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Pluto was discovered in 1930. After initial observations led to the belief that it was larger than Earth,[40] the object was immediately accepted as the ninth major planet. Further monitoring found the body was actually much smaller: in 1936, Ray Lyttleton suggested that Pluto may be an escaped satellite of Neptune,[41] and Fred Whipple suggested in 1964 that Pluto may be a comet.[42] The discovery of its large moon Charon in 1978 showed that Pluto was only 0.2% the mass of Earth.[43] As this was still substantially more massive than any known asteroid, and because no other trans-Neptunian objects had been discovered at that time, Pluto kept its planetary status, only officially losing it in 2006.[44][45]
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This definition is based in modern theories of planetary formation, in which planetary embryos initially clear their orbital neighborhood of other smaller objects. As described below, planets form by material accreting together in a disk of matter surrounding a protostar. This process results in a collection of relatively substantial objects, each of which has either "swept up" or scattered away most of the material that had been orbiting near it. These objects do not collide with one another because they are too far apart, sometimes in orbital resonance.[68]
The energetic impacts of the smaller planetesimals (as well as radioactive decay) will heat up the growing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by density, with higher density materials sinking toward the core.[130] Smaller terrestrial planets lose most of their atmospheres because of this accretion, but the lost gases can be replaced by outgassing from the mantle and from the subsequent impact of comets.[131] (Smaller planets will lose any atmosphere they gain through various escape mechanisms.[132])
All of the Solar System planets except Mercury[206] have substantial atmospheres because their gravity is strong enough to keep gases close to the surface. Saturn's largest moon Titan also has a substantial atmosphere thicker than that of Earth;[207] Neptune's largest moon Triton[208] and the dwarf planet Pluto have more tenuous atmospheres.[209] The larger giant planets are massive enough to keep large amounts of the light gases hydrogen and helium, whereas the smaller planets lose these gases into space.[210] The composition of Earth's atmosphere is different from the other planets because the various life processes that have transpired on the planet have introduced free molecular oxygen.[211]
No secondary characteristics have been observed around extrasolar planets. The sub-brown dwarf Cha 110913-773444, which has been described as a rogue planet, is believed to be orbited by a tiny protoplanetary disc[234] and the sub-brown dwarf OTS 44 was shown to be surrounded by a substantial protoplanetary disk of at least 10 Earth masses.[235] 041b061a72