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They are not planets, they are not asteroids or comets: they are the dwarf planets. For almost a century, starting from its discovery in 1930we have known only one object belonging to this category, Pluto. We learned to call it planetbut in fact, it was the discovery of other objects similar to him that triggered the chain of events that led to the 2006 to the much famous and hated definition of planetary objects.
In fact since then we define planets all those bodies that orbit around the Sun, are massive enough to have found their equilibrium in something that looks like a sphere, and were able to clear their orbit of other planetary bodies. If they meet only the first of these three requirements, then they are called minor bodiesbe they asteroids or comets. If, on the other hand, they are also spheroidal, then we call them dwarf planets. For an object to be spheroidal, it must have gravity strong enough to counteract gravity rigid body forces of the materials that compose it.
For ice-rich bodies such as those found beyond the orbit of Neptune, this transition occurs when the body size is approx 400 kilometres: it means that every time we find a planetary body that has at least that size, we know that it is most likely a dwarf planet. If the diameter is greater than 900 kilometers, then it is almost certainly a dwarf planet, whatever its composition. For now, however, there are only five officially recognized dwarf planets: in addition to Pluto and Ceresthere are Haumea, Eris and makemake. Five other objects are on the waiting list, but will most likely receive experimental confirmation in the coming years as their diameter is greater than 900 kilometers or close to this size, and they are Salacia, Gonggong, Orcus, quaar and sedna. There are also a few dozen other bodies that require further study and may be dwarf planets. However, it is estimated that they are in the hundreds, perhaps thousands the bodies of our planetary system that would fall under this category.
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