Diamond planets are forever

C&I Issue 11, 2012

US and French astronomers think they have discovered the first ever carbon-rich, diamond-encrusted planet. Known as 55 Cancri e, it is one of five planets that orbit the star 55 Cancri, which is around 40 light-years from Earth and can be seen with the naked eye in the constellation of Cancer.

By recording its passage in front of the star, astronomers have been able to determine that 55 Cancri e has around twice the diameter of Earth and roughly eight times its mass. This means that the planet can’t have the same composition as Earth, in which an iron core is surrounded by a silicate-based rocky mantle, as the mass is too low for the size of the planet.

Astronomers have suggested that the planet could possess a rocky interior if it is coated with a thick layer of water, which would be in a supercritical state at the high, 2000°C, temperatures expected at the planet’s surface. Now, a team led by Nikku Madhusudhan at Yale University, US, has suggested that the mass and diameter measurements would also make sense if 55 Cancri e consists mainly of carbon (Astrophysical Journal Letters, http://arxiv.org/abs/1210.2720).

Spectroscopy measurements support this idea by indicating that the planet’s host star is carbon-rich. To find out what kind of planets might have formed around such a carbon-rich star, the team combined structural models derived from the mass and diameter measurements. These revealed that 55 Cancri e could possess an iron core surrounded by a mantle of silicon carbide and covered with a thick crust of graphite and diamond that may account for around a third of its mass.

This kind of structure implies that the plate tectonics seen on Earth would not take place on 55 Cancri e, because of both the thick carbon crust and the lack of water. ‘Both these factors together may potentially inhibit plate tectonics on this planet, unlike oxygen-rich planets for which previous studies have suggested that plate tectonics is almost inevitable,’ Madhusudhan said.

He hopes to confirm this diamond planet status by conducting detailed observations of the star’s chemical composition and the planet’s atmosphere.

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