Searing ‘super-Earth’ 55 Cancri E may be a diamond planet

When astronomers first discovered 55 Cancri E back in 2004, they found a massive 'super-Earth' planet, twice the diameter of Earth and over eight times Earth's mass. It orbits so close to its Sun-like star, 55 Cancri A — around 25 times closer than Mercury is to our Sun — that a year there would be only 18 Earth-hours long, and its surface temperature would be a nearly 2000 degrees C.

As of May of this year, it was believed that the planet had a rocky granite core covered by a layer of 'supercritical' water — where, due to pressure and temperature, it exists in both liquid and gas phase at the same time — and surrounded by an atmosphere of steam. However, a new paper titled A Possible Carbon-rich Interior in Super-Earth 55 Cancri E, shows that the planet likely has no water at all, and is composed of something very different than what was originally thought.

Artist's concept of the planet called 55 Cancri E (NASA/JPL-Caltech)

"This is our first glimpse of a rocky world with a fundamentally different chemistry from Earth," said Dr. Nikku Madhusudhan, a postdoc researcher in Yale University's Department of Astronomy, who is the lead author of the paper. "The surface of this planet is likely covered in graphite and diamond rather than water and granite."

Normally, being able to see the light directly from a planet orbiting another star isn't possible. The planets are just too small compared to their star(s), and any light reflected by them gets 'washed out'. However, in 2005, astronomers used NASA's Spitzer Space Telescope to watch as two massive gas-giant planets slipped behind their respective stars — known as 'occultation' — and observe the difference in the amount of light they received. Comparing the light received before and after occultation with the light during occultation, astronomers were able to separate out exactly what light came specifically from being reflected by the planet. Examining the spectrum of that light gave them information about the planets' temperatures and even details about what is in their atmospheres.

For 55 Cancri E, a planet that is much smaller than a gas giant, it is next to impossible for us to see the light from it, even during occultation. The dip in the amount of light is just too small that it gets lost in the normal variations of the light from the star. However, 55 Cancri E happens to glow very brightly in the infrared spectrum. This made it much easier to use the same occultation method that works for much larger planets. As the Spitzer telescope watched 55 Cancri E pass behind its star, there was a dip in the amount of infrared light detected. The researchers then separated out the spectrum of light that was specifically from the planet, and they were able to use this to help figure out the composition of the planet.

Knowing the planet's mass and diameter from previous observations, Madhusudhan and his team ran through a series of different internal compositions for the planet, trying to figure out the right combination to account for all the information that has been gathered on the planet. Unlike Earth, which has a solid iron core, surrounded by the liquid iron outer core, the molten mantle, and the Earth's crust, the best fit they found for 55 Cancri E was to have a core of molten iron, surrounded by successive layers of silicon-based minerals, diamond, and a thin crust of graphite. They estimate that roughly one-third of the planet's mass — nearly three times the mass of Earth — is diamond.

From here, further observations of 55 Cancri E and its star should help confirm these findings, and it has not only opened the door to the possibility of finding 'diamond planets' around other stars, but also to a wider range of planetary compositions in general. Given the variety we have already found with planets and planetary systems in the stars that surround us, chances are we can look forward to many more amazing new discoveries yet to come.

The study was written by Madhusudhan, Kanani Lee from Yale University and Olivier Mousis from the Institut de Recherche en Astrophysique et Planetologie in France.