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An astronomical body orbiting the Sun, or another
star, the mass of which is too small for it to become a star itself
(less than about one-twentieth the mass of the Sun). Planets may be basically rocky objects, such as the
inner planets (Mercury,
Venus, Earth and
Mars), or primarily gaseous (liquid in the high pressure conditions of the interior) with a small solid core, like the outer planets (Jupiter, Saturn, Uranus and Neptune). These eight, together with Pluto, are the major planets of the solar system. Pluto, though more like the rocky planets, retains a significant amount of ices and is the sole example of an ice-dwarf major planet in the solar system. There are also within the solar system large numbers of minor planets, or asteroids, and a population of small ice dwarfs in the region beyond Neptune, known as the Kuiper Belt. So far it has not been possible to image directly any planet orbiting another star. However, the presence of planets around a number of stars other than the Sun has been inferred indirectly from the measurement of small cyclical changes in radial velocity, as revealed through the Doppler effect. Observations of discs around newly forming stars, which could provide the material to form planetary systems, strengthen the argument that planetary systems probably accompany at least some stars comparable with the Sun. In addition, there is strong evidence (from small variations in pulse frequency) that at least one pulsar has planetary-sized companions. See also: extrasolar planet. |