Seth Borenstein of the Associated Press is reporting that astronomers have discovered an extrasolar planet in the “goldilocks” zone of its star. That is, the planet appears to be in the circumstellar habitable zone where water can persist at liquid temperatures on its surface. The planet, named Gliese 581g, is a mere 20 light years away from Earth.
The article is referring to the circumstellar habitable zone, though presumably it is also in the galactic habitable zone since it’s so close to Earth. That means that Gliese 581g may have two of the major factors needed to make a planet hospitable to life.
Unfortunately, we’ve seen hundreds of reports like this, so I now read them with a bit of skepticism. For the last fifteen years, we’ve been hearing about supposedly Earth-like planets around other stars. The headline is so common that even Borenstein, in an otherwise breathless articles, admits: “Scientists have jumped the gun before on proclaiming that planets outside our solar system were habitable only to have them turn out to be not quite so conducive to life.”
The planet in question is tidally locked, so the same face perpetually faces its star. So it won’t have a pleasing climate. It’s about three times more massive than Earth, and it’s quite close to its star, which is an M dwarf. Such stars are probably not good hosts for habitable planets due to their high activity levels.
Whenever you read a story like this (and there will be many more in the next few years), it’s important to remember two things. First, Venus and Mars are much more Earth-like that this or any other extrasolar planet we’ve yet been able to detect. For instance, they’re around a star known to host a habitable planet, and they’re both quite close in orbit to that habitable planet. And yet, neither is home to life of any sort.
Second, even if a planet has all of the necessary conditions for hosting life, it doesn’t follow that it has life. There’s a difference between necessary and sufficient conditions. Astronomers often seem to forget this in the excitement of discovering extrasolar planets. In this story, Steven Vogt of the University of California at Santa Cruz (co-discover of the new planet) is quoted as saying: “It’s pretty hard to stop life once you give it the right conditions.” I have no idea what the empirical basis for such a claim would be. Water doesn’t spontaneously create reproducing organisms.
I’m not dismissing the possibility of life on extrasolar planets. I maintain an open mind on the question. Many people think that Guillermo Gonzalez and I argued in The Privileged Planet that life must be unique to Earth. Not so. We argued only that complex life, if it exists elsewhere, will be found only around very Earth-like planets. Nothing has caused us to doubt that claim. But while Gliese 581g may have a couple of features in common with Earth, it is still vastly more different than Earth compared to Venus and Mars.