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Scientists Reclassify Pluto as ‘Dwarf Planet’

Top international astronomers voted Thursday to strip Pluto's planetary status that it has held since its discovery in 1930. The celestial object is now redefined as a "dwarf planet," leaving just eight major planets in the solar system.

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    Nine planets no more. An international meeting in the Czech capital, Prague, has redefined what had been the furthest heavenly body orbiting the sun. Alan Boss is an astrophysicist and planet formation theorist at the Carnegie Institution in Washington. I spoke to him earlier today about how we wound up with one fewer planet today than yesterday.


    Alan Boss, welcome.

  • ALAN BOSS, Astrophysicist:

    Thank you.


    Why isn't Pluto recognized as a planet any longer?


    The reason is that Pluto is not the only major body in its area of the solar system. It's actually one of a whole swarm of objects which have been the Kuiper Belt objects. And, in fact, recently Michael Brown of Caltech discovered another object in the same swarm, which has been called Xena as a temporary name.

    And so we now realize that not only is Pluto not alone, there's even another object out there which is even bigger than Pluto. And so, in some sense, it's really just a member of a whole collection of objects rather than a splendid single isolation object like one of the major planets.


    But objects that don't cross a certain threshold of definition to be considered planets?


    Pluto is massive enough to have its figure pulled into a round shape because of gravity, and that has been proposed as a very basic criterion that any planet should hold. And that is part of the definition that was adopted today in Prague by the astronomers at the International Astronomical Union General Assembly.

    So, yes, being large enough to be massive enough to have a round shape is a part of the planet definition. And so the eight major planets share all that, Mercury through Neptune.

    But in addition, the IAU group today decided that there should be another criterion, as well, namely that you should be the biggest bad boy in your part of the neighborhood. And so certainly Earth, Venus, Jupiter, and Saturn, Uranus, Neptune, even Mercury and Mars meet that criterion, but Jupiter — sorry, but Pluto and Xena, the new object, do not.

    They are, as I said at the beginning, just a member of a whole swarm of objects. It's rather like the situation in the asteroid belt where Ceres is the largest asteroid. When Ceres was discovered back in 1800 or so, it was thought to be the missing planet between Mars and Jupiter. But then, a few years later, they discovered another asteroid called Vesta, and then another one called Pales, and then they realized, "Well, maybe we shouldn't call Ceres a planet after all." So they started calling those objects the asteroids or the minor planets, and we now know there are tens of thousands of those objects.