Earth twins found but too hot for life

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  • Published 21.12.11
Graphic displays the first two Earth-size extrasolar planets next to the Earth and Venus, ranked by their size. Credit: Tim Pyle

New Delhi, Dec. 20: Astronomers today announced their discovery of the first two Earth-size planets orbiting a distant star, but cautioned that both are so hellishly hot that they are unlikely to support life.

The planets Kepler-20e and Kepler-20f, orbiting a Sun-like star about 1,000 light years away, are the first true Earth-size planets found since astronomers began cataloguing planets beyond the solar system in the mid-1990s.

An international team of planet hunters used the US Kepler space observatory to study changes in light from the star Kepler-20 thought to be caused by the periodic transit of the planets in front of the star. The decrease in starlight during the transits suggests that Kepler-20e has a size 0.87 Earth radius, and Kepler-20f is even closer with 1.03 Earth radius. The findings will appear in the journal Nature tomorrow.

The two planets are among five orbiting Kepler-20, all of them closer to their parent star than the distance of Mercury from the Sun. Kepler-20e has a temperature of 767°C, while Kepler-20f is relatively cooler but still at a searing 432°C.

“The search for extrasolar planets has now crossed a threshold,” said Francois Fressin, a French astronomer at the Harvard Smithsonian Centre for Astrophysics in the US, and a member of the team.

The discovery demonstrates the Kepler observatory’s ability to detect Earth-size planets whose effects on starlight are so feeble that they are difficult to observe. “Now that we’ve found two, we hope to find more,” Fressin told The Telegraph.

“This is a big step towards finding a twin-Earth,” said Sujan Sengupta, associate professor at the Indian Institute of Astrophysics, Bangalore, who was not associated with the Kepler-20 search, but has been independently looking for extrasolar planets.

A planet’s habitability depends on its distance from the star and its size, he said.

“In addition to a planet’s surface temperature, a planet’s size determines its surface gravity, its internal structure and its atmospheric composition — and all of these can influence its habitability,” Sengupta told The Telegraph.

Two weeks ago, the Kepler search team had announced the discovery of Kepler-22b, a planet lying within the habitable zone of another Sun-like star, at just the right distance to have liquid water, but it was too large for other conditions to be right. That planet, dubbed superEarth, had 2.4 times Earth radius and is thus likely to be a gas giant similar to Neptune in the solar system.

Kepler-20e and Kepler-20f are the right size, but too hot. “Neither are true Earth analogs, and indeed finding such a true twin Earth is the ultimate goal of the Kepler mission,” said David Charbonneau, professor of astronomy at Harvard University, another team member.

The astronomers have used theoretical calculations to suggest that both the planets could have a 32 per cent iron core and a 68 per cent silicate mantle, an internal makeup similar to that of the Earth.

The calculations combined with certain other assumptions also suggest that Kepler-20f could have an atmosphere with water vapour, while Kepler-20e could be entirely rocky, as all of its water would have been lost through evaporation. The researchers now plan to try and measure the mass of the planets which will help them determine whether the composition is indeed iron and silicate.

“These planets are not habitable in their current conditions,” said Fressin. “But it is possible that they have migrated closer and closer to their parent over time — so they might have been habitable in the past — although we can’t say when and for how long.”