Astronomy Notes

One of the subgroups in International Astronomical Union (IAU) has now defined a new defination for Planet. If most of the positive votes go to this defination on 24 August in the meeting in Prague

(1) A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet. Make it simple, it’s the object must be in orbit around a star, while not being itself a star and the object must be large enough (or more technically correct, massive enough) for its own gravity to pull it into a nearly spherical shape.

(2) We distinguish between the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane, and other planetary objects in orbit around the Sun. All of these other objects are smaller than Mercury. We recognize that Ceres is a planet by the above scientific definition. For historical reasons, one may choose to distinguish Ceres from the classical planets by referring to it as a “dwarf planet.”

(3) We recognize Pluto to be a planet by the above scientific definition, as are one or more recently discovered large Trans-Neptunian Objects. In contrast to the classical planets, these objects typically have highly inclined orbits with large eccentricities and orbital periods in excess of 200 years. We designate this category of planetary objects, of which Pluto is the prototype, as a new class that we call “plutons”.

(4) All non-planet objects orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.

However, many are not satisfied with this defination. There is rumour that Moon will most likely be another planet as Moon orbits the Sun in a wobbly way just like the Earth wobbles around the Sun due to the Moon’s proximity. Therefore, Earth and Moon maybe another double planets.