Astronomers have discovered a surprising new planet, a rocky world with 17 times the mass of earth. There have been "super-earths" discovered before, but this one is in a league of its own. The scientists call it a "Mega-Earth".
Discovered by Nasa's Kepler Space Telescope and announced on Monday at an astronomy meeting in Boston, this new planet, officially named Kepler-10c, scrambles the equations that dictate how massive a rocky planet can be without ballooning into a Jupiter-like gas giant.
The theorists didn't see this coming. The orthodoxy was that, beyond about 10 earth masses, a planet would hold on to so much hydrogen gas that it would become like Jupiter or Saturn. Kepler-10c suggests plus-size planets can stay rocky, with clearly defined surfaces, rather than become gaseous and bloated.
That means there's more real estate out there for life as we know it on earth.
Kepler-10c is also very old, having formed about 11 billion years ago, less than three billion years after the birth of the universe. Rocky worlds weren't believed to have existed that long ago.
Kepler-10c, which orbits a star 560 light-years away in the constellation Draco, isn't likely to harbour life. It is too close to the parent star and the surface is thoroughly roasted.
Gravity at the surface is about twice that of earth's gravity. The planet is 2.3 times the diameter of earth, but is much denser. "It's still rock, but it's rock that's twice as dense as the rock we're used to," said Dimitar Sasselov, a professor of astronomy at Harvard and a co-author of the paper describing the "Mega-Earth".
The Kepler Space Telescope, launched in 2009, has found the faint signatures of thousands of planets, though some need more observation before their discovery can be confirmed. The telescope examines a small patch of the sky, taking images of stars and looking for periodic dimming of the starlight. If that dimming follows a regular pattern, it may be from a planet repeatedly passing across the face of the star as seen from the telescope.
Ground-based telescopes have followed up the Kepler leads and gathered new details about these planets. After the space telescope found Kepler-10c, a telescope on the ground measured its mass and discovered that it was a giant rocky world.
It now appears that planets are extremely abundant - virtually every star may have at least one planet. But the habitability of these worlds remains a mystery. No one has found an exact earth twin - a rocky, earth-sized world orbiting a sun-like star in the habitable zone.
One bulletin from the American Astronomical Society meeting in Boston offered a reminder that there are a lot of ways a planet can prove to be inhospitable to life. The "space weather", for example, might be ghastly.
Astrophysicist Ofer Cohen of the Harvard-Smithsonian Centre for Astrophysics modelled the environments of three candidate planets identified by the Kepler telescope, each apparently rocky like the earth, and orbiting their stars in what is deemed the "habitable zone" - not so close to the star that the planet gets baked and not so far away that surface water would likely be frozen.
All three of those parent stars studied by Cohen and his colleagues are common "red dwarfs", also known as "M-dwarfs", which account for about seven of every 10 stars in our galaxy. The "habitable zone" of these small stars is relatively close. But that brings into the equation another factor: The stellar wind, the particles streaming from the star's surface. Cohen concluded that the stellar wind would likely have stripped away the atmospheres of these planets.