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planet

Celestial body in our Solar System that is in orbit around the Sun, is large enough to be spherical, and which dominates its orbit (sweeping its neighbourhood clear). There are eight planets in the Solar System orbiting the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Neptune and Uranus. The inner four, called the terrestrial planets, are small and rocky, and have few natural satellites. The outer planets are called the major planets, and have denser atmospheres consisting mainly of hydrogen and helium gases, and many natural satellites. The largest planet in the Solar System is Jupiter (about 780 million km/490 million mi from the Sun) with a diameter of 140,000 km/87,500 mi, which contains a mass greater than all the other planets combined. The smallest (and closest to the Sun at 58 million km/36 million mi) is Mercury with a diameter of 4,800 km/3,030 mi.

As seen from the Earth, the five planets from Mercury to Saturn are conspicuous naked-eye objects moving in looped paths against the stellar background. The sizes of these loops, which are caused by the Earth's own motion around the Sun, are inversely proportional to the planet's distance from the Earth. Planets do not produce light, but reflect the light of their parent star.

Recent discoveries

Scores of planets orbiting stars other than the Sun have been discovered. Confirmed detections were first made in the early 1990s. Since then planets have been discovered by a variety of methods. A large planet causes a ‘wobble’ in the parent star's motion. A planet circling a pulsar may affect the timing of the pulsar's radio signals. A planet may pass between us and the parent star and dim its light. A few objects have been directly imaged – though these are large, and may be brown dwarfs rather than planets. The objects discovered range in mass from brown-dwarf size (about 5-90 times the mass of Jupiter) to a few times the mass of the Earth.

In 2005, US astronomers announced the discovery of a candidate for the title of planet in our Solar System. They observed an object approximately 3,000 km/1,800 mi wide orbiting the Sun three times as far away as Pluto. It was the largest object to be found in the Solar System since Neptune was discovered in 1846. The object, later named Eris, was given the temporary designation 2003 UB313 (it was originally observed in 2003 but only confirmed as a planet in January 2005). The astronomers used telescopes at the Palomar Observatory, California and Mauna Kea, Hawaii to make their discovery.

In 2006, Pluto was reclassified from planet to dwarf planet, leaving eight planets in our Solar System.

Space probes to planets include: Mariner 2 (launched in 1962 to Venus); Pioneer 10 (launched in 1972 to Jupiter); Mariner 10 (launched in 1973 to Venus and Mercury); Viking 1 and Viking 2 (launched in 1975 to Mars); Voyager 1 and Voyager 2 (launched in 1977) to Jupiter, Saturn, Uranus, and Neptune; Galileo (launched in 1989 to Jupiter); Cassini (launched in 1997 to Saturn); and numerous missions to Mars in the 1990s.