A volcanic super-Earth

With a radius twice the size of that of the Earth and a mass which is eight times greater, 55 Cancri e therefore remains a small planet, a lot less easy to detect than gas giants which are sometimes even bigger than Jupiter. Nonetheless, it is an ideal candidate for observation. Its star, which is very bright and almost a neighbor, is visible to the naked eye. Its signals are very easy to detect by telescope. The short orbit of the planet means that its transit can be observed at very frequent intervals. Finally, its proximity to its star means that it is a very hot object which therefore emits sufficient amounts of light for it to be detectable. Recently, large variations in temperature were also detected on its surface, leading the researchers to an exciting hypothesis. “This system contains other planets further out”, says Michaël Gillon. “They influence the orbit of 55 Cancri e, which is not perfectly circular but slightly elliptical. “Because it is very close to its star, the elliptical nature of the orbit generates intense tidal effects which result in a constant transfer of energy to its core by internal friction. This significant “tidal heating” could be the cause of colossal volcanic activity on its surface”.  

This plausible theory is not without basis. “When I said that 55 Cancri e had no counterpart in our solar system, this is true with regard to our planets”, clarifies the astronomer.  On the other hand, in terms of its structure, 55 Cancri e and its star show a certain similarity to Jupiter and Io, its closest satellite. Io’s orbit, influenced by Europe and Ganymede, two moons that are further away, is also elliptical. “This situation creates volcanic activity and intense tidal effects on Io”. Michaël Gillon is delighted with this somewhat encouraging analogy between the two structures. “We could only study this kind of phenomenon in our solar system. Today, we are on the brink of extending this study to other systems!"

An opportunity to be seized

The proximity of 55 Cancri e to its star traps it into what is called a “synchronous rotation”. That is to say, its period of revolution is identical to its orbital period so that it always presents the same side to its star, just as the Moon does to the Earth. The planet therefore has a side where it is always daytime and a side where it is always night. “This synchronous rotation presents an opportunity because it makes it possible to unambiguously determine the efficiency of the thermal energy distribution between the day and night sides of the planet which depends directly on the presence of an atmosphere and the dynamic of this atmosphere. If, as is the case on Earth, the two hemispheres were successively exposed to the star, the temperature around the globe would be a lot more uniform and it would therefore be more difficult to draw any conclusions”.   

For a short fraction of time during each of its orbits, the planet passes behind the star. It is therefore totally occulted and its contribution to the flux of the system disappears. Its emission, more precisely the emission of its day side, can therefore be measured in a negative way. “The phenomenon of synchronous rotation makes it possible to observe the planet from different angles during its entire orbit. If its thermal emission varies on its surface in terms of longitude, we will therefore record a modulation, a variation that belongs to the planet itself. By combining the two measurements, occultation and the orbital modulation curve, we can draw a longitudinal map of the thermal emission on the surface of the planet: The occultation gives us an absolute value for the thermal emission of the day side, and the orbital modulation shows us how this value varies according to the longitude”.