Quote:
Originally Posted by HarryT
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.
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Yes, I do count Eris as a planet, as well as Ceres, Haumea, Makemake, and probably Sedna. Pluto and Eris are not just two more members of the Kuiper Belt. They are both planets and Kuiper Belt Objects. The first tells us what they are; the second tells us where they are. The overwhelming majority of Kuiper Belt Objects are tiny, icy bodies nowhere near large enough to be rounded by their own gravity. Also, while Eris is more massive than Pluto, recent data from Eris' occulting a star in November 2010 shows that Pluto is actually larger in diameter, meaning Pluto is actually the ninth largest object direcly orbiting the Sun. Additionally, Pluto is estimated to be 70-75 percent rock, and Eris, given its higher mass, may be even more rocky.
My difference with the IAU definition, which was adopted by only four percent of its members (424 people) and opposed by an equal number of professional astronomers in a formal petition, is that I--like others who support the geophysical planet definition--see dwarf planets as a subclass of planets rather than as non-planets. There is no reason an object should have to clear its orbit to be considered a planet. In fact, the initial intent of Alan Stern, who coined the term dwarf planet, was to designate a third class of planets in addition to terrestrials and jovians, small planets large enough to be rounded by their own gravity but not large enough to gravitationally dominate their orbits. This would put the term dwarf planet in line with other uses of dwarf in astronomy, as dwarf stars are a subclass of stars, and dwarf galaxies are a subclass of galaxies.
If our solar system has 200 or more planets due to the large number of dwarf planets, what is wrong with that? No one tries to limit the number of Jupiter's moons to four because kids can't memorize 63. We don't ask kids to memorize all the rivers and mountains on Earth, and we don't need to ask them to memorize a list of planets either. It's more important that they understand the different kinds of planets and their characteristics. Trying to limit the number of solar system planets just so we have a number that is convenient is hardly a scientific argument.