Object: Earth-sized exoplanet
Composition: Rock and iron
Temperature: Scorching
[attachment=0]Kepler-78b-1200300dpiRGB.jpg[/attachment]
Earth now has a twin – but it's evil. There's a planet the same size as Earth in a distant solar system, and it shares our planet's mass and composition. However, the rocky exoplanet is so close to its star that
it orbits it once every 8 hours, making it hellishly hot, with almost no prospect of hosting life.
The planet, Kepler-78b, is not the first world to come within 20 per cent of Earth-like size or mass – those distinctions go to
Kepler-10b and
MOA-2007-BLG-192-L b, respectively.
It's not even the first to claim a rocky make-up, a title that went to
COROT-7b in 2009. Kepler-10b is also thought to be rocky. But it is the first to have all three characteristics at once, and raises astronomers' hopes that life-friendly Earth twins are out there.
"This is an existence proof," says
Andrew Howard of the University of Hawaii in Honolulu. "When you have one, you know that Nature can make Earth-sized rocky planets outside of the solar system."
Tiny wobbles
As its name suggests, Kepler-78b was first spotted by the now-defunct Kepler space telescope, which spent 3.5 years watching stars for regular dips in brightness, called transits, characteristic of a planet crossing in front of them. That revealed the planet's sizes. The more starlight it blocked, the bigger the planet.
But it gave no direct hint of any given planet's mass, and therefore the planet's density, a clue to composition.
The most successful technique so far for measuring planet masses is by watching the star for the tiny wobbles the planet's gravity induces. The larger and closer a planet is to its star, the bigger the wobble. Earth-mass planets are tiny enough that they would be difficult to detect unless they were close.
When Kepler-78b was discovered with an 8 hour year, two teams jumped at the chance to measure its mass. "Everybody instantly knew this was our one great shot," Howard says.
Iron and rock
Howard's team used the Keck telescope in Hawaii to observe the star. At the same time, a team led by
Francesco Pepe of the University of Geneva in Switzerland used the HARPS-N telescope on Spain's La Palma Island.
Howard's team now reports that the planet has a radius of 1.2 Earths and a mass of 1.69 Earths. Pepe's team calculates a radius of 1.16 Earths and a mass of 1.86 Earths. Both give bulk densities of roughly 5.5 grams per cubic centimetre, which is similar to that of Earth and implies a composition of iron and rock.
Unfortunately, it's so hot that much of that rock is probably molten. And it's going to be difficult to extend this technique to finding cooler, more comfortable Earth twins – because of their distance from their host star.
Ongoing mystery
"We didn't find a shortcut to Earth-sized planets in the habitable zone," Howard says. "It's going to require a new generation of ultra-stable spectrographs, probably on the world's biggest telescopes."
But Kepler-78b also poses a more immediate mystery: why does the planet exist at all? When the planetary system was forming, its young star was bigger than it is now, and would have engulfed Kepler-78b at its current orbit. That means the planet must have formed further away and travelled in towards the star since then. But if that is the case then it should have
fallen all the way in by now.
In any case, the planet's prognosis doesn't look good: theorists predicts that within 3 billion years, gravity will make Kepler-78b spiral into its star and be ripped to shreds.
