Published online 10 January 2011 | Nature 469, 143-144 (2011) | doi:10.1038/469143a

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Kepler's latest discovery is step closer to a true Earth analogue.

The Kepler craft was launched in 2009. NASA

Kepler, the space telescope considered most likely over the next few years to identify an Earth-like planet orbiting another star, has struck solid ground, mission scientists say.

Most of the hundreds of extrasolar planets discovered to date — including the eight previously reported by Kepler — are at least as large as Neptune and are mainly gas giants. But astronomers hope that the mission will eventually yield a trove of terrestrial planets, including some orbiting their stars at a distance that would allow their surfaces to host liquid water, and possibly life.

Now the team is a step nearer to that goal. It has found a Sun-like star 173 parsecs away — relatively close by galactic standards — harbouring a planet that is just 40% larger than Earth, although it is uninhabitably hot. Supporting measurements of the planet's mass, gathered at the Keck Observatory on Mauna Kea, Hawaii, suggest that the planet's density is 1.6 times that of Earth, implying a rocky composition. Although other telescopes have already spotted a handful of similar 'super-Earths', the first confirmed rocky planet to fall within Kepler's sights is an encouraging sign for the mission.

"It's a milestone," says co-investigator Natalie Batalha, an astronomer at San José State University in California, who unveiled news of the discovery during a talk on 10 January at the American Astronomical Society's annual meeting in Seattle, Washington.

Launched by NASA in 2009, Kepler's strategy is to point at a patch of sky in the plane of the Milky Way, where it can continuously monitor more than 100,000 stars, and to look for slight, periodic changes in their luminosity. Such dips in brightness often indicate the presence of planets repeatedly crossing in front of the stars they orbit. So far, after barely a year of observations, Kepler is limited to detecting short-period planets, those close to their stars.

“The first confirmed rocky planet to fall within Kepler’s sights is an encouraging sign for the mission.”



The new-found planet orbits its star — designated Kepler 10 — every 20 hours, at a distance of 2.5 million kilometres. This is less than 2% of the distance separating Earth and the Sun, ensuring that the planet is more 'terra lava' than terra firma, with a surface temperature of 1,800 °C, hot enough to melt silicate rock.

Astronomers at Keck were able to monitor how the planet tugs on the star, giving them its mass. Meanwhile, because the star is nearer than most of the stars in Kepler's search field, the Kepler team could record tiny variations in the star's luminosity due to sound waves rippling through its interior. The timing of these acoustic oscillations provided detailed information about the star's dimensions, much as the sound of a string on a bow reveals whether the instrument being played is a violin or a cello.

"Everything we measure about the planet is relative to the parent star," says Batalha. Armed with a precise figure for the star's diameter, she and her colleagues were able to nail down the planet's dimensions with similar precision.

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The planet's close orbit is similar to that of another rocky exoplanet, CoRoT-7b, discovered in 2009, says Greg Laughlin, an astronomer at the University of California, Santa Cruz, who was not involved in the work. He says the result shows how far astronomers' techniques for finding exo­planets of Earth-like proportions have advanced in recent years. "No one would have thought a decade ago that we'd be finding these ultra-hot rocky super-Earths," says Laughlin. Their existence suggests that cooler, smaller worlds await discovery as Kepler's monitoring time increases and it can identify planets in longer, more distant orbits, he adds.

In February, the Kepler mission is expected to release data on 400 additional planetary candidates. The team is excited, says Batalha, because it hopes that Kepler's growing number of detections will offer the first concrete estimates of the frequency of extrasolar planets — rocky and otherwise. "So far, it's been a deluge," says Batalha. "And we look forward to finding a lot more planets."