Is Earth a Planet?

Pluto Compared With Earth

Pluto Compared With Earth

Three years ago, with less than 4 percent of the membership voting near the end of a long meeting in Prague, after many attendees had already departed, a plurality of the remaining International Astronomical Union’s membership voted to declare that Pluto should no longer be considered a planet, but instead a “dwarf planet” or a “plutoid.” This was, at best, a less-than-overwhelmingly-popular decision, and conspicuously ill-timed. (See this item at, for instance.)

This next week the IAU’s general assembly will meet again for the first time since that vote. Will the question of Pluto’s status be revisited? See Is Pluto a Planet After All? in New Scientist.

(For our earlier brief comment on this topic see our post of March 18th on the 79th anniversary of Pluto’s discovery by Kansas native astronomer Clyde Tombaugh.)

For more information, particularly on the first NASA interplanetary(?!) probe to visit Pluto, the New Horizons spacecraft, see the New Horizons website.

Trans-Neptunian Objects

Published in: on July 27, 2009 at 3:40 pm  Comments (3)  

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3 CommentsLeave a comment

  1. Now, provide an illustration to scale of Jupiter and Earth side by side. Jupiter is far larger than Earth than Earth is larger than Pluto. Should Earth not be considered a planet?

    The IAU definition makes no sense because it states that dwarf planets are not planets at all, a departure from the use of the term “dwarf” in astronomy, where dwarf stars are still stars, and dwarf galaxies are still galaxies. Also, the IAU definition classifies objects solely by where they are while ignoring what they are. If Earth were in Pluto’s orbit, according to the IAU definition, it would not be considered a planet either. A definition that takes the same object and makes it a planet in one location and not a planet in another is essentially useless.

    The IAU should take responsibility for the highly flawed definition adopted by only four percent of its members, most of whom are not planetary scientists, in 2006. However, the IAU should not be viewed as the sole authority on the definition of planet. Many planetary scientists do not belong to the IAU. Should they not have a say in this matter? Something does not become fact simply because a tiny group that calls itself an authority says so. It is significant that hundreds of planetary scientists led by New Horizons Principal Investgator Alan Stern immediately signed a formal petition opposing the IAU definition.

    There are other venues through which a planet definition can be determined, such as last year’s Great Planet Debate at the Johns Hopkins University Applied Physics Lab.

  2. For another interesting sidelight on Pluto, see this item in Scientific American’s blog.

  3. […] for more on Pluto, you can visit our earlier posts Is Earth a Planet? and Pluto’s Anniversary. Published […]

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