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New Planets are Coming to your Neighborhood

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[A]stronomers are proposing to rewrite the textbooks to say that our solar system has 12 planets rather than the nine memorized by generations of schoolchildren. Much-maligned Pluto would remain a planet - and its largest moon plus two other heavenly bodies would join Earth's neighborhood - under a draft resolution to be formally presented Wednesday to the International Astronomical Union, the arbiter of what is and isn't a planet.

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If the resolution is approved, the 12 planets in our solar system listed in order of their proximity to the sun would be Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon, and the provisionally named 2003 UB313.

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[The AIU's] proposed new definition of a planet: any round object larger than 800 kilometers (nearly 500 miles) in diameter that orbits the sun and has a mass roughly one-12,000th that of Earth.

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"For the first time since ancient Greece, we have an unambiguous definition," he said. "Now, when an object is debated as a possible planet, the answer can be swift and clear."

New Planets

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Does their setting a definite quantitive boundry on what makes a planet seem somewhat arbitrary? Or is it necessary?

A planet has always been (vaguely in my mind) round, massive objects that orbit the sun. No one had ever thought of specifying a minimum diameter and minimum mass in the definition. In fact, a lot concepts, though omitting specific measurements and instead allowing any unit to have a measure within a certain RANGE, do not specify the boundry of those range of measures. Can it be valid to specify the boundry of those ranges? Can one include something like "a table's supporting leg(s) can be no shorter than X and no taller than Y" in its definition?

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Does their setting a definite quantitive boundry on what makes a planet seem somewhat arbitrary? Or is it necessary?

It would seem to be necessary, in my opinion, or else many of the comets and asteroids may be called 'planets.'

A planet has always been (vaguely in my mind) round, massive objects that orbit the sun. No one had ever thought of specifying a minimum diameter and minimum mass in the definition. In fact, a lot concepts, though omitting specific measurements and instead allowing any unit to have a measure within a certain RANGE, do not specify the boundry of those range of measures. Can it be valid to specify the boundry of those ranges? Can one include something like "a table's supporting leg(s) can be no shorter than X and no taller than Y" in its definition?

Remember, we are talking about a scientific definition, not a philosophic definition. A planet will still be "a large object revolving around the sun in the solar system" for common usage. When you use the term 'massive,' isn't that putting a boundary on planets? I am massive compared to an ant and I am orbiting the sun.

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It would seem to be necessary, in my opinion, or else many of the comets and asteroids may be called 'planets.'

Remember, we are talking about a scientific definition, not a philosophic definition. A planet will still be "a large object revolving around the sun in the solar system" for common usage. When you use the term 'massive,' isn't that putting a boundary on planets? I am massive compared to an ant and I am orbiting the sun.

But you're not round (I presume)! :) Yes, the term "massive" puts a boundry, but as I said, no specific measure is set as the boundry--unlike the proposed definition of a planet, which sets a minium mass (1/12,000 earth's mass) and diameter (800 km in diameter).

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On "Says You" on NPR last week-end, someone made a joke that the solar system would now include John Belushi as a planet. :)

I believe one astronomer called this decision the "no snowball left behind" definition.

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