Friday, September 08, 2006

What is the largest planet?

Lost in the debate over the (dwarf) planet Pluto's status in our solar system is perhaps a more important debate: What is the largest object that can be called a planet around another star?

For objects about 13 times the mass of Jupiter, deuterium (a heavy form of hydrogen) will burn in a nuclear reaction, though once the deuterium is burned up, all nuclear reactions stop. For that reason, most astronomers are comfortable calling objects this big "brown dwarfs," the term for wannabe stars.

What about for smaller objects? Gigantic planets can form in two ways. They can form like the giant planets in our solar system -- a disk of gas and dust around the parent star forms small asteroid-like objects that collide due to gravity to make larger and larger bodies. Once these are several times the mass of the Earth, their gravity is strong enough to scoop up gas, too, and the planets rapidly grow to hundreds of times the mass of the Earth, just like Jupiter and Saturn.

The other method is that, as the parent star forms out of a big cloud of gas, a hunk of that gas splits off and collapses on its own to form something 10 times larger than Jupiter. The only difference between a planet 10 times the mass of Jupiter formed this way and ten times the mass of Jupiter formed the other way is that the latter will have a rocky core several times the mass of the Earth, while the former will be all gas. From the outside, though, it is nearly impossible to tell the difference.

Today NASA announced the discovery of a brown dwarf about 12 times the mass of Jupiter around a nearby star. One of the co-discoverers, Kevin Luhman, a professor at Penn State, claims that this object must be a brown dwarf and not a planet because it is far enough away from the parent star that there could not have been a disk of dust and gas to form the object.

The problem is that, for many objects from about 10 times the size of Jupiter to 13 times the size of Jupiter, it is unclear which process formed the planet/brown dwarf. So, what are these objects to be called? Are they a fallen star or a risen planet? Like the case with Pluto, I don't know that there is a single answer that everyone will find satisfactory. My guess is that, eventually, we astronomers will tire of the controversy and call these something like "transition objects" or some other suitably vague term. Then we can stop arguing about what to call them and start arguing about true scientific topics -- how these things form, how they change with time, how common they are. And these topics may provide us with hints as to whether one formation scenario is favored, or if both seem to come in to play. And from that, we learn more about how stars and planets form.

1 comment:

  1. Don't forget what anniversary is September 8! I didn't.