The decision came from the International Astrophysical Union (IAU) today. Pluto is no longer considered a planet. This was a ruling that I had hoped for a long time they would make.
A week ago they came up with an unworkable proposal. This would keep the "traditional" nine planets and include Charon (so that Pluto would be a double planet), Ceres (the largest asteroid), and Xena (a Kuiper Belt object like Pluto, but somewhat larger) as planets. Their definition of planet was that it orbit the Sun, and that it is massive enough for gravity to have made it into a sphere. This was unworkable because it would have made too many planets. It's not just Pluto and Xena out there beyond Neptune. There are also the Easter Bunny, Santa, Orcus, Ixion, Varuna, Quaoar, 2002 FX25, Sedna and a host of other discovered and undiscovered objects in the Kuiper Belt larger than Ceres. All of these are almost certainly spheres, and they circle the Sun. So they would be planets. There would be a lot of planets out there. Boy would there ever be a lot of planets. Michael Brown of Cal Tech, the discoverer of Xena, said there would be 53 of them. Ask our high students to remember 53 planets.
As a result, a storm of protest arose in the astronomical community at the convention of the IAU in Prague, Czech Republic, which is considering the matter. Today an alternative resolution was considered and passed. It maintained the two requirements for a planet mentioned above, and it added a third one: that it "cleared the orbit of other objects". To me this means that it is more massive than everything else in its orbit put together. This means that Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune are planets; in fact, each has a "sphere of influence" - the orbit in which it moves. It also means that Pluto is not a planet (and neither is Charon), since its orbit goes all over the place in the Kuiper Belt and even overlaps Neptune's orbit. Ceres is not a planet, because the sphere in which Ceres is in is the asteroid belt and all the other asteroids put together is much larger than Ceres - in fact, it is about the mass of the Moon. So now there are only eight planets.
Actually, "planet" is a term used by us humans, and the only thing that has changed is what we call these objects in the Solar System. The objects have not changed themselves. To me, this entire debate is as much etymology as astronomy. People seem to behave as though "planet" was an inherent property of these objects, and that Pluto is a planet because it is a planet, and that it is that way in all our astronomical books, and there is a cartoon dog named after him and so forth. People are even getting attached emotionally to the far distant world. They say, "Poor Pluto", when actually the planet is not alive and could not care less. The astrologers take anything called a planet and proclaim influences of them over us, which somehow did not exist before. No, the entire thing was selecting words so that astronomers can logically refer to these objects. They have acted correctly in defining planets in terms of spheres of influence and in saying that Pluto is not a planet.
This should settle a few things now. For example, we have objects named Xena (who is not a mythological hero but a creation of today's hyperculture), Santa, and the Easter Bunny. These objects will now get permanent names. The rule is that if its orbital period is about 247 years, it is named after an underworld figure, else it is named after a creation deity. Xena would be renamed for a creation deity, and I hope it gets named Baiame, for the native Australian deity. Popular sentiment would select Persephone or Prosepina, the wife of Pluto or Hades. But these are names of asteroids. Kore is a possible name - Persephone's name as the Queen of the Underworld, and it is not the name of an asteroid. The Easter Bunny and Santa would also get official names.
But questions remain open for the future. For example, how about two Earth-sized worlds that orbit in overlapping orbits? Maybe such does not exist long; that eventually they would collide, as the Earth did with Theia to form the Moon. And shouldn't the Asteroid Belt (Mars-Jupiter) and the Kuiper Belt be declared something like planets, since they are zones or spheres of influence? Perhaps call these belts. There is one belt for each planet, and also the MJ Asteroid Belt and the Kuiper Belt. The Solar System has 10 belts.
Will the definition be viable in the future? The MJ asteroid belt is associated with the terrestrial planets, the largest of which is Earth, at about 8,000 miles in diameter. Ceres is 480 miles in diameter, so the ratio is about 17 to 1. In the same way the Kuiper Belt seems to be associated with the giant gas planets, the largest of which is Jupiter, at 88,000 miles. 1/17 of this is about 5200 miles, somewhat larger than Mars. This makes me think there is a really huge undiscovered Kuiper Belt object out there with a diameter of 5,200 miles. If such is discovered I would think it would have be called a planet, even though it would not dominate its orbit. But for now, the IAU has presented us with a reasonable definition of planet.
No comments:
Post a Comment