The Solar System

How many dwarf planets are in our solar system?
Answered by James L. Green
  • James L. Green

    James L. Green

  1. James L. Green Director, Planetary Science Division, National Aeronautics and Space Administration


    Well, right now, Pluto is a dwarf planet. In the Kuiper Belt we've observed well over a thousand other objects that are Pluto-like, and one of which, Eris, may be even bigger than Pluto. This is one of the reasons that called into question whether we should be calling Pluto a planet or not, because we're finding objects that are larger than it in a new environment.

    Let me give you a little perspective on that. If you walked into a library in 1850, pulled out the solar system book, went through it and began to memorize the planets, do you know how many planets you'd have to memorize? Twenty-three. Now why? Well starting in 1800 they were seeing Ceres, Vesta, Eros. They were finding objects in the asteroid belt and calling them planets. By 1852, it was clear they had seen so many of these things in the asteroid belt and began to characterize them better and they realized that they were basically debris and so you went from 23 down to eight, and then in 1930 or 1932 you discovered Pluto and you created nine.

    Then in the 1990s, you began to find objects that are Pluto-like beyond Pluto, and not a few, a lot. So, it's a whole new family of objects. We call those objects Kuiper Belt and so we now know Pluto is a Kuiper Belt object and it now has joined the rest of its family, where there are another thousand or more. As I said, we're using ground-based telescopes primarily funded through National Science Foundation to be able to see many more of these planets and understand what part of the solar system we didn't understand before, but yet exists and there's a lot of it.

    More answers from James L. Green »

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