A perfect target for this kind of measurement
Michaël Gillon continues, “More than seventy Super-Earths are known for the moment. The new generation instruments such as the NASA’s Kepler satellite make it possible to constantly find new ones. But most of them gravitate around weak stars that are very far away and invisible to the naked eye. In order to measure the emissions from such planets which are so small with regard to other observable stars, it would require gigantic and extremely accurate telescopes. We have not yet reached that stage”.
In the case of the present planet, everything fell into place to make it the ideal candidate for extra observations within the available technological limits. Not only is the star not too far from Earth in order to be easily detectable, but it is also very hot and very bright. The very close presence of 55 Cancri e in relation to its star implies that it is highly radiated which influences its actual temperature and which makes it a « hot » planet. By way of comparison, while the Earth has a temperature of 300 kelvins (K), and Jupiter of 100 kelvins, the exoplanet studied exceeds 2000 kelvins, as the measurement carried out by Spitzer shows. This temperature is higher than the melting point of most metals!
Finally, one last characteristic of the planet facilitated observation of its emission: its albedo. The albedo is a unit understood to be between 0 and 1 which measures the sunlight reflected back into space by a planet. The brighter the planet, like a block of ice for example, the more light it reflects and the less heat it keeps. It will have an albedo of around 1. By way of example, the houses with white facades in Greece insulate against heat by reflecting light. Conversely, the darker the planet, the more it will absorb light and heat, its albedo will be around 0.
As the temperature of 55 Cancri e is very high, this suggests that the reflectivity of the planet is weak. 55 Cancri e emits a very high heat and therefore suggests that the reflectivity of the planet is weak. “If the surface was very reflective, practically all the light from the star would be sent back into space and the temperature would not be as high. Our measurement shows that the planet is very hot which suggests therefore that it efficiently absorbs the light that comes from its star.” Its albedo is therefore very low, which very fortunately facilitated detection of its thermal emission.
Poor heat circulation
For a more precise study, it would be necessary to be able to measure the flux of the planet at many other wavelengths, which would make it possible to deduct the emission spectrum from it. This remains technologically impossible for the moment. A larger spectrum would make it possible to surmount other physical conditions such as variation in temperature according to altitude and composition of the atmosphere if there is one. “It is a first step in the detailed study of this Super-Earth, what we can already suppose from our measurement, is that there is a very weak redistribution of heat on this planet”, explains a delighted Michaël Gillon.
