Monday, September 13, 2004

Image of an Extra-Solar Planet?

Several news stories like this one have mentioned the recent announcement that a planet may have imaged around another star. If this is true, it would be the first direct picture of a planet in another solar system. So, is it true?

There are good reasons to be wary. Previous announcements have claimed to have imaged planets. Some have been shown to be background stars only appearing close to the "parent" star, others are still unconfirmed. This potential planet is somewhat exciting because the astronomers claim to have detected water. In order for water to form, the object has to be cooler than any known star.

So, how can we be certain that this is a planet orbiting another star? We could watch to see it orbit the parent star, but that could take hundreds of years. Or, we can wait a couple of years and look again -- stars all move relative to one another in the sky. If the purported planet and its parent star are moving the same direction at the same speed, then they are certainly travelling together. We must be patient!

This object raises some interesting questions. Assuming it is really a planet, the planet orbits a brown dwarf, a "failed" star that has too little mass to make its own light by nuclear fusion. But most astronomers would agree that a brown dwarf would not have enough material around it to form planets by "accretion" (Dust clumps together to form rocks, which clump together to form asteroids, which clump together to form planets.) So, if this is a planet orbiting a brown dwarf, it must have formed some other way.

Stars form when giant clouds of gas collapse due to gravity. It is possible that, when the gas cloud is collapsing, it will split into several smaller pieces. So it could be that the gas cloud that formed the brown dwarf split in two, with the larger clump forming the brown dwarf and the smaller lump the planet. But would you then call the "planet" a planet? It was made in a different way and likely has a different structure inside -- Jupiter probably has a center several times the size of the Earth made of rock, but a "planet" formed by splitting gas clouds would be almost pure gas, like the sun.

All this may be is a question of defining what a "planet" is. Of course, if the supposed planet turns out to be a star in the background, then the whole issue will go away. At least until the next possible planet is found!

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