Monday, February 18, 2008

Odds Look Good for Rocky Planets

In a NASA press release yesterday, two teams of astronomers announced complimentary research that suggests that earth-like planets around sun-like stars may be common.

The first team used the Spitzer space telescope to survey a bunch of sun-like stars of varying ages. They were looking for evidence of dust kicked up by the collision of planetesimals. Planets form in a disk of debris around their star. As smaller rocks collide, they stick together to form larger rocks. With more gravity, the larger rocks attract more mass, and you get runaway growth. It's a messy process though, and many of the collisions create small particles even as they add most of their mass to the growing planetesimals. It is these small dust particles that the Spitzer team searched for. Warmer dust is associated with formation of planets close to their star, like the earth. The astronomer found that 10 to 20 percent of young sun-like stars have warm dust, but stars older than 300 million years don't. This suggests that planet formation is common and that it runs to completion within the first couple hundred million years.

The second study is based on theoretical models of how planets form. They confirmed that dust is a by-product of rocky planet formation and offer up two ways of interpreting the dust observed by the Spitzer team. First, it is possible that the dust lingers for a while, so that even if planet formation happens in only ten million years, the dust sticks around and you see it even for older stars that have finished forming planets. If this is true, then about 20% of sun-like stars have planets. Another possibility is that big disks form planets fast and their dust gets eaten up, while less massive disks take longer, so their dust persists. Interpreted this way, the Spitzer dust observations could mean that up to 60% of sun-like stars form planets.

In reality, it is probably some combination of the two effects, which means that the likelihood of forming planets is somewhere between 20 and 60 percent. Pretty good odds, if you ask me.

For the original press release, check here.

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