A planet within our solar system is defined by the International Astronomical Union as a celestial body that is in orbit around the Sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and has cleared the neighbourhood around its orbit. Those bodies which fulfill the first two conditions but not the third are dwarf planets if they are not satellites.
There remains no universal definition to cover extrasolar planets, but it is likely similar criteria will apply. One division that remains unclarified is that between planets and stars. A body that is massive enough to undergo the fusion of hydrogen in its core is generally considered a star, although debate remains over objects that sustain deuterium fusion. There is also debate about whether secondary objects in planetary systems qualify as planets if they orbit a barycentre outside the primary's surface. The nature of such double planet systems has not yet been settled by the IAU.
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