Planet in the Sky With Diamonds

Carbon-rich world discovered 1,200 light-years away.

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By Marissa Cevallos, Science News

Planets studded with diamond mountains and filled with glass cores may be more than a psychedelic dream. A Jupiter-sized planet orbiting another star has a mostly carbon atmosphere, astronomers report online December 8 in Nature. The observation suggests that the rest of the planet may be mostly carbon, too.

The planet, WASP-12b, has an atmosphere containing at least twice as much carbon as scientists expected, an international team of researchers reports. This surprise invites new theories for how planets form, says astrophysicist Nikku Madhusudhan, a Princeton University astrophysicist who participated in the research while at MIT.

WASP-12b and its mother star, WASP-12, are 1,200 light-years from Earth. The planet is the size of Jupiter but at 2,500 kelvins, much hotter. Its orbit is only about one-fiftieth as far from its star as the Earth’s distance from the sun.

Using the Spitzer Space Telescope, Madhusudhan and colleagues recorded the infrared light from WASP-12b as the planet sneaks around its star. When the planet is in front of the star, the telescope records light from both the star and the planet’s atmosphere. When the planet is fully hidden behind the star, only the star’s light is recorded. Subtracting one measure from the other gives the light coming from just the planet.

Madhusudhan and colleagues compared their results to data simulated for millions of computer-generated planets with different molecules and temperatures in the atmosphere. The team found that a hot planet with at least as much carbon as oxygen fit the data best. In WASP-12b’s atmosphere, the carbon is found in the form of carbon monoxide, methane and carbon dioxide.

The existence of carbon planets has been proposed in the past, though how such a planet would form is still a mystery. Planets and their stars are thought to form from the same cloud of dust and gas at the same time, so astronomers have assumed in models that the atmospheres of giant planets would have compositions similar to the stars nearby. But WASP-12b has at least twice as much carbon in its atmosphere than the star it orbits—a mystery models can’t explain.

On lightweight planets like Earth, gases like hydrogen and helium escape the planet’s gravity, so the atmospheric chemistry doesn’t reflect the star’s composition. While such rocky planets are forming, if there is less available carbon than oxygen, the planet will be dominated by silicates, like the Earth.

But once carbon and oxygen are roughly equal, the rules of planet formation change.

“When the relative amount of carbon gets that high, it’s as though you flip a switch, and everything changes.   Everything would be different — like imagine, one day you’re a Yankees fan, the next day, Red Sox,” says Marc J. Kuchner of NASA’s Goddard Space Flight Center in Greenbelt, Md.

Though a giant carbon planet would be predominantly gas, a rocky, carbon-based body the size of Neptune—about 17 Earth masses—would probably have landscapes of diamond and graphite, says Kuchner.

Life on a carbon planet would look different too. Humans eat carbon and breathe oxygen on Earth, but that would be switched on a carbon planet, says Kuchner.

“You would probably eat rust and combine it with the air on this strange planet, which might be mostly carbon compounds,” says Kuchner.

More carbon planets probably exist, says Kuchner, and Jupiter may be one of them. Its carbon to oxygen ratio has been difficult to measure. When NASA’s Galileo probe visited Jupiter in the 1990s, it measured the atmosphere’s carbon-to-oxygen ratio as greater than one.

“Nobody believed it,” says Kuchner.

No model can yet explain how carbon planets form. They may be born in planetary disks with relatively high amounts of carbon, says Kuchner. The disk surrounding the star Beta Pictoris, 63 light-years away, for example, has 18 times as much carbon as the sun.

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