Astronomers have discovered a surprising 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 Monday at an astronomy meeting in Boston, this 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 that plus-size planets can stay rocky, with clearly defined surfaces, rather than becoming 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 3 billion years after the birth of the universe. Rocky worlds weren’t believed to have existed that long ago.
“Nature will do what she wants, regardless of earthling theorists,” said Sara Seager, a Massachusetts Institute of Technology planetary scientist who was not involved in the new discovery but said by e-mail that she finds it “incredibly exciting.”
Kepler-10c, which orbits a star 560 light-years away in the constellation Draco, isn’t likely to harbor life. It is too close to the parent star, and the surface is thoroughly roasted.
Gravity at the surface is nearly three times that of Earth. If there were creatures somehow bounding around, they would probably be rather squat. The planet is 2.3 times the diameter of Earth but is much denser, particularly toward the core.
“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 University and a co-author of the paper describing the “Mega-Earth.”
The Kepler telescope, launched in 2009, has found the faint signatures of thousands of planets, though some need additional observation before their discovery can be confirmed. The telescope examines a relatively 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 is 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-size world orbiting a sunlike star in the habitable zone.
One bulletin Monday from the American Astronomical Society meeting in Boston offered a reminder that there are a lot of ways a planet can prove inhospitable to life. The “space weather,” for example, might be ghastly.
Astrophysicist Ofer Cohen of the Harvard-Smithsonian Center for Astrophysics modeled the environments of three candidate planets identified by the Kepler telescope, each apparently rocky like Earth, and orbiting their stars in what is deemed the “habitable zone.” That’s the region in the Goldilocks position, not so close to the star that the planet gets baked and not so far away that water at the surface would probably 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 (but not our sun, which is a larger yellow dwarf). 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 probably would have stripped away the atmospheres of these planets.
“These planets don’t reside in a vacuum,” Cohen said. “They reside in a medium that has a continuous flow of particles, mostly protons, that are emitted by the star.”
This is what happened to Mars, he said. Long ago it had a protective magnetic field, as does Earth, and it held on to its atmosphere in the face of the solar wind. But Mars then lost its magnetic field, and solar wind stripped away the Martian atmosphere, he said.
This new research might alter the strategy of astronomers looking for truly Earth-like planets in habitable zones.
“Maybe we should not focus on M-dwarfs, even though those are so common,” Cohen said. “Maybe we should focus on the more sunlike stars.”