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Diamond planet discovered 40 light years from Earth

Illustration of the interior of 55 Cancri e — an extremely hot planet with a surface of mostly graphite surrounding a thick layer of diamond, below which is a layer of silicon-based minerals and a molten iron core at the center.

Haven Giguere

New research suggests that a massive, star-scorched planet in the Cancer constellation, not far from our solar system, is made in large part of diamonds, opening new vistas on the way scientists understand how planets are created.

The planet, 55 Cancri e, is twice the size of Earth and so rich in carbon that it holds at least three times our planet's mass in diamonds, according to a new paper that has been accepted for publication in the journal Astrophysical Journal Letters.

On 55 Cancri e, the surface is covered in graphite, the dark stuff of pencils, with diamonds pushing through from layers underneath, said the paper's lead researcher, Nikku Madhusudhan, a fellow at Yale University's Center for Astronomy & Astrophysics.

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"We're talking about a rocky planet. Not much gas, almost entirely solid. The outermost layer is at an extremely high temperature ... There could also be a lot of diamonds on the surface because of the geophysical movements in the interior. The diamonds could come to the surface very easily," Dr. Madhusudhan said in an interview.

"But just below the surface there is a very thick layer, about a third of the whole radius [of the planet], just in diamonds."

That rocky, mind-boggling world broils under 2,150C heat because the planet is so close to its star.

"There's not even a climate left. It's rubbed off by the heat. It's a permanent barbecue," said co-author Olivier Mousis, a French astronomer at Université de Franche-Comté.

The planet is the closest of five that orbit around 55 Cancri A, a star located 40 light years from Earth, close enough that it can sometimes be seen with the naked eye.

"In astronomy terms, it's like one of our suburbs. It's closer to us than you and I are on the phone," said Dr. Mousis, who was speaking from his office in Besançon, near the French-Swiss border.

The planet was detected in 2004 and its radius was estimated through transiting, meaning that scientists deduced 55 Cancri e's size by measuring the decrease in luminosity when the orbiting planet transited in front of its star.

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At first, researchers assumed that the planet's composition was similar to the Earth, with iron, silicates and water.

However, Dr. Madhusudhan said, that model didn't properly fit because, to match 55 Cancri e's mass, an Earth-like model would required a massive layer of water that would have been in a supercritical state, neither liquid nor gas, because of the extreme conditions on the planet.

"That type of explanation wasn't adequate ... it was a vision that was too anthropocentric," Dr. Mousis said.

Their suspicions that another model should apply was further buttressed when spectrographic data showed that 55 Cancri e's star was also very rich in carbon.

Presuming that the star and its planets were created from the same original chemical cloud, the researchers tested a number of chemical combinations that could better explain the planet's mass.

The carbon-rich scenarios fit their calculations best, signalling a fundamental shift in the way faraway planets should be analyzed, Dr. Madhusudhan said.

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"When we are looking for planets in outer solar systems, we now have to be very open with regards to their chemistry. We cannot presume that they are Earth-like."

Baking so close to its star, 55 Cancri e could not host life but Dr. Madhusudhan said its existence raised questions about other high-carbon, low-water planets.

"We have to be open with regards what kind of biology could exist ... if we had the same planet at a cooler temperature, the life forms there could be very different from what we know. There wouldn't be enough water or oxygen for them to survive so they would have to adapt to something else."

The researchers are hoping to gain better insight on 55 Cancri e with more data from sources such as the Hubble telescope, NASA's Spitzer Space Telescope, which works in the infrared spectrum, and the Canada-France-Hawaii Telescope, which is run jointly with the National Research Council of Canada.

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