Indeed a planet that is so close to its star is trapped in a state of resonance. That is to say that it turns on itself at the same speed as its orbital period and thus always shows the same face to the sun, exactly like the moon in relation to the Earth. “Which necessarily means that there is a side which is constantly irradiated and a hemisphere in perpetual darkness, between these two hemispheres, there is necessarily a great difference in temperature according to the efficiency of the heat transfer from the bright side to the dark side? The emission that we are measuring corresponds to the bright side, and its higher value favors a great difference in temperature from the dark side and therefore an inefficient heat transfer between the two hemispheres. Indeed, if this transfer was efficient, the dark side would not retain so much heat and its emission would be weaker”, the researcher points out.

Pioneer in the study of terrestrial planets

The luminous emission had already been measured on much bigger planets of the size of Neptune or Jupiter. But this is the first time that this measurement has been made possible for a Super-Earth. “In order to study planets that are so far away and even smaller, we must wait for the activation of the JWST spatial telescope, the replacement for Hubble. Its diameter will be 6.5 meters wide while Spitzer is only 85 centimeters. JWST, which will not be launched into space before 2018 at the earliest, promises to provide much more precise measurements and covering a much larger spectrum of wavelengths. This would make a more detailed study of the planets like 55 Cancri e, and also the study of smaller, colder and more distant planets from Earth. This would be a veritable revolution, just as Spitzer initiated a revolution in its time in the study of planets in transit”. This is a study which even today remains one of the most efficient ways to discover new planets. “Of course, there are direct imagery techniques which succeed in separating the star and the planet in the same image. Therefore in these cases there is a possibility of detecting the emission without needing to observe the transit of the planet. But these are techniques which are slowly being perfected and which currently can only study giant, very hot and massive planets relatively close to us, orbiting very far from their stars. These technologies would not, for example, have made possible our discovery as the planet is so small and so close to its star to be able to clearly distinguish the two fluxes on the same image”, exclaims a delighted Michel Gillon.

Nevertheless, by studying 55 Cancri e, the astrophysicist’s team opens the way to a new area of astrobiology, that is the study of small terrestrial planets. “This result is stimulating and encourages us to redouble our efforts. On one hand, we must of course continue to push out the boundaries of our instruments to make advances in the study of small exoplanets. Yet the fact that this first detection of the emission of a Super-Earth concerns a planet in orbit around a near star is not by chance: the brighter the host star the more precise our observations can be. It is therefore necessary to concentrate our research on transiting planets around stars that are nearest the solar system. This is the only way we will find planets similar to Earth that we could really study in detail with our future instruments. In this context, we are campaigning for strong participation by Belgium in the CHEOPS project which is managed by Switzerland and which aims to put a super-precise spatial telescope in orbit devoted to the search for terrestrial planets in orbit around stars neighboring the sun”.