Pair of planets found that resemble Earth

After finding more than 340 planets orbiting other stars, astronomers have found two that are the most similar to Earth so far.

The most recently discovered one is less than two times as large as Earth, making it the smallest exoplanet — or planet outside our solar system — found to date. The other one was found in 2007, but new observations have shown that it is the only exoplanet to date that orbits its star in the so-called habitable zone, where water remains a liquid. Thus, it is the only exoplanet discovered that is likely to have oceans.

Intriguingly, both orbit the same star, a dwarf 20 light-years from Earth called Gliese 581, European researchers said Tuesday.

The identification of the small planet "is a remarkable discovery and bodes well for our eventual discovery of a true Earthlike, habitable planet," astronomer Alan Boss of the Carnegie Institution of Washington said in an e-mail.

It "is the most exciting discovery in exoplanets so far," added astronomer Geoff Marcy of the University of California, Berkeley via e-mail. "It shows that nature makes such small planets, probably in large numbers."

The small planet is the fourth discovered circling Gliese 581 by a team of astronomers working at the European Southern Observatory's 3.6-meter telescope at La Silla, Chile. They identified the planets by detecting and analyzing slight wobbles in the star's path as the planets orbit it.

The small planet, called Gliese 581 e, has an estimated mass equal to 1.9 Earths and orbits its sun every 3.15 days, the team reported at a meeting at the University of Hertfordshire in Britain. Because it is so close to Gliese 581, it is blisteringly hot and any gases or liquids that it might have carried have long since boiled away.

In February, French astronomers said they had discovered an even smaller planet, called CoRoT-Exo-7b, that has an estimated mass equal to 1.7 Earths, circling a different star. But experts said the data for Gliese 581 e is more convincing.