edge of seventeen
It is amazing that in all the history of astronomy, one of the oldest of sciences, there has never been an official definition of the word "planet".
Depending upon how the International Astronomical Union defines "planet" in the spring of 2006, our sun might have 8, 9 or even seventeen planets. If they go with the definition that a planet is any round object greater than 1000 km in diameter, then Pluto will remain a planet - and several Kuiper Belt Objects will be officially planets as well: Sedna, Quaoar, Xena, Buffy, Santa, EasterBunny, Orcus, and Ixion. (They are going to have to come up with official names for some of these as well... while Xena might pass muster, I doubt that Santa or EasterBunny will become the official names of those objects.)
Of course, the IAU might decide to demote Pluto and group it within the "trans-Neptunian objects", a decision which would surely anger a great many people.
I find the definition noted above to be somewhat lacking. The 1000 km value is completely arbitrary. Personally I prefer the "gravitational rounding" definition of a planet: any object which orbits a star that is large enough for gravity to force it into a spherical shape, but too small for gravity to ignite hydrogen fusion. Of course, under this particular definition, the sun would have nineteen (known) planets: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Ixion, Quaoar, Varuna, Santa, EasterBunny, Orcus, Buffy, Xena, and Sedna. Under this definition, the sun may in fact have dozens or even hundreds of planets, the vast majority of them so far undiscovered. Some more asteroids besides Ceres might also be considered planets under the gravitational rounding definition (we really don't know much about Pallas, for instance).
Hey, as far as I'm concerned, the more the merrier.
(h/t to Chris at Spacecraft)
Technorati Tags: Space
2 comments:
Based on that particular definition,
"any object which orbits a star that is large enough for gravity to force it into a spherical shape, but too small for gravity to ignite hydrogen fusion. "
the Moon would also be a planet. So would Europa, Io, Titan, etc... I guess they would put in a provision for moons.
Unless you mean orbits just a star and nothing else. In a manner of speaking the Moon orbits the Earth and the Sun.
Yeah, Dan, I meant something which orbits a star and nothing else. But yes, the earth and the moon do orbit each other, even if the mutual center of mass of the system is within the earth's volume; similarly, Jupiter and Ganymede orbit each other, but the center of mass of the system is within Jupiter. A separate definition would have to be made for "moon" to account for this.
Post a Comment