After locating more than 340 planets orbiting other stars, astronomers have identified two that are the most similar to Earth so far.
The most recently discovered one is almost twice as large as Earth, making it the smallest exoplanet -- for extra-solar planet -- found to date. The second 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 Earth-like, habitable planet," astronomer Alan Boss of the Carnegie Institution of Washington wrote in an e-mail.
It "is the most exciting discovery in exoplanets so far," added astronomer Geoffrey W. Marcy of UC 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 with 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 an astronomical 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 dissipated, leaving only uninhabitable rock.
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.
The other three planets in the Gliese system have masses of 16, five and seven Earths. The one with a mass of seven Earths, called Gliese 581 d, was initially thought to have an orbital period of 80 days, which would put it just on the outer edge of the habitable zone.
Recent refinements of the data, however, show that it has an orbit of only 66.8 days, which places it well within the habitable zone, astronomer Stephane Udry of Geneva University told the meeting. Because of its distance from Gliese 581, moreover, it must have a significant amount of water and other gases, he added. It could have oceans thousands of meters deep, he said.
The team is continuing to monitor Gliese 581 in hopes that the orbital planes of the planets will bring them between the star and Earth, which will allow astronomers to learn more about their composition.