Press release

TRAPPIST-1 is gradually emerging

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Illustration showing the seven Earth-size planets of TRAPPIST-1. The image does not show the planets' orbits to scale, but highlights possibilities for how the surfaces of these intriguing worlds might look. ©NASA/JPL-Caltech

Intensive observations made by terrestrial telescopes (TRAPPIST and SPECULOOS) and space telescopes (Hubble and Spitzer) allow us to know a little more about TRAPPIST-1 and its seven planets. Michaël Gillon (ULiège - STAR Institute) and his collaborators publish four important papers (1,2,3,4) that refine our knowledge of the exoplanets and their stars. Mass, radii, first atmospheric constraints: all these new results confirm the terrestrial, rocky nature of the planets, while suggesting compositions significantly richer in water than that of our Earth. Researchers are now eagerly awaiting the launch of the James Webb Space Telescope, which will gather more information on the planet's best known exoplanetary system.


ne year ago, an international team of astronomers led by Michaël Gillon (STAR Institute) announced at a NASA press conference one of the most remarkable exoplanetary discovery : seven planets the size of the Earth had been detected around a cold dwarf star about 40 light-years away from our solar system, all sufficiently temperate to be "potentially habitable", i. e. capable - in theory - of harboring liquid water on their surface. The key to life as we know it on Earth! Today, four studies involving astronomers from the University of Liège present new and important constraints on the nature of the planets of TRAPPIST-1 and their potential habitability.

The first study, published in The Astrophysical Journal (1) and directed by Valérie Van Grootel (STAR Research Institute), focuses on TRAPPIST-1, the star of the system. Thanks to new observations, theoretical models, and analysis techniques, the team was able to refine the determination of the star's properties, making it possible to estimate more accurately its distance, temperature, radius and mass. The mass being about 10% higher than previously estimated." The better you know a star, the better you can know its planets. It is therefore an essential step towards a better understanding of the planets of TRAPPIST-1 and their potential habitability," explains the astronomer from Liège, first author of the publication.

The second study, published in Astronomy & Astrophysics (2) reveals that the seven planets of TRAPPIST-1 are globally rocky and richer - up to 5% - in water than the Earth. "It's much more water than our oceans, which make up only 0.02% of our planet's mass!" says Simon Grimm, a post-doctoral fellow at the University of Bern. This was made possible by the accurate measurement of planetary masses and dimensions, including a large dataset collected by NASA's Spitzer telescope.


This graph presents known properties of the seven TRAPPIST-1 exoplanets (labeled b thorugh h), showing how they stack up to the inner rocky worlds in our own solar system. The horizontal axis shows the level of illumination that each planet receives from its host star. The vertical axis shows the densities of the planets. Density, calculated based on a planet's mass and volume, is the first important step in understanding a planet’s composition.

The third study, conducted by Julien de Wit, a researcher at MIT (EAPS Lab) and published in Nature Astronomy (3), used NASA/ESA's Hubble Space Telescope to observe the four planets of the system most likely to be habitable, in order to detect atmospheric traces. For three of the planets, the data obtained exclude the presence of a hydrogen-rich atmosphere. With regard to the fourth, the presence of such an atmosphere is not yet ruled out.  The presence of extensive and hydrogen dominated atmospheres on the four planets would have presented them as gaseous and inhospitable worlds like Neptune. Which is not the case." Gathered together, the results of our researches confirm the terrestrial nature and potential habitability of the planets of TRAPPIST-1 system," says Julien de Wit, principal author of the study.

The fourth and final study, published in Monthly Notices of the Royal Astronomical Society (4), looked at new data collected by NASA's Spitzer Space Telescope that refined the radius of the star and the seven planets that make up the system." In this study, we also show that future studies of the planetary atmosphere will not be significantly affected by the activity of the star, which is encouraging for the future," enthuses Laetitia Delrez, first author of the paper and ULiège post-doctoral fellow at the University of Cambridge.

« The next important step in studying the planets of TRAPPIST-1 will be the spectroscopic observation of their atmospheres using the new space telescope James Webb, which will be launched next year by NASA and ESA, says Michaël Gillon, astronomer at the University of Liege and co-author of the four publications. The James Webb will enable us to study in detail the atmospheres of these planets, notably to measure their composition and detect possible molecules of biological origin."

The best is yet to come in the study of this unique exoplanetary system.


Not so strange worlds

A journey to the center of the TRAPPIST-1 exoplanetary system revealed in February 2017. These illustrations present the new data collected by the Spitzer Space Telescope and analyzed by an international team of researchers, including researchers from the STAR Research Institute. ©NASA/JPL Caltech

The SPECULOOS Observatory

The discovery of the TRAPPIST-1 system is part of a wider project, called SPECULOOS (Search for habitable Planets EClipsing Ultra-cOOl Stars). This project has been funded by the European Union through an ERC Starting Grant awarded to Michaël Gillon by the European Research Council (ERC). While the TRAPPIST telescopes focus on about a hundred ultra-cold stars, the SPECULOOS telescopes will target about a thousand stars. The first SPECULOOS observatory, consisting of four telescopes, is currently being installed at the European Southern Observatory of Paranal (ESO) in Chile. Within a few months, he will begin his intensive search for planetary systems similar to that of TRAPPIST-1. 

More about speculoos

Scientific references

(1) Valérie van Grootel et al., 2018, Stellar Parameters for TRAPPIST-1, The Astrophysical Journal, 853,30.

(2) Simon L. Grimm et al. 2018, The Nature of TRAPPIST-1 Planets, Astronomy & Astrophysics, 2018.

Read Press release from ESO

(3) Julien de Wit et al. 2018, Atmospheric Reconnaissance of Habitable-zone Earthsized

Exoplanets, Nature Astronomy, 2018

(4)  Laetitia Delrez et al, 2018, Early 2017 observation of TRAPPIST-1 with Sptizer, MNRAS, 1-24.


Michaël GILLON – STAR Research Institute  - Université de Liège I 04 366 97 43  I 04 73 346 402

Valérie VAN GROOTEL – STAR Institute  - Université de Liège I 04 366 9730 I 0498 639 635

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