Discoveries in search for twin Earths

Astronomers have detected the first Earth-sized planets orbiting a distant star. The discovery is a landmark in the search for a “twin Earth” that might harbour life like ours.

The 700 or so extra-solar planets found previously were all considerably larger than Earth. The two new discoveries, made with the US Kepler space observatory, are about the same volume and mass.

“The goal of Kepler is to find Earth-sized planets in the habitable zone [neither too near or too far from their parent star],” said François Fressin of the Harvard-Smithsonian Centre for Astrophysics, lead author of an online Nature paper describing the new planetary system. “Proving the existence of Earth-sized exoplanets is a major step toward achieving that goal.”

The new worlds orbit Kepler-20, a Sun-like star 950 light years away. They are too close to their parent star – and therefore too hot – to be habitable now by life as we know it, but both appear to be rocky planets with a geological composition dominated by iron and silicate minerals, similar to Earth and other planets in the inner solar system.

The outer planet of the pair, Kepler-20f, may have a thick water-vapour atmosphere. “Although 20f is not habitable now, it could have been habitable for a long period in the past,” said Linda Elkins-Tanton of the Carnegie Institute for Science, another member of the team.

There has already been considerable publicity about other “twin Earths” discovered by Kepler, a space telescope that the US space agency Nasa launched in 2009.

In particular a planet called Kepler-22b is at the right distance from its parent star for liquid water to exist. The announcement of its discovery two weeks ago led to speculation that it might have continents, oceans and even life.

But Dr Fressin said Kepler-22b could not be regarded as Earth-like. “You could fit 13 Earths inside Kepler-22b,“ he said. “The most likely thing is that it's simply a mini-Neptune, not suitable for life. Just because a planet lies within the habitable zone that doesn't mean it is habitable.”

What will have astronomers really cheering is the discovery of planets that are not only Earth-sized but also within their star’s habitable zone – and that is just a matter of time, said David Charbonneau, astronomy professor at Harvard University.

All Kepler’s exoplanet discoveries are made indirectly, by looking at the very slight dimming that occurs when a planet crosses the face of its planet star. The observation can then be confirmed by a tiny wobble in the star’s position due to the gravitational pull of the planet.

Astronomers deduce the size, position and nature of each planet from these signals, which are much more obvious for large than small planets. It will be several years before telescopes are powerful enough to produce direct images of planets beyond the solar system or to analyse the composition of their atmosphere for biological molecules.

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