The solar system consists of the Sun; the nine planets, sixty eight (68) satellites of the planets, a large number of small bodies (the comets and asteroids), and the interplanetary medium. The inner solar system contains the Sun, Mercury, Venus, Earth and Mars:

   The planets of the outer solar system are Jupiter, Saturn, Uranus, Neptune and Pluto:

   The orbits of the planets are ellipses with the Sun at one focus, though all except Mercury and Pluto are very nearly circular. The orbits of the planets are all more or less in the same plane (called the ecliptic and defined by the plane of the Earth's orbit). The ecliptic is inclined only 7 degrees from the plane of the Sun's equator. Pluto's orbit deviates the most from the plane of the ecliptic with an inclination of 17 degrees. The above diagrams show the relative sizes of the orbits of the nine planets from a perspective somewhat above the ecliptic (hence their non-circular appearance). They all orbit in the same direction (counter-clockwise looking down from above the Sun's north pole); all but Venus, Uranus and Pluto also rotate in that same sense.

 

 

   The above composite shows the nine planets (click to enlarge) with approximately correct relative sizes  

   One way to help visualize the relative sizes in the solar system is to imagine a model in which it is reduced in size by a factor of a billion . Then the Earth is about 1.3 cm in diameter (the size of a grape). The Moon orbits about a foot away. The Sun is 1.5 meters in diameter (about the height of a man) and 150 meters (about a city block) from the Earth. Jupiter is 15 cm in diameter (the size of a large grapefruit) and 5 blocks away from the Sun. Saturn (the size of an orange) is 10 blocks away; Uranus and Neptune (lemons) are 20 and 30 blocks away. A human on this scale is the size of an atom; the nearest star would be over 40000 km away.

   Not shown in the above illustrations are the numerous smaller bodies that inhabit the solar system: the satellites of the planets; the large number of asteroids (small rocky bodies) orbiting the Sun, mostly between Mars and Jupiter but also elsewhere; and the comets (small icy bodies) which come and go from the inner parts of the solar system in highly elongated orbits and at random orientations to the ecliptic. With a few exceptions, the planetary satellites orbit in the same sense as the planets and approximately in the plane of the ecliptic but this is not generally true for comets and asteroids.

Classification

   The classification of these objects is a matter of minor controversy. Traditionally, the solar system has been divided into planets (the big bodies orbiting the Sun), their satellites (a.k.a. moons, variously sized objects orbiting the planets), asteroids (small dense objects orbiting the Sun) and comets (small icy objects with highly eccentric orbits). Unfortunately, the solar system has been found to be more complicated than this would suggest:

	there are several moons larger than Pluto and two larger than Mercury;
	there are several small moons that are probably captured asteroids;
	comets sometimes fizzle out and become indistinguishable from asteroids;
	the Kuiper Belt objects and others like Chiron don't fit this scheme well;
	The Earth/Moon and Pluto/Charon systems are sometimes considered "double planets".

   The nine bodies conventionally referred to as planets are often further classified in several ways:

	by composition: Terrestrial or rocky planets: Mercury, Venus, Earth, and Mars: The terrestrial planets are composed primarily of rock and metal and have relatively high densities, slow rotation, solid surfaces, no rings and few satellites. Jovian or gas planets: Jupiter, Saturn, Uranus, and Neptune: The gas planets are composed primarily of hydrogen and helium and generally have low densities, rapid rotation, deep atmospheres, rings and lots of satellites. Pluto.
	by size: small planets: Mercury, Venus, Earth, Mars and Pluto. The small planets have diameters less than 13000 km. giant planets: Jupiter, Saturn, Uranus and Neptune. The giant planets have diameters greater than 48000 km. Mercury and Pluto are sometimes referred to as lesser planets (not to be confused with minor planets which is the official term for asteroids). The giant planets are sometimes also referred to as gas giants.
	by position relative to the Sun: inner planets: Mercury, Venus, Earth and Mars. outer planets: Jupiter, Saturn, Uranus, Neptune and Pluto. The asteroid belt between Mars and Jupiter forms the boundary between the inner solar system and the outer solar system.
	by position relative to Earth: inferior planets: Mercury and Venus. closer to the Sun than Earth. The inferior planets show phases like the Moon's when viewed from Earth. Earth. superior planets: Mars thru Pluto. farther from the Sun than Earth. The superior planets always appear full or nearly so.
	by history: classical planets: Mercury, Venus, Mars, Jupiter, and Saturn. known since prehistorical times visible to the unaided eye modern planets: Uranus, Neptune, Pluto. discovered in modern times visible only with telescopes Earth.                Planet Distance (000 km) Radius (km) Mass  (kg) Discovered by: Date Mercury 57,910 2439 3.30e23     Venus 108,200 6052 4.87e24     Earth   149,600 6378 5.98e24     Mars 227,940 3397 6.42e23     Jupiter  778,330 71492 1.90e27     Saturn 1,426,940 60268 5.69e26     Uranus 2,870,990 25559 8.69e25 Herschel 1781 Neptune 4,497,070 24764 1.02e26 Galle 1846 Pluto  5,913,520 1160 1.31e22 Tombaugh  1930                            Earth, Sun and Moon Solar System Earth Mars Pluto's Moon Charon Uranus' Moon Bianca Halley's Comet Mars' Moon Deimos Saturn Uranus' Moon Caliban Jupiter Mars' Moon Phobos Saturn's Moon Dione Uranus' Moon Cordelia Jupiter's Moon Ananke Mercury Saturn's Moon Enceladus Uranus' Moon Cressida Jupiter's Moon Callisto Meteor Showers Calendar Saturn's Moon Hyperion Uranus' Moon Desdemona Jupiter's Moon Carme Moon Saturn's Moon Iapetus Uranus' Moon Juliet Jupiter's Moon Elara Neptune Saturn's Moon Mimas Uranus' Moon Oberon Jupiter's Moon Europa Neptune's Moon Despina Saturn's Moon Phoebe Uranus' Moon Ophelia Jupiter's Moon Ganymede Neptune's Moon Galatea Saturn's Moon Rhea Uranus' Moon Portia Jupiter's Moon Himalia Neptune's Moon Larissa Saturn's Moon Tethys Uranus' Moon Puck Jupiter's Moon Io Neptune's Moon Naiad Saturn's Moon Titan Uranus' Moon Rosalind Jupiter's Moon Leda Neptune's Moon Nereid Solar System Uranus' Moon Sycorax Jupiter's Moon Lysithea Neptune's Moon Proteus Sun Uranus' Moon Titania Jupiter's Moon Pasiphae Neptune's Moon Thalassa Uranus Uranus' Moon Umbriel Jupiter's Moon Sinope Neptune's Moon Triton Uranus' Moon Ariel Venus Leonids Pluto Uranus' Moon Belinda   Space radar images of Earth from the Shuttle Latest GIF from NOAA's GOES-8 satellite Declassified Spy Satellite Photographs Earth Science Pictures Earth Observing System Educators' Visual Materials Center for Earth and Planetary Study http://www.nasm.si.edu/ceps/research/research.htm The Center for Earth and Planetary Studies (CEPS), which is part of the Smithsonian's National Air and Space Museum, "conducts an active research program in planetary and terrestrial geology and geophysics using remote sensing data from Earth-orbiting satellites and manned and unmanned space missions." The Research portion of the site allows visitors to view the many projects that have taken place, such as the Topographic Mapping of Mercury project being conducted by Dr. Anthony Cook.  Each page contains a description of the project; a link to each researcher's staff page; and links to images, graphs, maps, and more.