Rocky exoplanet milestone in hunt for Earth-like worlds

An artist's conception shows how the star-facing side of Kepler 10b may look

Astronomers have discovered the smallest planet outside our Solar System, and the first that is undoubtedly rocky like Earth.

Measurements of unprecedented precision have shown that the planet, Kepler 10b, has a diameter 1.4 times that of Earth, and a mass 4.6 times higher.

However, because it orbits its host star so closely, the planet could not harbour life.

The discovery has been hailed as "among the most profound in human history".

The result was announced at the 217th annual meeting of the American Astronomical Society in Seattle, US, by Nasa's Kepler team.

The Kepler space telescope, designed to look for the signs of far-flung planets, first spotted the planet 560 light years away, alongside hundreds of other candidate planets.

Kepler relies on the "transiting" technique, which looks for planets that pass between their host star and Earth.

A tiny fraction of the star's light is blocked periodically, giving a hint that the star has a planet orbiting it.

The radius of the planet correlates to exactly how much light is blocked when it passes.

Follow-up measurements by a telescope at the Keck observatory in Hawaii confirmed the find of Kepler 10b by measuring how the planet pulls to and fro on its parent star as it orbits.

These measurements also bore out the fact that the parent star was about eight billion years old - a grandfather among stars of its type.

Crucially, this meant that the star was free of the optical and magnetic activity that have introduced some uncertainty into the measurements of previous candidates for rocky exoplanets, such as Corot-7b, announced in early 2009.

Ever-expanding fields

This cosmic dance causes tiny changes in the colour of the starlight that is measured by telescopes.

However, what completed the suite of measurements for the Kepler team was the use of asteroseismology - a study of distant stars that is akin to the study of earthquakes on the Earth.

The oscillations that occur within a star - as within the Earth - affect the frequencies of the light that the star emits in a telltale sign of the star's size.

With the size of the host star, the details of the planet's and star's mutual dance, and the planet's radius, the density of the planet can be calculated.

"All of our very best capabilities have converged on this one result and they all converge to form a picture of this planet," said Natalie Batalha, a San Jose State University professor of astrophysics who helps lead the Kepler science mission for Nasa.

Hot rocks

Professor Batalha told BBC News that the result was unique in an ever-expanding field of exoplanet discoveries, with smaller and smaller exoplanets discovered as experimental methods improve.

"We're always pushing down toward smaller and less massive, so it's natural that we're arriving there," she said.

"But perhaps what's not so natural is that we've pinned down the properties of this planet with such fantastic accuracy that we're able to say without a doubt that this is a rocky world, something that you could actually stand on."

One could, that is, if it were not so close to its host star that its daytime temperature exceeds 1,300C - so Kepler 10b is not a sensible candidate to host life. However, as Professor Batalha explained, it is a significant step in Kepler's mission.

"We want to know if we're alone in the galaxy, simply put - and this is one link in the chain toward getting to that objective.

"First we need to know if planets that could potentially harbour life are common, and we don't know if that's true - that's what Kepler is aiming to do."

A pioneer of the hunt for exoplanets, Geoffrey Marcy, from the University of California Berkeley, said that Kepler 10b represented "a planetary missing link, a bridge between the gas giant planets we've been finding and the Earth itself, a transition... between what we've been finding and what we're hoping to find".

"This report... will be marked as among the most profound scientific discoveries in human history," he said.