The researchers calculated the habitability of 370 exoplanets discovered so far, on a scale of 0 to 1. (David A. Aguilar/Harvard-Smithsonian Center for Astrophysics)An Earth-sized planet that could support liquid water — and therefore life — has more than a 50 per cent chance of being discovered in the first half of 2011, two U.S. researchers predict.

Samuel Arbesman, a computational biologist at Harvard Medical School, and Gregory Laughlin, an astronomer who specializes in numerical simulations and modelling, based their predictions on the properties of the exoplanets — planets outside our solar system — discovered so far.

The paper detailing their method was posted this week on arXiv, a public server that archives and distributes research articles. The posting noted that the article been accepted for publication in the journal PloS ONE.

The researchers developed a formula for calculating the habitability of a planet on a scale of 0 to 1, where 0 is unhabitable and 1 is Earth-like. They based on the surface temperature at the poles and the equator — which must be within the range that allows for water to exist as a liquid — and the planetary mass, which determines the force of gravity at the surface.

They did the calculation for 370 planets that have been discovered, and plotted each planet's discovery date against its habitability. They then extrapolated the data to a habitability of 1 while using a statistical method called bootstrapping, where subsets of the data are sampled to get a better picture of the distribution within the sample. They found there is:

• A 50 per cent chance a habitable planet will be discovered by May, and the likeliest discovery date is early May.
• A 2/3 probability a habitable planet will be discovered by the end of 2013.
• A 75 per cent chance a habitable planet will be discovered by 2020.
• A 95 per cent chance a habitable planet will be discovered by 2264.

A graph from the paper shows past planetary discoveries as dots, with the black ones having the highest habitability among all the planets discovered that year. The sloping black line is a best-fit line for the black dots. The horizontal grey line indicates a habitability of 1. (ArXiv)The authors acknowledged their prediction ignores a lot of factors, including technological advancement, but they don't think that will alter the existing pattern of increasing planetary discoveries.

They also noted that predicting future scientific and technological progress is a "slippery and difficult process" and many past predictions haven't panned out. On the flip side, publicizing predictions can influence them to come true, the paper said.

"However, due to the large number of observations and long periods of time required to confirm an extrasolar planet discovery," it added, "it is unlikely that our prediction at this time will appreciably affect the announcement of the discovery of an Earth-like planet."