In this system with 7 exoplanets, at least 3 could shelter liquid water on their surface.
NASA revealed the discovery by the Spitzer space telescope of 7 exoplanets of mass comparable to Earth, orbiting the star Trappist-1. Three of them would be in the habitable zone of the star (the Goldylocks zone).
In May 2016, the Belgian telescope TRAPPIST (for „TRansiting Planets and PlanestIsimals Small Telescope“) managed by the University of Liège and based at the Observatory of La Silla in Chile, found a rare pearl. A solar system relatively close to ours (about forty light-years) consisting of an ultra-cold dwarf star (whose temperature is less than 2500K, ie 2200 ° C).
This type of star is usually neglected as part of the search for exoplanets because of the very little light and therefore low visibility.
The „transits method“ for detecting exoplanets
The observation (infrared) of the light emitted by this star (called Trappist-1) via ground-based telescopes made it possible to detect variations in luminosity characteristic of the passage of several celestial bodies between the star and the observer. This detection technique, known as the „transits method“, has been used to ascertain the presence of three planets (named b, c and d), all of a size comparable to that of the Earth, orbiting close to this tiny star . The first two turn so close to the star that they make the turn in respectively 1.51 and 2.42 Earth days.
„But the last one, the exoplanet D, was a problem for us, we could not figure out exactly what it was,“ says Martin Turbet, a researcher at the Dynamic Meteorology Laboratory and Ph.D. student at the Pierre et Marie Curie University, Paris. And for good reason, it was not a single exoplanet but five. „An important discovery around which the US Space Agency carefully maintained the suspense until Wednesday, February 22, 2017.
A system of exoplanets at 40 light-years from Earth
The news of the system discovery of planets gravitating around Trappist-1, a tiny star barely larger than the planet Jupiter, was already a major discovery in May 2016. First, because the planets in question are rocky, an element indispensable to the emergence of life as we know it on Earth), but above all because the plane of their orbit is ideally oriented to allow observing their passage in front of their star (hence the „method of transits „Described above).
„And the fact that these exoplanets gravitate around a dwarf star is a valuable asset,“
says Martin Turbet, a researcher, co-signing an article describing the discovery.
Certainly, this system of exoplanets that orbit Trappist-1 is not the closest to our Earth. It is 40 light years away from us. A distance 10 times greater than the one separating us from Proxima b, the nearest exoplanet identified to date, gravitating to 4.24 light years away from us, around the star Proxima of the Centaur.
„But the orbit of Proxima b does not allow us to observe the exoplanet’s passage in front of its star, so it will be much more difficult to gather information about the composition of its possible atmosphere,“ continues Martin Turbet.
On the other hand, although it is a little farther away (but not as much as it is on an astronomical scale) the planetary system around the star Trappist-1 promises a real harvest of data in comparative planetology.
A system that could hold … Between Mercury and the Sun!
„It is necessary to imagine a system with 7 planets with approximately the diameter of the Earth (to within 15%) and orbiting very close to their small star, and this solar system could hold between our sun and Mercury, the planet with the closest orbit to our Sun „describes the researcher. They are in fact 20 to 90 times closer to their star than the Earth is to the Sun.
„And according to these first measurements, it would seem that, although these exoplanets all have a diameter comparable to that of the Earth, the majority of them is less dense than our planet,“ explained the astronomer.
In order to explain such a low density, we may suppose the presence of volatile compounds and perhaps a great deal of water on their surface or in the interior.
One of the scenarios imagined by the researchers is that these planets formed far enough from their star, trapping a large amount of ice crystals during their accretion, before migrating to their current position near their star.
At least three of the planets (e, f and g) have insolations compatible with the existence of surface liquid water for a wide range of atmospheric compositions.