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Mapping a super-Earth

Exoplanetology never ceases to surprise and is constantly progressing. Recently, a team of researchers from several institutes, including the University of Liege, established a longitudinal thermal brightness map of 55 Cancri e, a “super-Earth”, which is approximately forty light-years away from Earth. The challenge was to measure the evolution of the brightness of the planet during its entire orbit around the host star. This data already exists for some gas giants, but is totally new for smaller planets. The data were obtained by photometric analysis of images taken by Spitzer, one of NASA’s space telescopes. These images made it possible to observe the hottest and coldest regions of the planet and to provide an estimate of some of its surface properties. The discovery marks the beginning of a new direction of research within the science of astrophysics: the study of the geological and atmospheric dynamics of small-sized exoplanets. 

Since it began in 1995 with the discovery of a first planet outside the solar system, exoplanetology has led to the compilation of a list of thousands of planets orbiting hundreds of stars. Today, it is estimated that there are tens of billions of them in our Galaxy alone. Some observational methods already allow us to obtain quite a quite detailed, though somewhat inert, picture of these planets. Measuring the radial velocities of their host star, for example, can help to determine their mass, and photometric observation of their transit reveals information about their size. The combination of the two methods reveals their density, thus providing a crucial clue as to their geological composition (gas, rock, metal and ice), and eventually, about the possible presence of an atmosphere. These methods of detection remain indirect. The vast majority of these exoplanets are too small and distant to be directly observable. In short, it is the behavior of their star or variations in the brightness of the entire system that reveals their presence. These constraints mean that it is not easy to get a clear picture of any activity that may exist on these planets.

These rather static portraits do not do justice to these planets. Just like the planets in our solar system, as they should be subject to significant physical and geological dynamics. The study of these planets marks a new direction in exoplanetology which has been enthusiastically initiated by Michaël Gillon, a research associate at the FNRS in the Origins in Cosmology and Astrophysics Laboratory of the University of Liege. In collaboration with the University of Cambridge and other institutes, he has just participated in a pioneering study that has been published in the journal Nature (1). The study involved the establishment of a longitudinal map of temperature differences across the entire surface of 55 Cancri e, a super-Earth which is forty light years away from Earth and which orbits its star in only eighteen hours (See: “A super-Earth is highlighted” and “55 Cancri e: enormous variations in temperature!”). 

Brightness 55Cancrie

These data were obtained by photometric analysis of several complete orbits by the planet around its star. Such ground-breaking detail concerning a small planet also enables us to learn more about the possible presence of an atmosphere. However, as is always the case with any promising new discovery, a cautious approach is advised. Michaël Gillon explains, “The thermal emission and its distribution between the two hemispheres of an exoplanet had already been measured for gas giants but these planets are essentially big balls of gas, the complete opposite of smaller planets. The latter can present a large range of possibilities in terms of their composition and surface conditions. For example, we do not know if 55 Cancri e is composed of rock or ice, if it is a truly telluric planet or the residual core of a giant planet, if it has always been close to its star or if it formed further away before eventually migrating towards its star. There are many possible scenarios, both in terms of its history and its composition”.  Of course, researchers have their own ideas on the subject. But given that this is only a first study, there is no available statistical knowledge to test their theories. Michaël Gilloncontinues, “In addition, there are no super-Earths in our solar system. It only has giant planets which are much bigger, and telluric planets which are much smaller. Unfortunately, we cannot therefore compare 55 Cancri e to a solar specimen that it is possible to study in situ”. 

(1) Brice-Olivier Demory, Michael Gillon, Julien de Wit, Nikku Madhusudhan, Emeline Bolmont, Kevin Heng, Tiffany Kataria, Nikole Lewis,Renyu Hu, Jessica Krick, Vlada Stamenković, Björn Benneke, Stephen Kane & Didier Queloz, A map of the extreme day-night temperature gradient of a super-Earth exoplanet. Nature, DOI: 10.1038/nature17169

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