched series in the history of public broadcasting. A few astronomers scoffed at a scientist who spent so much time in the public eye, but it is probably fair to say that Sagan\u2019s enthusiasm and skill as an explainer won more friends for the science of astronomy than anyone or anything else in the second half of the twentieth century.<\/p>\n<p id=\"fs-id1170326169519\" class=\"\">In the two decades since Sagan\u2019s death, no other scientist has achieved the same level of public recognition. Perhaps closest is the director of the Hayden Planetarium, Neil deGrasse\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term350\" class=\"no-emphasis\" data-type=\"term\">Tyson<\/span>, who followed in Sagan\u2019s footsteps by making an updated version of the\u00a0<em data-effect=\"italics\">Cosmos<\/em>\u00a0program in 2014. Tyson is quick to point out that Sagan was his inspiration to become a scientist, telling how Sagan invited him to visit for a day at Cornell when he was a high school student looking for a career. However, the media environment has fragmented a great deal since Sagan\u2019s time. It is interesting to speculate whether Sagan could have adapted his communication style to the world of cable television, Twitter, Facebook, and podcasts.<\/p>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h3 class=\"textbox__title\">Link to Learning<\/h3>\n<\/header>\n<div class=\"textbox__content\">\n<p>Two imaginative videos provide a tour of the solar system objects we have been discussing. Shane Gellert\u2019s\u00a0<a href=\"https:\/\/openstax.org\/l\/30needsomespace\" target=\"_blank\" rel=\"noopener nofollow noreferrer\">I Need Some Space<\/a>\u00a0uses NASA photography and models to show the various worlds with which we share our system. In the more science fiction-oriented\u00a0<a href=\"https:\/\/openstax.org\/l\/30wanderers\" target=\"_blank\" rel=\"noopener nofollow noreferrer\">Wanderers<\/a>\u00a0video, we see some of the planets and moons as tourist destinations for future explorers, with commentary taken from recordings by Carl Sagan.<\/p>\n<\/div>\n<\/div>\n<h3 data-type=\"title\">A Scale Model of the Solar System<\/h3>\n<p id=\"fs-id1170326093910\" class=\"\">Astronomy often deals with dimensions and distances that far exceed our ordinary experience. What does 1.4 billion kilometers\u2014the distance from the Sun to Saturn\u2014really mean to anyone? It can be helpful to visualize such large systems in terms of a scale model.<\/p>\n<p id=\"fs-id1170326386156\" class=\"\">In our imaginations, let us build a scale model of the solar system, adopting a scale factor of 1 billion ([latex]{10^9}[\/latex])\u2014that is, reducing the actual solar system by dividing every dimension by a factor of [latex]{10^9}[\/latex]. Earth, then, has a diameter of 1.3 centimeters, about the size of a grape. The Moon is a pea orbiting this at a distance of 40 centimeters, or a little more than a foot away. The Earth-Moon system fits into a standard backpack.<\/p>\n<p id=\"fs-id1170326042510\" class=\"\">In this model, the Sun is nearly 1.5 meters in diameter, about the average height of an adult, and our Earth is at a distance of 150 meters\u2014about one city block\u2014from the Sun. Jupiter is five blocks away from the Sun, and its diameter is 15 centimeters, about the size of a very large grapefruit. Saturn is 10 blocks from the Sun; Uranus, 20 blocks; and Neptune, 30 blocks. Pluto, with a distance that varies quite a bit during its 249-year orbit, is currently just beyond 30 blocks and getting farther with time. Most of the moons of the outer solar system are the sizes of various kinds of seeds orbiting the grapefruit, oranges, and lemons that represent the outer planets.<\/p>\n<p id=\"fs-id1170326128525\" class=\"\">In our scale model, a human is reduced to the dimensions of a single atom, and cars and spacecraft to the size of molecules. Sending the Voyager spacecraft to Neptune involves navigating a single molecule from the Earth\u2013grape toward a lemon 5 kilometers away with an accuracy equivalent to the width of a thread in a spider\u2019s web.<\/p>\n<p id=\"fs-id1170326087667\" class=\"\">If that model represents the solar system, where would the nearest stars be? If we keep the same scale, the closest stars would be tens of thousands of kilometers away. If you built this scale model in the city where you live, you would have to place the representations of these stars on the other side of Earth or beyond.<\/p>\n<p id=\"fs-id1170326286048\" class=\"\">By the way, model solar systems like the one we just presented have been built in cities throughout the world. In Sweden, for example, Stockholm\u2019s huge Globe Arena has become a model for the Sun, and Pluto is represented by a 12-centimeter sculpture in the small town of Delsbo, 300 kilometers away. Another model solar system is in Washington on the Mall between the White House and Congress (perhaps proving they are worlds apart?).<\/p>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h3 class=\"textbox__title\">Link to Learning<\/h3>\n<\/header>\n<div class=\"textbox__content\">\n<p>This\u00a0<a href=\"https:\/\/openstax.org\/l\/30modsolsys\" target=\"_blank\" rel=\"noopener nofollow noreferrer\">model of the solar system<\/a>\u00a0shows all orbits and sizes to scale, and it lets you fly between the planets at an enhanced speed.<\/p>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h3 class=\"textbox__title\">Making Connections<\/h3>\n<\/header>\n<div class=\"textbox__content\">\n<h4 id=\"21\" class=\"os-subtitle\" data-type=\"title\"><span class=\"os-subtitle-label\">Names in the Solar System<\/span><\/h4>\n<p id=\"fs-id1170326097579\" class=\"\">We humans just don\u2019t feel comfortable until something has a name. Types of butterflies, new elements, and the mountains of Venus all need names for us to feel we are acquainted with them. How do we give names to objects and features in the solar system?<\/p>\n<p id=\"fs-id1170326454356\" class=\"\">Planets and moons are named after gods and heroes in Greek and Roman mythology (with a few exceptions among the moons of Uranus, which have names drawn from English literature). When William Herschel, a German immigrant to England, first discovered the planet we now call Uranus, he wanted to name it Georgium Sidus (George\u2019s star) after King George III of his adopted country. This caused such an outcry among astronomers in other nations, however, that the classic tradition was upheld\u2014and has been maintained ever since. Luckily, there were a lot of minor gods in the ancient pantheon, so plenty of names are left for the many small moons we are discovering around the giant planets. (Appendix G\u00a0lists the larger moons). More recently, the names of dwarf planets and their moons have been drawn from the mythology of other cultures besides Greek and Roman.<\/p>\n<p id=\"fs-id1170326478164\" class=\"\">Comets are often named after their discoverers (offering an extra incentive to comet hunters). Asteroids are named by their discoverers after just about anyone or anything they want. Recently, asteroid names have been used to recognize people who have made significant contributions to astronomy, including the three senior authors of this book.<\/p>\n<p id=\"fs-id1170326146431\" class=\"\">That was pretty much all the naming that was needed while our study of the solar system was confined to Earth. But now, our spacecraft have surveyed and photographed many worlds in great detail, and each world has a host of features that also need names. To make sure that naming things in space remains multinational, rational, and somewhat dignified, astronomers have given the responsibility of approving names to a special committee of the\u00a0<span id=\"c9239bd9-a44b-4187-a3f9-d0a5511b45e4_term351\" class=\"no-emphasis\" data-type=\"term\">International Astronomical Union<\/span>\u00a0(IAU), the body that includes scientists from every country that does astronomy.<\/p>\n<p id=\"fs-id1170326066472\" class=\"\">This IAU committee has developed a set of rules for naming features on other worlds. For example, craters on Venus are named for women who have made significant contributions to human knowledge and welfare. Volcanic features on Jupiter\u2019s moon Io, which is in a constant state of volcanic activity, are named after gods of fire and thunder from the mythologies of many cultures. Craters on Mercury commemorate famous novelists, playwrights, artists, and composers. On Saturn\u2019s moon Tethys, all the features are named after characters and places in Homer\u2019s great epic poem,\u00a0<em data-effect=\"italics\">The Odyssey<\/em>. As we explore further, it may well turn out that more places in the solar system need names than Earth history can provide. Perhaps by then, explorers and settlers on these worlds will be ready to develop their own names for the places they may (if but for a while) call home.<\/p>\n<p id=\"fs-id1170326096369\" class=\"\">You may be surprised to know that the meaning of the word\u00a0<em data-effect=\"italics\">planet<\/em>\u00a0has recently become controversial because we have discovered many other planetary systems that don\u2019t look very much like our own. Even within our solar system, the planets differ greatly in size and chemical properties. The biggest dispute concerns Pluto, which is much smaller than the other eight major planets. The category of dwarf planet was invented to include Pluto and similar icy objects beyond Neptune. But is a dwarf planet also a planet? Logically, it should be, but even this simple issue of grammar has been the subject of heated debate among both astronomers and the general public.<\/p>\n<\/div>\n<\/div>\n<h3 data-type=\"footnote-refs-title\">Footnotes<\/h3>\n<ul data-list-type=\"bulleted\" data-bullet-style=\"none\">\n<li id=\"fs-id1170326145109\" data-type=\"footnote-ref\"><a role=\"doc-backlink\" href=\"https:\/\/openstax.org\/books\/astronomy\/pages\/7-1-overview-of-our-planetary-system#footnote-ref1\">1<\/a> <span data-type=\"footnote-ref-content\">The generic term for a group of planets and other bodies circling a star is\u00a0<em data-effect=\"italics\">planetary system<\/em>. Ours is called the\u00a0<em data-effect=\"italics\">solar system<\/em>\u00a0because our Sun is sometimes called\u00a0<em data-effect=\"italics\">Sol<\/em>. Strictly speaking, then, there is only one solar system; planets orbiting other stars are in planetary systems.<\/span><\/li>\n<li id=\"fs-id1170326168588\" data-type=\"footnote-ref\"><a role=\"doc-backlink\" href=\"https:\/\/openstax.org\/books\/astronomy\/pages\/7-1-overview-of-our-planetary-system#footnote-ref2\">2<\/a> <span data-type=\"footnote-ref-content\">An AU (or astronomical unit) is the distance from Earth to the Sun.<\/span><\/li>\n<li id=\"fs-id1170326176843\" data-type=\"footnote-ref\"><a role=\"doc-backlink\" href=\"https:\/\/openstax.org\/books\/astronomy\/pages\/7-1-overview-of-our-planetary-system#footnote-ref3\">3<\/a> <span data-type=\"footnote-ref-content\">We give densities in units where the density of water is 1 g\/cm<sup>3<\/sup>. To get densities in units of kg\/m<sup>3<\/sup>, multiply the given value by 1000.<\/span><\/li>\n<\/ul>\n<\/div>\n<\/div>\n<\/figure>\n<\/div>\n<div class=\"textbox\">This book was adapted from the following: Fraknoi, A., Morrison, D., &amp; Wolff, S. C. (2016). 7.1 Overview of Our Planetary System. In <i>Astronomy<\/i>. OpenStax. https:\/\/openstax.org\/books\/astronomy\/pages\/7-1-overview-of-our-planetary-system under a <a href=\"http:\/\/creativecommons.org\/licenses\/by\/4.0\/\" target=\"_blank\" rel=\"noopener noreferrer\">Creative Commons Attribution License 4.0<\/a><\/div>\n<div>Access the entire book for free at <a href=\"https:\/\/openstax.org\/books\/astronomy\/pages\/1-introduction\">https:\/\/openstax.org\/books\/astronomy\/pages\/1-introduction<\/a><\/div>\n","protected":false},"author":33,"menu_order":1,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[48],"contributor":[],"license":[],"class_list":["post-327","chapter","type-chapter","status-publish","hentry","chapter-type-numberless"],"part":323,"_links":{"self":[{"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/pressbooks\/v2\/chapters\/327","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/wp\/v2\/users\/33"}],"version-history":[{"count":5,"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/pressbooks\/v2\/chapters\/327\/revisions"}],"predecessor-version":[{"id":849,"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/pressbooks\/v2\/chapters\/327\/revisions\/849"}],"part":[{"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/pressbooks\/v2\/parts\/323"}],"metadata":[{"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/pressbooks\/v2\/chapters\/327\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/wp\/v2\/media?parent=327"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/pressbooks\/v2\/chapter-type?post=327"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/wp\/v2\/contributor?post=327"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/wp\/v2\/license?post=327"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}