For the first time, a team of scientists of the Universities of Cambridge and Liège (Michaël Gillon, FNRS Research Associate at the Origins in Cosmology and Astrophysics Laboratory - ULg) have detected atmospheric variability on a rocky planet outside the solar system, and observed a nearly threefold change in temperature over a two year period (1). They have indeed measured an increase in the global temperature of substantially 300% between 2012 and 2013. Although the researchers are quick to point out that the cause of the variability is still under investigation, they believe that it could be due to massive amounts of volcanic activity on the surface. These results demonstrate for the first time that terrestrial planets orbiting stars other than the Sun can also have a significant geological activity and atmospheric variability. It's sort of the first "exo-planetary weather report" that has just been drawn.

55 Cancri e, Michaël Gillon, FNRS qualified researcher at the ORigins in  Cosmology and Astrophysics Laboratory (ULg) knows it well: in 2012, the Liège astrophysicist had co-signed an important article in The Astrophysical Journal Letters (2) which reported the first observation of the thermal emission of a super-Earth, in this case 55 Cancri e, exoplanet discovered in 2005 by ground-based telescopes. It orbits a Sun-like star (55 Cancri A) located 40 light years away in the constellation of Cancer. To be able to measure the thermal emission (brightness) of the exoplanet, Michaël Gillon and colleagues had previously used the Spitzer Space Telescope (see article A super-Earth highlighted). As the planet is very close to its star, it is impossible to obtain a direct image with current technologies. Indeed, its light is blended with the one of its star. Nevertheless, the fact that it is hidden by its star once per orbit, allows the indirect detection of its light emission, by the precise monitoring of the combined flux of the star and its planet.  One can indeed record a decline of the brightness of the system when the planet is hidden by the star, an event called "occultation". This drop of brightness corresponds to the contribution of the planet, and its detection therefore provides access to the measurement of the planet's light emission. However, as the contrast between the planet and the star is tiny in the visible spectrum, in the range of 1 ppm, which is undetectable by current instruments, scientists had made their measurement in the infrared, where the contrast is much more favorable, on the order of a hundredth of %. In this part of the electromagnetic spectrum, the emission of the planet is dominated by its own thermal emission and not by the starlight that it reflects.

Diamond?

As reported by Michaël Gillon, everything matched for 55 Cancri e to be the ideal candidate for additional observations. First, the star is not too far from our Earth and is therefore very bright. The very close presence of 55 Cancri e to its star means that it is heavily irradiated, which affects its effective temperature, and makes it a “hot” planet. By comparison, the earth has a temperature of 300 kelvins (K), and Jupiter 100 kelvins, while the exoplanet studied exceeds 2,000 kelvins, as shown in the measurement made by Spitzer. This temperature is above the melting point of most metals!


These features have indeed attracted many teams around the world that have focused their attention on the planet. A team of Yale University (3) even speculated, still in 2012, that in view of its temperature, and of the carbon-enrichment of its host star, it could be composed of graphite and diamonds! Hypothesis that seems not to be popular today ... The new study also shows that the radius of the planet is smaller than estimated earlier, and is consistent with an Earth-like composition, much less exotic.

(1) Demory B-O., Gillon M., Madhusudhan, Queloz D., Variability in the super-Earth 55 Cnc e, Monthly Notices of the Royal Astronomical Society, may 2015 http://arxiv.org/abs/1505.00269
(2) 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.
(3) Madhusudhan N. et al. A possible Carbon-rich Interior in Super-Earth 55 Cancri e, The Astrophysical Journal Letters, 2012.