An international research group has, on the initiative of astrophysicist Michaël Gillon from Liege, observed the emission of a super-Earth for the first time, that is to say its own light. This news opens the way to a more in-depth study of planets which are relatively small.

Since 1995, exoplanets are continually being discovered. Thanks to the combined efforts of teams from across the globe and the powerful telescopes they use there are now several hundred of them. As technology has developed, these planets have been yielding up their secrets and more and more discoveries are being made. After the gas giants, the terrestrial planets have come out of the shadows. Today, we can affirm that the majority of stars present in our galaxy have their own planetary systems. Yet, despite this, the curiosity of researchers is far from being satisfied. Indeed, it is not merely a case of detecting an exoplanet in order to unravel all its secrets. It must then be characterized in detail which is an extremely arduous task with current technology.

Yet astrophysicists are making progress step by step and are refining their methods. A recent discovery is now opening the way for a new era, that of the characterization of exoplanets that are comparable to Earth. On the initiative of Michaël Gillon, an astrophysicist and research associate at the F.R.S-FNRS (astrophysics and image processing laboratory of the University of Liège), an international team of researchers has, for the first time, observed the emission and luminous flux of 55 Cancri e, a Super-Earth much bigger than our planet but which has many points in common with it (1).

55 Cancri e

Thanks to previous observations of the radial velocity of the star and the transit of the planet, it has been established that 55 Cancri e has a radius two times greater than the Earth and a mass that is eight times greater. This allowed astrophysicists to get an approximate idea of its density and therefore its most probable composition. Michaël Gillon explains, “55 Cancri e is an object that is composed mainly of rock, with some more volatile components. Hypothetically, we are inclined to think that there is an external layer of water ice in the supercritical state, given the very high temperature on the surface and the immense pressure due to gravity. We could find other volatile components there such as methane, for example. The only place where water could be found in the gaseous state is on the surface, forming a secondary atmosphere surrounding the planet. The mix between rock and ice would suggest an object resembling what we imagine the cores of Neptune and Uranus to be, without their gas envelope composed mainly of hydrogen and helium.”

This does not mean that this layer of ice is white and comparable to what we have in our freezers. In reality, the surface of the planet is very dark, in comparison with its ability to store the heat from the radiation of its star. This particularity may be due to the composition of the vapor, to the presence of certain gases or otherwise to the very high temperature which alters the composition of the atmosphere.

(1) Demory B.-O., Gillon M., Seager S., Benneke B., Deming D. & Jackson B., Detection of Thermal Emission from a Super-Earth, The Astrophysical Journal Letters, 2012.