The TRAPPIST-1 system, discovered in 2017 by a team led by University of Liege astronomer Michael Gillon, includes seven Earth-sized planets orbiting a low-mass red dwarf star. These planets are prime candidates for atmospheric studies, utilizing the James Webb Space Telescope's (JWST) advanced spectroscopic capabilities. Recent analysis of the innermost planet, TRAPPIST-1 b, focused on its mid-infrared emissions to assess the likelihood of an atmosphere.
According to Elsa Ducrot, co-lead author of the study and assistant astronomer at CEA Paris, "Planets orbiting red dwarfs are our best chance of studying for the first time the atmospheres of temperate rocky planets, those that receive stellar fluxes between those of Mercury and Mars." The TRAPPIST-1 planets serve as an ideal laboratory for this crucial research.
Prior data at 15 microns suggested that a thick CO2-rich atmosphere was unlikely, favoring a model where the planet is a "bare dark rock" without an atmosphere. However, the new study expanded measurements to 12.8 microns, combining these results with detailed atmospheric and surface models.
Pierre Lagage, co-lead author and head of the astrophysics department at CEA Paris, noted the importance of emission measurements in mitigating stellar contamination, a common issue in transit spectroscopy around red dwarfs. He stated, "Emission quickly became the preferred method for studying rocky exoplanets around red dwarfs during the first two years of JWST."
The findings challenge the initial bare rock model, suggesting two possible scenarios: a surface made of ultramafic rocks or an atmosphere rich in CO2 and haze. The presence of haze could lead to thermal inversion, where a warmer upper atmosphere absorbs starlight, raising questions about haze formation and climate stability.
Dr. Michiel Min from SRON Netherlands Institute for Space Research explained, "These thermal inversions are quite common in the atmospheres of Solar System bodies, perhaps the most similar example being the hazy atmosphere of Saturn's moon Titan." However, he emphasized that the chemistry of TRAPPIST-1 b's atmosphere is expected to differ significantly from Titan or other rocky bodies in the Solar System.
While the potential for an atmosphere exists, the research team believes the bare rock scenario remains more probable based on current data. Future observations, including phase curve analyses to monitor heat distribution, will provide further clarity.
Professor Michael Gillon stated, "By analyzing the efficiency with which heat is redistributed on the planet, astronomers can deduce the presence of an atmosphere." If an atmosphere is present, heat should be distributed from the day side to the night side; without it, heat redistribution would be minimal.
JWST's ongoing Rocky Worlds program, which allocates 500 hours to observing terrestrial exoplanets around red dwarfs, will yield additional data to enhance our understanding of TRAPPIST-1 b. These results will be pivotal in determining whether these distant rocky worlds possess atmospheres or remain barren, silent rocks in space.