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Planets
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Planets are bodies orbiting a star that are not stars themselves, and that are not too small. How small a body can be and still deserve to be called a planet is somewhat arbitrary: Small objects without atmospheres are called minor planets, or more commonly, asteroids. The smallest of the planets, Pluto, has a diameter of 2,500 kilometers; the largest of the asteroids, Ceres, has a diameter of 940kilometers–but some astronomers consider Pluto an asteroid.
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Planets ... have varying degrees of axial tilt; they lie at an angle to the plane of the their stars' equators. This causes the amount of light received by each hemisphere to vary over the course of its year; when the northern hemisphere points away from its star, the southern hemisphere points towards it, and vice versa. Each planet therefore possesses seasons; changes to the climate over the course of its year. The point at which each hemisphere is farthest or nearest from its star is known as its solstice. Each planet has two in the course of its orbit; when one hemisphere has its summer solstice, when its day is longest, the other has its winter solstice, when its day is shortest. Jupiter's axial tilt is very small, so its seasonal variation is minimal; Uranus, on the other hand, has an axial tilt so extreme it is virtually on its side, which means that its hemispheres are either perpetually in sunlight or perpetually in darkness around the time of its solstices.[51] Among extrasolar planets, axial tilts are not known for certain, though most hot Jupiters are believed to possess negligible to no axial tilt, as a result of their proximity to their stars.[52]
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Discoverers of the Numbered Minor Planets The numbered minor planets have been discovered by a large number of individuals and teams. Lists of the discovery statistics are available ordered by number of discoveries and alphabetically. Also available is a list of the number of discoveries made at the most prolific discovery sites. Discovery circumstances are ... available, as is a breakdown of numbered discoveries by year.
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Planets are created in flat disks of gas and dust, called protoplanetary disks, which swirl around young, cool stars. According to standard planet-formation theory, over millions of years the particles clump together and grow in size like dust bunnies.
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How many planets would this make? The nine planets that everyone knows are all round, so they are clearly planets. Ceres, the largest asteroid, is ... round and would become a planet (the fifth). The big question, then, is how many new planets are there in the Kuiper belt, a region of rocky/icy bodies beyond Neptune, and the home of Pluto and 2003 UB313 ("the 10th planet").
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By definition, all dwarf planets are members of larger populations. Ceres is the largest body in the asteroid belt, while Pluto is a member of the Kuiper belt and Eris is a member of the scattered disc. According to Mike Brown there may soon be over forty trans-Neptunian objects that qualify as dwarf planets under the IAU's recent definition.[34]
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