Utilising the James Webb space telescope(JWST), an international team has mapped the climate of rocky exoplanets with masses similar to Earth in the TRAPPIST-1 system.

First discovered in 2000, TRAPPIST-1 is a red dwarf star and is thought to be about 3 billion years older than the Solar System. Further discoveries in 2016 and 2017 identified at least seven exoplanets in the star’s orbit, all likely tidally locked and with circular orbits between 1.5 and 19 days.

At least four of the planets in TRAPPIST-1’s orbit- TRAPPIST-1d, TRAPPIST-1e TRAPPIST-1f and TRAPPIST-1g- are considered potentially hospitable to life, as their orbital distance allows the existence of liquid water, although there is no evidence of atmospheres on any of the planets in the system.

The two planets studied, TRAPPIST-1b and TRAPPIST-1c, are believed to have temperatures differences at day and night in excess of 500 degrees Celsius and so likely do not have atmospheres.

“The TRAPPIST-1 system is incredible! Seven planets, some with masses similar to Earth’s, orbit the same star. At least three planets are located in the star’s habitable zone, where the surface temperatures would allow for the presence of liquid water. It is the perfect playground for comparative planetology, unraveling the mysteries of this type of planet and testing our hypotheses about the development of life around these stars,” enthuses Emeline Bolmont, associate professor in the Department of Astronomy at the Faculty of Science, director of the Centre for Life in the Universe (CVU) at the UNIGE, and co-author of the study.

As published in Nature Astronomy, an international team of the University of Bern (UNIBE) and the University of Geneva (UNIGE), both members of the National Centre of Competence in Research PlanetS, marked the tenth anniversary of the discovery of the TRAPPIST-1 system with an observation campaign.

Red dwarf stars are common- and very active

Cooler and smaller than our Sun, red dwarfs make up 75% of the stars in our galaxy. Many have exoplanets (planets located outside the Solar System), but the conditions of these worlds are vastly different to our own.

Red dwarf stars are known to exude high levels of ultraviolet radiation and energetic particle fluxes, which erode planetary atmospheres and reduce the chances of life surviving. Exoplanets in the habitable zones of red dwarfs are positioned extremely close to their star- for example, all of TRAPPIST-1’s planetary orbits would fit inside the distance between the Sun and Mercury- and are tidally locked, meaning their axis rotation is in sync with their orbit and one side of the planet is permanently facing the star, fixing the sides of the planet into permanent day and night.

“The presence of an atmosphere around these tidally locked planets could allow for energy transfer between the day and night sides, resulting in more moderate temperatures across the planet, which would have a significant impact on their potential habitability,” adds Brice-Oliver Demory, professor and director of the Center for Space and Habitability at UNIBE, and co-author of the study. “Successfully detecting the atmosphere of one of these planets has therefore become a key objective for our community, highlighting the importance of the TRAPPIST-1 system with the JWST,” he explains.

Sixty hours of observations were able to rule out the presence of any atmospheres on the two rocky exoplanets

The JWST continuously observed the two rocky exoplanets closest to the star in infrared light over a full orbit. Sixty hours of measuring the light flux from TRAPPIST-1 and planets b and c allowed astronomers to calculate the surface temperature and climate of each exoplanet on both the day and night sides.

TRAPPIST-1b and TRAPPIST-1c exhibit a significant temperature difference between their two hemispheres, suggesting a lack of energy redistribution between the two sides of each exoplanet, which implies a lack of atmosphere. During the day, the surface temperatures of the two planets exceed 200°C and nearly 100°C, respectively, while their nights are plunged into frigid temperatures below -200°C. If the planets ever had atmosphere, it is likely they were stripped away by the forces emitted by the red dwarf star.

The telescope is now focusing on TRAPPIST-1e

The lack of a dense atmosphere on the two inner planets of the TRAPPIST-1 system supports the hypothesis that intense radiation and energetic ejections play a significant role in the evolution of planets around red dwarf stars. But planets further away in the system may not suffer the same severe impacts, as the further study intends to explore. JWST is now observing planet “e”, which exists in the so-called Goldilocks zone, where planetary atmosphere conditions permit the existence of liquid water.

“TRAPPIST-1 serves as a reference system. Our theoretical models show that the outermost planets of the TRAPPIST-1 system can possess an atmosphere despite the absence of one on the two inner planets. This is similar to Mercury, the closest planet to our Sun, which has no atmosphere, while Venus and Earth have retained theirs. We look forward to continuing the exploration of the TRAPPIST-1 system!”, concludes Emeline Bolmont.