Study: Fewer Earthlike Planets Than Previously Thought

Scientists figure there's about 2 billion, give or take

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Into the continuing saga of the search for Earthlike planets, a new study has fallen. It turns out there are probably fewer of them out there than previously thought.

A quick history: Last fall, astronomers announced that they found an Earthlike planet, Gliese 581 g, that was similar in mass to our own and inhabited the critical "Goldilocks Zone" (not to far away or too close to its theoretical Sun). Naturally, earthlings were giddy about our potential planetary kin, and even though Gliese was purported to be 20 light years away, the discovery was hailed as a major breakthrough for science.

Then we got cynical. Only a month later, some scientists doubted that the planet was ever there at all. In an assessment of opinions at the time, The Wire wrote "at the moment no one can say for sure whether the planet exists or not."

Now, scientists are estimating that there could be up to 2 billion Earthlike planets housed among the approximately 100 billion sunlike stars in the Milky Way galaxy. And they caution that such a low number (again, think, relative to the expanse of the seeming infinity) "could make it hard to find other 'Earths' in our galaxy."

How did scientists settle on the 2 billion theoretical earths estimation--especially when they haven't found them yet?

National Geographic reports that scientists parsed data collected from NASA's Kepler space telescope using two measures: "a transiting planet's size and the distance at which the planet orbits its star" to determine its likeness to Earth by placing it in the just-right "Goldilocks zone" temperature. That means they took a small sample size of about 156,000 stars in Kepler's field of view, did mathematical calculations to determine what percentage of sunlike stars have earthlike planets and extrapolated their findings to the rest of the entire galaxy.

Some of the scientists involved have qualified their findings a bit. MIT planetary sceintist Sara Seager, talking to National Geographic, made a comparison using the easier-to-wrap-your-mind around example of random polling: "Say you're doing a census of the United States," she explained. "If you go to California and knock on every door, you can then extrapolate out to the rest of the country. That's what Kepler's doing." There's also this to keep in mind:

The other big concern is that, with the data Kepler can collect, it's impossible to say whether a given planet is truly Earthlike.

Size alone, for example, doesn't say enough about habitability. "Earth and Venus are about the same mass and size," she said—and by some definitions both worlds fall in our sun's habitable zone.

We're niether astronomists nor pollsters, but that doesn't exactly sound like it bodes well for anything nearing a complete count of theoretical Earthlike planets. Guess we'll have to wait till the scientists actually find one.

Pictured above: the one earth-like planet we know definitely exists, Earth, being circled by the space shuttle Discovery on its final mission in February 2011.

This article is from the archive of our partner The Wire.