Detecting the 55 Cancri e emission

In the present case, the planet is so close to its star that it is impossible to obtain an image of it with current technology. Indeed, its light is drowned out by that of its star. Nevertheless, the fact that it is hidden by its star once per orbit makes the indirect detection of its light emission possible, by precise measurement of the combined flux of the planet and the star (photometry). Indeed, by following the photometric development of the system, we can in theory measure a drop in the flux recorded by the detector when the planet is hidden by the star, that is to say, when its emission is occulted. This drop in the flux corresponds to the contribution of the planet, and its detection therefore gives access to measurement of the light emission of the planet .

“The expected contrast between the planet and the star is miniscule in the visible spectrum, in the order of one millionth, which is not detectable by our current instruments. This is why we conducted our measurements in infrared, where the contrast is much more favorable, in the order of one hundredth of a per cent. More concretely, we carried out our measurement at 4.5 microns (light visible to the human eye covering a range of 0.4 to 0.7 microns, Editor’s note.) At that wavelength, the emission from the planet is dominated by its own thermal emission and not by the starlight it reflects”.

This measurement remains a feat of exceptional precision which would have required observation of four occultations of the planet by the most efficient infrared spatial telescope. The weakness of the signal detected illustrates well the fact that such a measurement would be impossible for the moment on a small planet which does not show conditions as favorable as those encountered in the present case. “Imagine that we have to measure the flux variation on a smaller and colder planet, when a warm planet only influences the overall flux by 1/10 000th. For example, for a twin planet of the Earth, the expected signal would be ten times weaker at least”.