The unique architecture of TRAPPIST-1


An international team of astronomers, including some scientists from the University of Liège, has used data gathered by the Kepler Space Telescope to observe and confirm details of the outermost of seven exoplanets orbiting the star TRAPPIST-1. They confirmed that the planet, TRAPPIST-1h, orbits its star every 18.77 days, is linked in its orbital path to its siblings and is frigidly cold. Far from its host star, the planet is likely uninhabitable — but it may not always have been so.

"TRAPPIST-1h was exactly where our team predicted it to be," said Rodrigo Luger, a doctoral student at University of Washington (USA) and first author of the paper published on May 22nd in the journal Nature Astronomy. Thanks to the data collected by NASA’s Kepler space telescope, the researchers discovered a mathematical pattern in the orbital periods of the inner six planets, which was strongly suggestive of an 18.77 day period for planet h.

"It had me worried for a while that we were seeing what we wanted to see. Things are almost never exactly as you expect in this field — there are usually surprises around every corner, but theory and observation matched perfectly in this case."

The planets of the TRAPPIST-1 system are detected when they transit, or pass in front of, their host star, blocking a measurable portion of the light.  The team of the University of Liège was able to observe only a single transit for TRAPPIST-1h, the farthest-out of the star's seven progeny, prior to the data from K2, the second mission of the Kepler Space Telescope. These new data included four transits of TRAPPIST-1h across its star.

The team used also the K2 data to further characterize the orbits of the other six planets, help rule out the presence of additional transiting planets, and learn the rotation period and activity level of the star.

"TRAPPIST-1 is active with important eruption happening every three days, which is actually rather calm when compared to other ultracool stars", comments Catarina S. Fernandes, PhD student at the University of Liège and co-author of the article published in Nature Astronomy. "The stellar activity is important as it can also play a role in planet habitability and composition of their atmosphere. This is even more so for stars like Trappist-1 where the exoplanets are close in orbit.”

Astronomers have also discovered that the seven planets of TRAPPIST-1 are linked in a complex dance known as orbital resonance where the respective orbital periods are mathematically related and influence each other slightly.

"These resonances are dynamical connections between the planets that must have developed early in the life of the TRAPPIST-1 system", says Michaël Gillon, astronomer at the University of Liège, also co-author of the publication. "They point to an interesting history in which the planets formed further out from the star, and migrated inward. This suggests that their initial compositions could have been much richer in water than Earth's, which is extremely exciting from an astrobiological point of view!"

It also means that while TRAPPIST-1h is now probably colder than Earth - depending on its geological activity and atmospheric composition -  it likely spent several hundred million years in a much warmer state, when its host star was younger and brighter.

"The TRAPPIST-1 planets form a unique system not only because of their fascinating dynamical architecture, but also because they are prime targets for detailed characterization with current and upcoming telescopes, which should enable us to learn about their atmospheric composition and the possible existence of life on their surfaces. Currently, there is just no better target to search for life beyond our solar system", outlines Michaël Gillon.

The TRAPPIST-1 system

TRAPPIST-1 is a middle-aged, ultra cool dwarf star, much less luminous than the sun and only a bit larger than the planet Jupiter. The star, which is nearly 40 light-years or about 235 trillion miles away in the constellation of Aquarius, is named after the ground-based Transiting Planets and Planetesimals Small Telescope (TRAPPIST), the facility managed by the University of Liège that first found evidence of planets around it in 2015.

The TRAPPIST survey is led by Liège astronomer Michaël Gillon of the University of Liège, who is also a coauthor on this research. In 2016, Gillon’s team announced the detection of three planets orbiting TRAPPIST-1 and this number was upped to seven in a subsequent 2017 paper. Three to four of TRAPPIST-1's planets appear to be within the star's habitable zone, that swath of space around a star where a rocky planet could have long-lived large amounts of liquid water over its surface. Still, the seven planets of the system have insolation low enough to make possible the existence of liquid water on a least a fraction of their surfaces, depending on their compositions and atmospheric properties.

(1) Luger R. et  al., A seven-planet resonant chain in TRAPPIST-1, Nature Astronomy 1, 0129 (2017) | DOI: 10.1038/s41550-017-0129 *


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Fig 1 : The orbits of the seven planets around the star TRAPPIST-1. The grey region is the zone, where liquid water could exist on the surface of the planets. On planet TRAPPIST-1 h liquid water is possible under a thick layer of ice. (1 AU is the distance between the Sun and the Earth.) Credit: A. Triaud

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Fig.2 : Artist’s impression of TRAPPIST-1h (Credit: NASA/JPL-Caltech)

Link to the animation

The animation shows a simulation of the planets of TRAPPIST-1 orbiting for 90 Earth-days. After 15 Earth days, the animation focuses only on the outer three planets: TRAPPIST-1f, TRAPPIST-1g, TRAPPIST-1h. The motion freezes each time two adjacent planets pass each other; an arrow appears pointing to the location of the third planet. This complex but predictable pattern, called an orbital resonance, occurs when planets exert a regular, periodic gravitational tug on each other as they orbit their star. The three-body resonance of the outer three planets causes the planets to repeat the same relative positions, and expecting such a resonance was used to predict the orbital period of TRAPPIST-1h.Daniel Fabrycky / University of Chicago; with reference to Luger et al. 2017, Nature Astronomy

More informations

About Michaël Gillon :

About Emmanuël Jehin :

About Valérie Van Grootel :

* Contributing to this discovery are researchers at the University of Bern in Switzerland; Paris Diderot and Paris Sorbonne Universities in France; the University of Liège in Belgium; the University of Chicago; the University of California, San Diego; California Institute of Technology; the University of Bordeaux in France; the University of Cambridge in England; NASA's Ames Research Center, Goddard Space Flight Center, and Johnson Space Center; Massachusetts Institute of Technology; the University of Central Lancashire in England; King Abdulaziz University in Saudi Arabia; Cadi Ayyad University in Morocco; and the University of Geneva in Switzerland.


Press contacts

STAR Institute, University of Liège

Michaël Gillon, FNRS Research associate


+32 473 34 64 02

Emmanuel Jehin, FNRS Research associate


+32 495 23 72 98

Valérie Van Grootel, FNRS Research associate



Catarina S. Fernandes, PhD Student


+32 4366 9721

or via the Press office of the University of Liège


+32 4 366 52 17

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