TRAPPIST-1 is five years old

Five years to the day after the discovery of the fascinating TRAPPIST-1 exoplanetary system, Michaël Gillon - Senior Research Associate at the ULiege, who was responsible for the discovery of the system - and other colleagues involved in scientific missions that are interested in the star and the seven planets of the system, take stock of what we know and what we are still waiting for.

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ust five years ago, the discovery made headlines around the world when it was announced: astronomers had discovered a unique exoplanetary system of seven rocky planets, all the size of Earth, locked in tight orbits around a red dwarf star called TRAPPIST-1 (as the first of these planets were discovered by the ULiege TRAPPIST-South telescope in Chile). Although these tantalizing planets have since yielded some of their secrets, they remain enigmatic. The question that continues to be asked is: Are any of these worlds habitable? And if so, do they harbor life?

"I don't think we know that at all," says Sean Carey, head of the Exoplanet' Science Institute at NASA's Jet Propulsion Laboratory, who processed data from the now-retired Spitzer space telescope at the time of the discovery. "That people are even able to ask the question of whether a planet around another star is habitable - that boggles my mind. But on the fifth anniversary of the discovery, scientists may be on the verge of an answer. The long list of tasks for the James Webb Space Telescope (JWST), which was recently launched on 25 December 2021 - far more powerful than any previous space telescope - includes searching for signs of an atmosphere on these seven extrasolar worlds.

A prime target is the fourth planet, TRAPPIST-1 e, which lies right in the middle of what scientists call the 'habitable zone', an area neither too close nor too far from the star where the temperature is high enough to theoretically allow liquid water on the surface of an orbiting planet. Although the planets in the TRAPPIST-1 system are very close to their star, the star is a 'red dwarf', meaning that it is not only much cooler than our Sun, but also much smaller - only slightly larger than Jupiter. By comparison, if the entire TRAPPIST-1 system was placed in our own solar system, it would fit into the orbit of our innermost planet, Mercury.

The habitable zone is only a first step. Such a planet would also need a suitable atmosphere, and JWST, especially in its early observations, will probably only be able to obtain a partial indication of the presence of an atmosphere. But the discovery of an atmosphere on a TRAPPIST-1 planet, however modest, would perhaps be comparable to the discovery of the system itself five years ago. "What is at stake here is the first characterization of the atmospheres of potentially habitable Earth-sized exoplanets", says Michaël Gillon, astronomer and FNRS Senior Research Associate at the Astrobiology Research Unit (Faculty of Science) of the University of Liège and lead author of the study which, on 22 February 2017, revealed the seven sister planets orbiting a star 40 light-years away from Earth.

In this context, measurements already made by the Hubble Space Telescope have shown great promise. They have revealed that none of the seven planets in the system has an extended hydrogen-rich atmosphere that would make it unsuitable for hosting habitable conditions on its surface. "This is great, but now we need a space telescope much more powerful than Hubble, one that can detect dense, compact atmospheres like Earth's around these planets and reveal their composition. That telescope is James Webb! "says Elsa Ducrot, a researcher at the CEA (Commissariat à l'Énergie Atomique et aux Energies Renouvelables) in Paris, who is heavily involved in the study of the system and did her thesis at ULiège.

The size of the TRAPPIST-1 planets could also help strengthen the case for habitability, although the research is far from conclusive. These planets are comparable to Earth not only in diameter but also in mass. Determining their masses was possible thanks to their clustering around TRAPPIST-1: squeezed shoulder to shoulder, they jostle each other, allowing scientists to measure their masses from their mutual gravitational perturbations. "We were able to deduce their sizes and masses very precisely," says a delighted Elsa Ducrot. "This means that we also know their densities very precisely, and they suggest compositions similar to that of the Earth.

For Michaël Gillon, the societal significance of TRAPPIST-1 is another positive aspect of the system's discovery. "We have seen TRAPPIST-1 included in some artistic works; in music, science fiction novels, comic books. It's really something that we've enjoyed a lot over the last five years. It's as if this system now has a life of its own in the collective imagination.