• Astronomical body larger than an asteroid but too small to support fusion reactions and so qualify as a star. We have direct knowledge of only the eight planets of the solar system. Pluto was formerly classified as the ninth planet of the solar system, but in 2006 it was re-classified as a dwarf planet. Several of the major planetary satellites, especially Titan, are so large that they would be viewed as planets if they were in their own orbits around the Sun – Titan is quite a lot larger than Mercury. The solid solar system planets all seem to have their own internal processes which are more important than meteorite impact in forming their surfaces, and are massive enough to take up near-spherical shapes. Planets are aggregations of material formed in the early days of the solar system, while the comets, meteorites and asteroids are fundamental pieces of the early solar system never gathered into planets. It seems from close observation of their proper motions that other nearby stars may have large planets much bigger than Jupiter, the largest planet in the solar system, although this method does not allow smaller planets to be detected. But nobody knows how often planets might form near other stars and whether multiple star systems, which are very common, could form planets as the Sun, a lone star, did some 4.5 billion years ago. Detecting stars around other planets relies on hunting for small Doppler effects in their spectra. A planet orbiting a star displaces it away from and towards the Earth during its orbit. This movement can be detected by blue or red shiftsin starlight received at the Earth. But the efect is very small and means detecting movement of under 100m/s which has only become possible in recent years. Stars around which planets have been detected include 51 Pegasi, a Sun-like star with a planet at least half as massive as Jupiter orbiting nearer than Mercury’s orbit around the Sun. Three planet-sized objects, two larger than the Earth and one smaller, have also been detected orbiting the pulsar 1257+12. The star 70 Virginis has been tentatively allocated a planet 6.5 times as massive as Jupiter, orbiting at about 0.4AU, while 47 Ursae Majoris is thought to have a planet about twice as massive as Jupiter in a 2.1AU orbit, and a planet has been claimed around the star HR3522. The bright star Beta Pictoris, which is surrounded by a dust cloud from which planets may be forming, also shows slight changes in its radial velocity which suggest the presence of a planet.

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