Astrophysicists from the University of Liege using the TRAPPIST telescope have discovered a planetary system formed of three Earth-sized planets around an « ultracool red dwarf star » slightly bigger than Jupiter in size. While these planets are close to their host star, they are not exposed to very high temperatures and could harbor areas on their surfaces that are suitable for life. The new planetary system is only 40 light years away from Earth. This proximity should allow the future generation of large telescopes to study the atmosphere of these planets with precision. As of today, these planets are the most promising candidates for having liquid water or even life outside the solar system. Even though the news has just been published in the journal Nature, it is probably a safe bet that teams of astrophysicists over the world will be tripping over each other to study this planetary system. In the meantime, the team that made the surprising discovery only has eyes for "its" small nearby star...

The news is incredible. Michaël Gillon and Emmanuël Jehin, both of whom are FNRS research associates at the Origins in Cosmology and Astrophysics Laboratory of the University of Liege make no effort to hide their joy and amazement at the discovery. They lead the TRAPPIST project which has just discovered a new system which is composed of three Earth-sized planets (1). Better still, these planets could very likely be habitable and are currently the most likely candidates for the detection of life outside our solar system that we know of.

This discovery is great news for the astronomers from Liege because it augurs well for their new SPECULOOS project which will be officially launched in 2017. Financed for the most part by Europe, this unique experiment led by Michaël Gillon, will pursue an already promising intuition despite the fact that it goes against current beliefs, that ultracool red dwarf stars, which are a lot smaller, colder and frequent than sun-like stars, could harbor rich, complex and numerous planetary systems. SPECULOOS consists of four robotic telescopes with a diameter of 1 meter that will observe around 500 of these small cold stars located in the neighborhood of the sun, that is to say, less than 100 light years away.  In the preliminary phase of this mission, the team of astronomers has been observing fifty target stars since 2011 using the University of Liege’s TRAPPIST telescope which is based in Chile (see article Astrophysicists from Liege in seventh heaven). This preparation should provide a statistical basis to determine whether this idea has any chance of success. After 5 years, which is a very rapid result given the small number of stars studied, three planets in orbit around their sun have just been detected at a distance of only 40 light years from Earth in the constellation Aquarius. Their name of the host star is 2MASS J23062928-0502285. Following the discovery, it is now also officially named TRAPPIST-1.

Planetary systems that are still unknown

It may seem astonishing that the planetary systems of ultracool red dwarf stars had not been studied earlier, especially given the fact that these stars are very frequent in the Galaxy, much more so than sun-like stars. In fact, most of the new stars that form in the universe are small in size, much smaller than the sun. Moreover, the lifespan of stars increases the smaller they are. Stars produce their own energy by nuclear fusion in their core. Smaller stars are subjected to weaker internal temperatures and pressures, and for this reason, they consume their fuel a lot less quickly. By way of comparison, the sun will last 10 billion years while an ultracool red dwarf can live more than 10 times longer. What is also very interesting is the fact that these stars are less luminous and their habitable zone is therefore much closer to their surface. A planet could complete an orbit in just a few days while offering mild temperatures. A greater orbital frequency drastically increases the chance of detecting a planet because it will have a lot more transits. And transits by planets orbiting around these small stars will be easier to detect given that a greater fraction of the stellar disc will be obscured when the planet passes between it and the Earth.

(1) M. Gillon, E. Jehin, et al. Temperate Earth-sized planets transiting a nearby ultracool dwarf star, Nature, May 2016