Mercury, shown at left in a Mariner 10 image, is the closest planet to the Sun - it orbits at an average distance of 57,910,000 km, or 0.38 AU from Sun. (An AU is an Astronomical Unit; it is equal to the distance between the Sun and the Earth.) Mercury's diameter is 4,880 km and it has a mass of 3.3023 kg. Mercury has been known since at least the time of the Sumerians (3rd millennium BC). Although the Greeks had two names for it, Apollo for its apparition as a morning star and Hermes as an evening star, they knew that the two names referred to the same object. We know this planet as Mercury - the name of the Roman counterpart to Hermes.

At right is a Galileo image of Venus. Venus is located 108,200,000 km (0.72 AU) from the Sun. It has a diameter of 12,103.6 km and a mass of 4.86924 kg. Venus' rotation is somewhat unusual in that it is both very slow (243 Earth days per Venus day, slightly longer than Venus' year) and retrograde (east to west, rather than west to east like Earth's). The pressure of Venus' atmosphere at the surface is 90 atmospheres (about the same as the pressure at a depth of 1 km in Earth's oceans). It is composed mostly of carbon dioxide. There are several layers of clouds many kilometers thick composed of sulfuric acid. These clouds completely obscure our view of the surface. However, we do have photographs of its surface taken by the 1970s Venera missions. The dense atmosphere produces a run-away greenhouse effect that raises Venus' surface temperature by about 400 degrees to over 740 K (a temperature hot enough to melt lead). Venus' surface is actually hotter than Mercury's despite being nearly twice as far from the Sun.

In Roman mythology, Venus is the goddess of love and beauty. The planet is named after her, most likely because it is the brightest of the planets that was known to the ancients. (With a few exceptions, the surface features on Venus are named for famous women.) Venus has been known since prehistoric times. It is the brightest object in the sky except for the Sun and the Moon. Since Venus is closer to the Sun than Earth is, it shows phases much like our Moon does.

Earth is located a distance of 149,600,000 km (1.00 AU) from Sun. It has a diameter of 12,756 km and a mass of 5.9736 x 1024 kg. Earth is the only planet whose English name is not derived from Greek/Roman mythology. The name derives from Old English and Germanic. There are, of course, hundreds of other names for the planet in other languages.

More information about Earth.

Mars, shown in this Hubble Space Telescope image, is 227,940,000 km (1.52 AU) from the Sun. It has a diameter of 6,794 km and a mass of 6.421923 kg. Mars is the Roman god of war - most likely the planet got its name from its red color. Though Mars is smaller than Earth, its surface area is about the same as the land surface area of Earth. Except for Earth, Mars has the most highly varied and interesting terrain of any of the terrestrial planets, some of it quite spectacular.

Olympus Mons is the largest mountain in the Solar System - rising 24 km (7,800 ft.) above the surrounding plain. Its base is more than 500 km in diameter and is rimmed by a cliff 6 km (20,000 ft.) high, shown right. There is also the Tharsis Bulge, 10 km high and spread across 4000 km of Martian surface. Valles Marineris is a canyon system that dwarfs Earth's Grand Canyon - it is 4000 km long and from 2-7 km deep! Hellas Planitia is an impact crater located in Mars' southern hemisphere - it is over 6 km deep and 2000 km in diameter! Much of the Martian surface is very old and cratered, but there are also much younger rift valleys, ridges, hills and plains.

Gaspra and Ida are main belt asteroids.	Left is a Galileo image of the asteroid 243 Ida and its small satellite, Dactyl, taken 13.5 minutes before closest approach from a distance of 10,300 km. This image was taken through the infrared (8890 nm) filter. Ida is 58 km along its long axis. Dactyl is the small object to the right of Ida.
Right is a Galileo clear filter image of asteroid 951 Gaspra taken 10.5 minutes before closest approach. About 15 km of the 19 km asteroid can be seen in this frame, which has a resolution of about 100 meters and was taken from 5,300 km. Many craters can be identified on the surface.

Jupiter is the most massive planet in our Solar System. In fact, it is more than twice as massive as all the other planets combined, and 318 times more massive than Earth. Its orbit takes it 778,330,000 km (5.20 AU) from the Sun. Jupiter's equatorial diameter is 142,984 km and its mass is 1.927 kg. The planet was named after the Roman king of gods, Jupiter.

Jupiter is the fourth brightest object in the sky (after the Sun, the Moon and Venus; at some times Mars is also brighter). It has been known since prehistoric times. Galileo discovered its four major moons in 1610. That is why Io, Europa, Ganymede and Callisto are known as the Galilean Moons. There are 12 other smaller moons. Jupiter is famous for its Great Red Spot - a huge storm that is twice as large as the Earth. Jupiter is now known to have a ring system.

Saturn, the second largest planet in our Solar System, is 1,429,400,000 km (9.54 AU) from the Sun. It has an equatorial diameter of 120,536 km and a mass of 5.6826 kg. In Roman mythology, Saturn is the god of agriculture.

Saturn is most famous for its incredible ring system. Two prominent rings (A and B) and one faint ring (C) can be seen from the Earth. The gap between the A and B rings is known as the Cassini division. Saturn's rings, unlike the rings of the other planets, are very bright. Though they look continuous from the Earth, the rings are actually composed of innumerable small particles each in an independent orbit. They range in size from a centimeter or so to several meters. A few kilometer-sized objects are also likely. Saturn's rings are extraordinarily thin: though they're 250,000 km or more in diameter they're no more than 1.5 kilometers thick. Despite their impressive appearance, there's really very little material in the rings -- if the rings were compressed into a single body it would be no more than 100 km across. The ring particles seem to be composed primarily of water ice, but they may also include rocky particles with icy coatings.

Alone of all the satellites in the Solar System, Saturn's moon Titan has a significant atmosphere. At the surface, its pressure is 50% higher than Earth's. Its atmosphere is composed primarily of molecular nitrogen (as is Earth's) with no more than 6% argon and a few percent methane. Interestingly, there are also trace amounts of at least a dozen other organic compounds (i.e. ethane, hydrogen cyanide, carbon dioxide) and water. The organics are formed as methane, which dominates in Titan's upper atmosphere. The result is similar to the smog found over large cities, but much thicker. In many ways, this is similar to the conditions on Earth early in its history when life was first getting started.

Uranus, shown left in this Hubble Space Telescope image, is 2,870,990,000 km (19.218 AU) away from the Sun, has a diameter: 51,118 km (equatorial), and a mass of 8.68325 kg. It was the first planet discovered in modern times; it was found by William Herschel on March 13, 1781. Uranus has been visited by only one spacecraft, Voyager 2 on Jan 24 1986. Most of the planets spin on an axis nearly perpendicular to the plane of the ecliptic but Uranus' axis is almost parallel to the ecliptic. In effect, Uranus, rolls around the Solar System on its side!

Like the other gas planets, Uranus has rings. Like Jupiter's, they are very dark but, like Saturn's, are composed of fairly large particles ranging up to 10 meters in diameter in addition to fine dust. There are 11 known rings, all very faint; the brightest is known as the Epsilon ring. The Uranian rings were the first after Saturn's to be discovered. This was of considerable importance since we now know that rings are a common feature of planets, not a peculiarity of Saturn alone. Uranus has approximately 10 small moons and 5 larger ones. It is likely that there are several more tiny satellites within the rings.

Neptune, smaller in diameter but larger in mass than Uranus, orbits the Sun at a distance of 4,504,000,000 km (30.06 AU) from the Sun. It has an equatorial diameter of 49,532 km, and a mass of 1.024726 kg. In Roman mythology, Neptune was the god of the sea. We know that Neptune's blue color is the result of absorption of red light by methane in its atmosphere. Like a typical gas planet, Neptune has rapid winds confined to bands of latitude and large storms or vortices. Neptune's winds are the fastest in the Solar System, reaching 2000 km/hour!

Neptune also has rings. Earth-based observations showed only faint arcs instead of complete rings, but Voyager 2's images showed them to be complete rings with bright clumps. One of the rings appears to have a curious twisted structure (left). Like Uranus and Jupiter, Neptune's rings are very dark but their composition is unknown.

Pluto (left)is the farthest planet from the Sun (usually) and by far the smallest. It orbits the Sun at a distant average distance of 5,913,520,000 km (39.5 AU) from the Sun. It has a diameter of 2274 km and a mass of 1.2722 kg. Unlike the other planets, which orbit roughly along the same plane, Pluto's orbit is inclined 17 degrees from this "ecliptic". In Roman mythology, Pluto is the god of the underworld. The planet was discovered in 1930 by a fortunate accident. Calculations based on the motions of Uranus and Neptune, which later turned out to be incorrect, had predicted a planet beyond Neptune. Not knowing of the error, Clyde W. Tombaugh at Lowell Observatory in Arizona did a very careful sky survey which turned up Pluto anyway. After the discovery of Pluto, it was quickly determined that Pluto was too small to account for the discrepancies in the orbits of the other planets. The search for Planet X continued but nothing was found. Nor is it likely that it ever will be: the discrepancies vanish if the mass of Neptune determined from the Voyager 2 encounter with Neptune is used. There is no tenth planet.

Pluto is the only planet that has not been visited by a spacecraft. Even the Hubble Space Telescope can resolve only the largest features on its surface. Pluto has a satellite, Charon (right). Charon is about half the size of Pluto - it has a diameter of 1172 km, and orbits Pluto at 19, 640 km. Charon is unusual in that it is the largest moon with respect to its primary planet in the Solar System (a distinction once held by Earth's Moon). It was named after the mythological figure who ferried the dead across the River Styx into Hades (the underworld). It was discovered by Jim Christy and Bob Harrington in 1978. Prior to that it was thought that Pluto was much larger since the images of Charon and Pluto were blurred together.

For detailed, up-to-date, information about our Solar System, see the wonderful "Nine Planets" page, written by Bill Arnett. Much of the text in this section of the page is courtesy of him.

See also NASA JPL's page on the planets.

Some of these images were created by Space Telescope Science Institute, operated by the Association of Universities for Research in Astronomy, Inc., from NASA contract NAS5-26555, and is reproduced with permission from AURA/STScI.

The Cosmic Distance Scale was created by Maggie Masetti. Scientific Oversight was provided by Dr. Koji Mukai.

This file was last modified on Tuesday, 11-Mar-2008 16:02:51 EDT