Quote:
Originally Posted by laurele
Better Branson than the IAU. Pluto is a planet, always was, always will be. Please do not blindly accept the controversial demotion of Pluto, which was done by only four percent of the International Astronomical Union, most of whom are not planetary scientists. Their decision was immediately opposed in a formal petition by hundreds of professional astronomers led by Dr. Alan Stern, Principal Investigator of NASA’s New Horizons mission to Pluto. Stern and like-minded scientists favor a broader planet definition that includes any non-self-luminous spheroidal body in orbit around a star. The spherical part is important because objects become spherical when they attain a state known as hydrostatic equilibrium, meaning they are large enough for their own gravity to pull them into a round shape. This is a characteristic of planets and not of shapeless asteroids and Kuiper Belt Objects. Pluto meets this criterion and is therefore a planet.
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The problem with this approach is that we now know that Pluto is merely one (large) member of the "Kuiper Belt" - the asteroid belt beyond the orbit of Neptune. More than 1000 Kuiper Belt objects have currently been discovered, of which the most massive now known is not Pluto, but the object designated 163199 Eris, which is 27% more massive than Pluto and is estimated to have a diameter of around 2400km. Logically, if you call Pluto a planet, you have to call Eris one too. Do you want to do that?
The term "dwarf planet" has now been adopted by the IAU for non-satellite objects which are massive enough to have become spherical, but not massive enough to have cleared other objects out of their vicinity. Dwarf planets include the Kuiper Belt objects Pluto, Eris, Haumea, and Makemake, plus of course Ceres. It's estimated that there may well be as many as 200 dwarf planets in the solar system.