Friday, August 05, 2005

Planet X

Naturally, with three similar objects announced in a couple days, there is bound to be some confusion. On top of that, there seems to be no end of confusion as to what is a planet, and who gets to say if an object is one or not.

The three new objects are 2003UB313, 2003EL61, and 2005FY9. There. Is that all clear?

2003UB313 is currently 97 AU from the Sun in a 36 AU by 97 AU orbit. It's diameter is thought to be in the range from 2300 km to 3200 km. Larger than Pluto. Look for it in 280 years at only 36 AU. It should be at least 4 times as bright - perhaps 16th magnitude. I can hardly wait!

2003EL61 is currently 51 AU from the Sun. Its about 1600 km in diameter - 70% the diameter of Pluto. It's mass is 32% the mass of Pluto. The mass is known because it has a little moon in a 49 day orbit.

2005FY9 was also announced. Its also about three quarters the size of Pluto.

The newly discovered objects are not planets, at least not yet, according to the IAU - the International Astronomical Union. We could each decide what is and isn't a planet, but the resulting confusion of terminology would be bad - perhaps the end of life as we know it. The IAU hasn't had a chance to admit more objects into the planet club as yet, at least in our solar system.

What will the IAU say? Well, for one thing, there is no IAU approved definition for the term planet at the moment. IMO, this is bad. For one thing, it means you can't predict how the politics will play out. So, all you can do is prepare several new versions of your basic astronomy textbook, and release the correct one when the IAU gets around to voting. If you want to preprint them, expect to send many of them to the recycling bin.

For the ancients, a planet was a wandering star. This definition worked pretty well. You look up into the sky, and if it looked like a dot, and it moved around, then it was a planet. That implies the following definition: planets are naked eye point objects that move. By this definition, Mercury, Venus, Mars, Jupiter and Saturn are planets. If the ancients had noticed Uranus, it would be one too. Of note is that Pluto, Neptune, and the Earth are not. The ancients also considered the Sun and the Moon as of the seven heavenly objects. Again, Dirt... I mean Earth wasn't one of them.

With the advent of the telescope, and some hard work, people got the idea that planets are spherical things that orbit the Sun. The proposed definition that i like preserves this, but adds some constraints. One does not want every grain of sand that happens to orbit the Sun be called a planet. One convenient lower limit is the size a body must be before it collapses to at least more or less a sphere under it's own gravity. That happens at around a diameter of 700 km (434 miles). At the large end, if an object is big enough, it undergoes fusion, and therefore is a star. That happens (with Deuterium?) at about 13 times the mass of Jupiter. So, an object that orbits the Sun, but not also another body, that is at least 700 km, but less than 13 Jupiter masses is a planet. The shortened version is "a spherical non-fusor in orbit around a fusor". Under this definition, the current nine planets remain planets. Ceres, Varuan, Quaoar, Sedna, and at least two of the new ones, 2003UB313 and 2003EL61, and a few others are planets. Vesta isn't, as it is only 525 km, for example, even though it's pretty spherical, as far as i know.

Other people say that Pluto shouldn't be a planet, but rather a Kuiper Belt object. IMO, fooey. Jupiter, Saturn, Uranus and Neptune are gas giants, but they are planets too. Mercury, Venus, Earth and Mars are terrestrial planets (rocky), but they are planets too. There isn't any reason that Pluto can't be studied as a Kuiper Belt Object (KBO) along with other KBOs even if it keeps it's membership in the planetary club. IMO, the concept and definition for planet-hood is a public sort of thing, and ought to have the simplicity that the public can cope with. IMO, it's a good thing that the public has some sort of clue that planets are like the Earth, only "out there", rather than that they are "points of light that move". For one thing, it brings a little bit of astronomy down to Earth.

As for Planet X, planets don't really have numbers. Pluto was closer to the Sun than Neptune from 1979 to 1999. During that time, it was the 8th planet. If my favorite definition is approved by the IAU, Ceres becomes the 5th planet from the Sun, and Pluto becomes Planet X, the 10th planet from the Sun. However Pluto is currently at 39 AU and 2003UB313 will be only 36 AU in 280 years, so the numbering will continue to change.

If you disagree, well, YOU'RE WRONG ...i mean, sure, i'd like to hear your opinion.


FreeThinker said...

Wow, you look so young for 46!

Stephen said...

Clean living, plenty of exercise, lots of water.