National Aeronautics and Space Administration






www..gov Humans have gazed at the heavens and tried to understand solid with icy surfaces. NASA are en route to two of continue to send data until at least 2020. It will be thousands of the cosmos for thousands of years. Ancient civilizations placed the dwarf to study them — the Dawn mission will visit years before the two Voyagers exit the enormous Oort , a great emphasis on careful astronomical observations. Early in 2015 and the mission will reach in vast spherical shell of icy bodies surrounding the solar system. Greek were among the first to leave a written re- that same year. Neither Ceres nor Pluto has been previously vis- As we explore the , we wonder: Are there other planets cord of their attempts to explain the cosmos. For them, the uni- ited by any spacecraft. verse was Earth, the , the , the , and five glowing where life might exist? Are we alone? These are the great ques- points of light that moved among the stars. The Greeks named , rings, and magnetic fields characterize . tions that science is now probing. Only recently have astrono- the five points of light — called planetos, or wanderers — after There are 145 known planetary moons, with at least 22 moons mers had the tools to detect large planets around other stars in their gods. The Romans later translated the names into Latin — awaiting official recognition. (Three of the dwarf planets also other solar systems using on Earth and in space. , , Mars, Jupiter, and — and these are the have moons: Pluto has three, has one, and has FAST FACTS names astronomers use today. Planetary features are named two.) The planetary moons are not all alike. One moon (Saturn’s by the International Astronomical Union, founded in 1919. For ) has a thick ; another has active volcanoes Mean Distance more information about names of planets, moons, and features, (Jupiter’s ). Equatorial from the Sun Radius km, mi, consult the Gazetteer of Planetary website at Rings are an intriguing planetary feature. From 1659 to 1979, Body km mi millions millions Moons* planetarynames.wr.usgs.gov. Saturn was thought to be the only with rings. NASA’s Sun 695,500 432,200 — — — Ancient observers believed that the Sun and all the other ce- Voyager missions to the outer planets showed that Jupiter, Uranus, and Neptune also have ring systems. Mercury 2,440 1,516 57.91 35.98 0 lestial bodies revolved around Earth. But astronomers gradually Venus 6,052 3,760 108.21 67.24 0 realized that the Earth-centered model did not account for the Most of the planets have magnetic fields that extend into space Earth 6,378 3,963 149.60 92.96 1 motions of the planets. In the early 17th century, Gali- and form a around each planet. These magneto- Moon 1,737 1,080 ** ** — lei’s discoveries using the recently invented strongly spheres rotate with the planet, sweeping charged particles with Mars 3,397 2,111 227.94 141.63 2 supported the concept of a “solar system” in which all the plan- them. Jupiter 71,492 44,423 778.41 483.68 49† ets, including Earth, revolve around a central — the Sun. Saturn 60,268 37,449 1,426.73 886.53 53‡ Planetary moons, the , and more planets were How big is our solar system? To think about the large distances, Uranus 25,559 15,882 2,870.97 1,783.94 27 eventually discovered: Uranus (in 1781) and Neptune (1846). The we use a cosmic ruler based on the (AU). One Neptune 24,764 15,388 4,498.25 2,795.08 13 largest known , Ceres, was discovered between Mars AU is the distance from Earth to the Sun, which is about 150 mil- *Known moons as of September 2009. The moons are not and Jupiter in 1801. Originally classified as a planet, Ceres is lion kilometers or 93 million . The area of the Sun’s influ- included in this list. now designated a dwarf planet (but retains its asteroid label), ence stretches far beyond the planets, forming a giant bubble **Mean Earth–Moon distance: 384,400 kilometers or 238,855 miles. †Jupiter has 13 moons awaiting official confirmation, bringing the total to 62. along with Pluto, which was discovered in 1930; Eris, found in called the heliosphere. The enormous bubble of the heliosphere ‡Saturn has 9 moons awaiting official confirmation, bringing the total to 62. 2003; Haumea, found in 2004; and , found in 2005. is created by the solar , a stream of charged gas blowing outward from the Sun. As the Sun the center of the Milky ABOUT THE ILLUSTRATION Our solar system formed about 4.6 billion years ago. The four Way, the bubble of the heliosphere moves also, creating a bow planets closest to the Sun — Mercury, Venus, Earth, and Mars — shock ahead of itself in interstellar space — like the bow of a The planets are shown in the correct order of distance from the are called the terrestrial planets because they have solid, rocky ship in — as it crashes into the interstellar gases. The area Sun, the correct relative sizes, and the correct relative orbital surfaces. Two of the outer planets beyond the of Mars — where the is abruptly slowed by from gas distances. The sizes of the bodies are greatly exaggerated rela- Jupiter and Saturn — are known as gas giants; the more distant between the stars is called the termination shock. tive to the orbital distances. The faint , Uranus, Uranus and Neptune are called giants. and Neptune are not shown. Eris, Haumea, and Makemake do A spacecraft that reached the termination shock would be able not appear in the illustration owing to their highly tilted orbits. Earth’s atmosphere is primarily and oxygen. Mer- to measure the slowing effect, and that is exactly what happened The dwarf planet Ceres is not shown separately; it resides in the cury has a tenuous atmosphere, while Venus has a thick when began sending unusual data to Earth in late between Mars and Jupiter. atmosphere of mainly . Mars’ carbon dioxide 2003. In December 2004, scientists confirmed that Voyager 1 atmosphere is extremely thin. Jupiter and Saturn are composed had crossed the termination shock at about 94 AU, approxi- FOR MORE INFORMATION mostly of and , while Uranus and Neptune are mately 13 billion kilometers (8.7 billion miles) from the Sun, ven- solarsystem.nasa.gov/planets/profile.cfm?Object=SolarSys composed mostly of water, , and , with icy turing into the vast, turbulent expanse where the Sun’s influence mantles around their cores. The Voyager 1 and 2 spacecraft diminishes. , 16 billion kilometers (10 billion miles) solarsystem.nasa.gov/education/ visited the gas giants, and Voyager 2 flew by and imaged the from Voyager 1, crossed the termination shock in August 2007. ice giants. Ceres and the outer dwarf planets — Pluto, Eris, Voyager 1 may reach interstellar space sometime between 2014 Haumea, and Makemake — have similar compositions and are and 2017; the spacecraft should have enough electrical power to

LG-2009-09-563-HQ — JPL 400-1344B 09/09