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Shine on, you crazy diamond

Artist's impression of 55 Cancri e

Artist’s impression of 55 Cancri e – graphite surrounding diamond, then silicon and a molten iron core. (Courtesy: Haven Giguere, Yale University)

By Tushna Commissariat

There’s nothing quite like a planet made mostly of diamond to get everybody’s attention, and that is what a team of astronomers from Yale says it might have found. The researchers say that 55 Cancri e, a rocky super-Earth, is mainly made of carbon – in the form of diamond and graphite.

This is not the first time a “diamond planet” has hit the headlines. Last year an international team of researchers found a pulsar, with an orbiting planet about the mass of Jupiter, that seemed to be made entirely of diamond. Further research revealed that “the planet” was, in fact, the pulsar’s companion star – an ultralow-mass carbon white dwarf that just about survived being completely destroyed by the pulsar. The core of the remnant would mostly be carbon and some oxygen, but thanks to the near-Jupiter mass of the companion star, its own gravity could crystallize it to form diamond – just how carbon is transformed into diamond deep within the Earth. You can take a look at the paper about that research here.

This time, astronomers seem much more certain that what they are dealing with is indeed a planet. 55 Cancri e belongs to the 55 Cancri star system, which is a mere hop, skip and jump away from Earth in astronomical terms at a distance of 41 light-years. Indeed, the system – with five known planets that orbit a parent star – can be seen with the naked eye on a clear, dark night. Interestingly, 55 Cancri e is the closest planet to its parent star, with a dizzying 18-hour orbit – the shortest orbit known for an exoplanet – and is tidally locked, so one side always faces the star. Until now, it was thought to have a substantial amount of super-heated water on its surface and was believed to have a similar chemical composition to Earth.

However, new data and research have shown that the planet contains no water at all, and appears to be composed primarily of carbon, iron, silicon carbide and, possibly, some silicates. The study, led by Yale postdoctoral researcher Nikku Madhusudhan and colleagues, estimates that at least a third of the planet’s mass – the equivalent of about three Earth masses – could be diamond.

“This is our first glimpse of a rocky world with a fundamentally different chemistry from Earth,” says Madhusudhan. “The surface of this planet is likely covered in graphite and diamond rather than water and granite.” In 2011 Madhusudhan revealed the first discovery of a carbon-rich atmosphere in a distant gas-giant planet, opening the possibility of long-theorized carbon-rich rocky planets or “diamond planets”.

The conformation of this carbon-rich super-Earth now means that the many rocky exoplanets thought to exist can no longer be assumed to have chemical constituents, interiors, atmospheres or biologies similar to those of Earth, according to Madhusudhan. A carbon-rich composition could influence the planet’s thermal evolution and plate tectonics, for example, with implications for volcanism, seismic activity and mountain formation.

This is the first time astronomers have identified a likely diamond planet around a Sun-like star and specified its chemical make-up. Further spectroscopic analysis of the planet’s atmosphere and its parent star’s composition will be necessary to ascertain 55 Cancri e’s “priceless” composition.

A paper on the work has been accepted for publication in the journal Astrophysical Journal Letters and an arXiv preprint is available here.

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