The odds are that a habitable exoplanet will be found by the middle of next year if the rate of discovery continues at its current pace, two scientists say.

Using data on the exoplanets discovered so far, Samuel Arbesman, a computational biologist at Harvard Medical School and astronomer Gregory Laughlin of the University of California say there’s a 50:50 chance that an Earth-sized planet capable of holding liquid water will be found during the first half of 2011.

They created a scale for the habitability of a planet based on its surface temperature at the poles and equator, and its mass, which determines gravity, and applied this to 370 previously discovered planets.

They then plotted each planet’s habitability against its date of discovery. Extrapolating this data, they concluded that there’s a 50 percent chance that a habitable planet will be discovered by May, and that the likeliest date of discovery is early May.

They say there’s a two-thirds chance that a habitable planet will be doscovered by the end of 2013, and a 75 percent chance of finding one by 2020.

“In the past decades, the number of known extrasolar planets has ballooned into the hundreds, and with it the expectation that the discovery of the first Earth-like extrasolar planet is not far off,” say the authors.

“Using a bootstrap analysis of currently discovered exoplanets, we predict the discovery of the first Earth-like planet to be announced in the first half of 2011.”

More than 450 exoplanets have been discovered so far, but most are gas giants like Jupiter with no solid surface. In June, a team using the Kepler telescope said they’d found another 750 possible exoplanets, some of which may be Earth-like.

The paper’s available here.