A planet is a relatively cold body that orbits a star. Planets are thought to have formed from the same gas and dust that condensed to make the parent star. They can be seen by eye and telescope because of the light they reflect from their star. The planets themselves often have orbiting moons and dust rings.
The nine planets in our solar system that are in elliptical orbits near the ecliptic plane are divided into two classes: the inner and outer planets. The inner planets (Mercury, Venus, Earth, and Mars) are made of rocky material surrounding an iron-nickel metallic core. Earth and Venus have substantial cloud-forming atmospheres, and Mars has a thin atmosphere similar in composition to the of Venus.
The outer planets (Jupiter, Saturn, Uranus, Neptune, and Pluto) are, with the exception of Pluto, large masses of hydrogen in gaseous, liquid, and solid form surrounding Earth-size rock plus metal cores. Pluto, made of ice and rock, is probably an escaped moon of Neptune.
It is likely that other stars have planets orbiting them since the star- and planet-formation mechanisms are similar throughout the universe. When stars form the leftover gas and dust accumulate by mutual gravitational attraction into planetesmals. Observation of disk-shaped dust clouds around newly formed stars are an indication of planet formation in progress.
Planetary astronomy is a very active field, thanks to new space probes like the Galileo unmanned spacecraft. In 1995 scientists found evidence that Jupiter's moon Europa has a liquid ocean and, perhaps, the right conditions for life. The Mars Pathfinder mission landed a small roving vehicle on the planet in 1997, providing up-close pictures suggesting that liquid water had once scoured the surface. Pathfinder's roving vehicle Sojourner also performed soil chemistry analysis, and other probes like the Mars Polar Lander will continue to provide new information about planetary surfaces.
Astronomers have also found planets circling stars other than our own. The first was in 1995, when Michel Mayor and Didier Queloz found a planet around star 51 Pegasi, an almost perfect twin of the Sun. Since then nearly two dozen "extrasolar" planets had been discovered by 1999. These new planets are usually large, like Jupiter. They cannot be seen directly, but are inferred from the wobble seen on some stars, as observed from large telescopes on Earth. The wobble is caused by the gravitational pull of large planets near the star. Because these planets are big, gassy, and close to their star, they are not likely to contain any life, but their existence shows that there is nothing special about the fact that planets circle our Sun.
Other special arrangements have been found in the 1990s. The Hubble Space Telescope captured an image of a dust ring around the star HR 4796A, 220 light-years from Earth. The ring roughly resembles that of Saturn, but on a vastly larger scale. Some objects in the rings could be planets, or the slender shape of the ring may be influenced by nearby planets.
One extrasolar planet has been found only 15 light-years from Earth, circling the star Gliese 876. This is much closer than other extrasolar planets, which mostly lie at a distance of 40 to 80 light-years. Gliese 876 is a small star, less than 1/3 the mass of the Sun, suggesting that extrasolar planets are anything but rare.
In 1999 astronomers announced the first-ever detection of an entire solar system around a star. Only 44 light-years from Earth, three large planets were found circling the star Upsilon Andromedae, a sun-like star visible to the naked eye on Earth. Again the presence of the planets was inferred from gravitational wobbling. Astronomers suspect the planets are similar to Jupiter and Saturn—huge spheres of gas without a solid surface. One of them completely circles its star in only 4.6 Earth days. Such discoveries show that planetary science will likely be a fruitful and surprising field for years to come.