Eight 9-hour observations distributed over a period of one month made it possible to record these variations in emission and to verify that they were repeated in accordance with the orbital phases of the planet. The astrophysicists were therefore able to assemble the data and establish the longitudinal map of the thermal emissions of the planet. The map reveals an enormous temperature gradient between the day and night sides:  The day side has a temperature rising as high as 2700 Kelvin as opposed to 1300 Kelvin for the night side”. “Such a temperature gradient indicates an inefficient circulation of heat which corresponds to a rocky planet without an atmosphere. In fact, it is essentially the atmosphere which distributes heat on the surface of a planet by means of dynamic phenomena (winds). Without an atmosphere, the temperature gradient between the day and night sides will be very significant”. This is true, and yet…

The hotspot paradox

In theory, without an atmosphere, the hottest part of the planet should be the point nearest the star (the substellar point). Therefore, on the graph showing variations in the light of the planet (see figure above), the peak of brightness should be at the point nearest the occultation, given that these are the moments when the telescope captures the largest part of the day side. However, the brightness curve reveals a peak that has shifted, occurring at a time well before the occultation. “The hottest point on the planet is therefore not at a point exactly below the star, says Michaël Gillon, but has shifted some thirty degrees towards the East. “This observation would be easy to understand in the presence of an atmosphere, which, thanks to strong winds blowing towards the East would enable a transfer of heat to take place leading to the hot point being located at a point other than the substellar point. And yet, in the case of 55 Cancri e, this observation is difficult to reconcile with the significant thermal gradient between the two hemispheres”.  

A theory that needs to be proved seems to evade this paradox. It is to be remembered that the very close proximity of the planet to its star coupled with the influence of other planets in the system means it has an elliptical orbit. This generates considerable tidal effects. “The temperature during the day is higher than that at which rocks melt. We can imagine that these rocks form “oceans” of magma. Under the tidal effects, there could be an “oceanic” transfer, a flow of magma towards the East causing a delay between the peak in irradiation and the peak in heat”.  

A second theory involves the presence of a very particular atmosphere which conceals unusual phenomena. The astrophysicist continues, “But, on this subject, we are really still in the realm of speculation. One thing is certain, a hydrogen-based atmosphere, such as those observed around gas giants is highly unlikely. This element is too light and would quickly be blown out of the gravitational field of the planet, or attracted by the star’s magnetic field. We could imagine a secondary atmosphere, composed of heavier elements resulting in a constant degassing on the surface due to the massive irradiation of the planet. It would be constantly destroyed and renewed. If 55 Cancri e is essentially composed of ice, this atmosphere should be rich in oxygen and carbon monoxide. If it is essentially rocky, which we believe today to be the case, the secondary atmosphere could be rich in silicates and metals”.