A new mathematical analysis predicts the first truly habitable exoplanet will show itself by early May 2011.

Well, more or less. "There is some wiggle room," said Samuel Arbesman of the Harvard Institute for Quantitative Social Science, lead author of a new paper posted online and to be published in PLoS ONE Oct. 4. His calculations predict a 50 percent probability that the first habitable exoplanet will be discovered in May 2011, a 66 percent chance by the end of 2013 and 75 percent chance by 2020.

"This is, as far as we can tell, right around the corner," said exoplanet expert Greg Laughlin of the University of California, Santa Cruz, co-author of the paper.

Astronomers have found 490 planets outside our solar system to date, and those planets have been getting steadily smaller and more Earth-like. But none so far actually resemble Earth in its most important property: the ability to support life.

So Arbesman and Laughlin devised a mathematical way to define habitability using the techniques of scientometrics, the scientific study of science itself.

The pair considered a planet's mass and its surface temperature at the points in its orbits when it is closest and furthest away from its star, and calculated which of these properties would be friendliest to liquid water (and, therefore, presumably, life). Then they plotted their habitability function on a scale of 0 to 1, where 0 is uninhabitable and 1 is a clone of Earth.

Next, the researchers turned to the exoplanets that have already been found. They calculated the habitability metric for 370 exoplanets whose masses and distances from their stars are relatively well-known, and plotted that number against the planet's date of discovery. Then they used a statistical method called bootstrapping, which looks at subsets of data to get a better idea of the overall distribution, to extrapolate forward to a planet with a habitability value of 1.

The median date for this planet to make its grand entrance, they found, is early next May. And the planet-hunting Kepler spacecraft may not be the one to find it, the researchers add.

"To find the really good stuff that Kepler is going to detect is going to take a few years," Laughlin said. "Because the mission has only been flying for a bit more than a year, they just haven't had time to find the planets that are genuinely habitable. Though they will."

"We simply wanted to say it's an open field, we don't know who's going to win," Arbesman said. "But it seems like whoever does win, it's going to happen soon."

Exoplanet expert Sara Seager of MIT says she's not surprised.

"They made a prediction you could probably make without all that probability," she said. "People are specifically searching for planets that have liquid water. Just knowing how many people are looking and how many stars they're looking at.... If you want a big Earth around a small star, that could happen any day."

Arbesman and Laughlin admit their habitability metric is a little optimistic and their analysis leaves out factors like the march of technology. "It's not a scientific result, it's not a discovery," Laughlin said. "It's just something to spark discussion, to point out an interesting trend."

And if they're wrong, he adds, we'll know soon enough.

Image: Artists rendition of nearby star Gliese 581 and its planets. Some astronomers think one of its planets, Gliese 581d, could be habitable, but others are skeptical. Credit: ESO/L. Calcada

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