It's the fourth planet-planet e-that the scientists suggest might be another life-bearing world, even though it's about four times as massive as Earth. If you lived there, you'd see a yellow sun in the sky, but your year would last just 168 days. That's because Tau Ceti lies somewhat closer to its star than Venus does to the sun and thus revolves faster than Earth. The fifth and outermost planet, designated Tau Ceti f, completes an orbit every 640 days and is slightly closer to its star than Mars is to the sun.
However, Tuomi's team warns that disturbances on the star itself, rather than orbiting planets, may be producing the small velocity changes in Tau Ceti. "They're really digging deep into the noise here," says Sara Seager, an astronomer at the Massachusetts Institute of Technology in Cambridge who was not part of the team. "The [astronomical] community is going to find it hard to accept planet discoveries from signals so deeply embedded in noise."
"They're pushing the envelope," says Gregory Laughlin, an astronomer at the University of California, Santa Cruz. "Some or even many of these planets could go away. But I think that they've done absolutely the best job that you can do, given the data." Laughlin says it's frustrating that the most interesting planets-small ones like Earth-are so challenging to detect: "You have to get tons and tons and tons of velocity measurements over many years, and then you really, really have to take extreme care-as this Tuomi et al. paper does-to get rid of all the systematic noise."
Team member Chris Tinney, an astronomer at the University of New South Wales in Sydney, Australia, acknowledges the problem. "It's certainly very tantalizing evidence for potentially a very exciting planetary system," Tinney says, but he adds that verifying the discovery may take 10 years, and the scientists didn't want to wait that long. "We felt that the best thing to do was to put the result out there and see if somebody can either independently confirm it or shoot it down."
If the planets exist, they orbit a star that's about twice as old as our own, so a suitable planet has had plenty of time to develop life much more advanced than Homo sapiens. That may just explain why no one from Tau Ceti has ever contacted beings as primitive as us.
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This is adapted from ScienceNOW, the online daily news service of the journal Science. http://news.sciencemag.org