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Ask Dr. SETI ®

Chapter 1: Astrophysics

Signal from a Habitable Planet?

Dear Dr. SETI:
I have been told that a SETI candidate signal analyzed by Dr. Ragbir Bhathal in Australia came from the vicinity of Gliese 581g, a recently discovered Earth-like planet. Is there any truth to this?

Joe S.

The Doctor Responds:
Well, Joe, yes and no. Ragbir's telescope was indeed pointed in the vicinity of the star Gliese 581, when a potential (though inconclusive) SETI candidate signal was detected. But, "vicinity" was probably the wrong word to choose. "General direction of" would be more apt. This does not necessarily mean the signal was coming from the newly discovered planet (or, in fact, from any of the five previously discovered planets orbiting that particular star -- or even from the general neighborhood of that star!)

Consider that the Milky Way galaxy contains 400 billion stars, more or less. Consider that the largest radio telescope on Earth, the Arecibo Observatory, can view only one part in 16 million of the sky at any given time. Divide the one number by the other, and you can see that, for any observation, there are, on average, about 25 thousand stars within its field of view -- most of which are (a) invisible to us on Earth, and (b) completely unknown to us, not even appearing in our star catalogs. The same principle is true, more or less, for any telescope, optical or radio. So, should a signal be detected coming from any direction, we have perhaps about a one in 25 thousand chance of saying which star was the source of that signal!

By the way, notwithstanding the comments of Steve Vogt, the discoverer of the exoplanet in question, Gliese 581g is not particularly Earthlike. Yes, it happens to be in its star's habitable zone, but there the similarity ends. This planet is tidally locked, with one side always facing its Sun, and the other side always pointed away from it. If Earth were in a similar predicament, we wouldn't even be having this conversation.

To be sure, Gliese 581g is a noteworthy discovery, being in its star's Habitable Zone. Unfortunately, the publicity and speculation surrounding this discovery is a fine example of journalistic exaggeration.

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