An international team including the University of Bern (UNIBE) and the University of Geneva (UNIGE), members of the National Centre of Competence in Research PlanetS, has succeeded in mapping the climate of rocky exoplanets with masses similar to Earth for the first time. This major breakthrough is based on continuous observations using the James Webb Space Telescope. The two planets studied belong to the iconic TRAPPIST-1 planetary system, discovered ten years ago. This system of seven planets is a laboratory for scientists studying life in the Universe, particularly around red dwarf stars. The two planets do not appear to have atmospheres, as observations show temperature differences between day and night exceeding 500 degrees Celsius. The results are published in Nature Astronomy.
Red dwarf stars—cooler and smaller than our Sun—make up more than 75% of the stars in our Galaxy. Astronomers have shown that small, Earth-like planets are common around this type of star. Consequently, the question of the emergence of life on these worlds, so different from our own, quickly became a central question.
Among the planetary systems discovered around red dwarfs, TRAPPIST-1, which celebrates its tenth anniversary this year, holds a prominent place in scientific research. Astronomers marked this anniversary with an observation campaign using the James Webb Space Telescope (JWST), focusing on the two innermost planets in the system (closest to the star), TRAPPIST-1b and TRAPPIST-1c. These continuous observations have ruled out the hypothesis of dense atmospheres on the two planets, confirming that the harsh conditions around these stars can influence planetary evolution.
“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.
Energy bombardments
While red dwarf stars and their planets are common in our Galaxy, their habitability is not necessarily guaranteed. First, these stars are very active and bombard their planets with intense ultraviolet radiation and energetic particle fluxes, which could erode their atmospheres and eradicate any life that might exist.
Second, planets in the habitable zone of a red dwarf orbit very close to their star, and tidal forces synchronize their rotation with their orbital period, much like the Moon with the Earth. These planets therefore complete one rotation on their axis at the same time as they orbit their star. The result is a permanent day on one side and permanent night on the other.
“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 TRAPPIST-1 observations
The observations with the JWST involved continuously observing the two planets closest to the star, and therefore most exposed to its influence, in infrared light over a full orbit. These 60 hours of observations allowed scientists, for the first time, to map the climate of Earth-sized planets. By measuring the light flux from TRAPPIST-1 and the planets "b" and "c," astronomers were able to determine the surface temperatures of both planets with great precision, on both their day and night sides.
TRAPPIST-1b and TRAPPIST-1c exhibit a significant temperature difference between their two hemispheres. 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. This enormous contrast suggests a lack of energy redistribution between the two sides of the planets, and therefore the absence of atmospheres. If the two planets possessed atmospheres during their formation, these were completely stripped away by the extreme conditions imposed by their star.
The search continues
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 from red dwarfs play a significant role in the evolution of planets around this type of star.
What about the slightly more distant planets located in the habitable zone? The JWST is currently observing the system's planet "e," which lies within the star's habitable zone—the region where liquid water can exist on the surface.
"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.