The TRAPPIST-1 system, discovered in 2017 by a team led by University of Liege astronomer Michael Gillon, contains seven Earth-sized planets orbiting a very low-mass red dwarf star. As prime candidates for atmospheric studies, these planets are the focus of JWST's revolutionary spectroscopic capabilities. The innermost planet, TRAPPIST-1 b, has been analyzed extensively in the mid-infrared range to determine the likelihood of an atmosphere.
"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," explained Elsa Ducrot, co-lead author of the study and assistant astronomer at CEA Paris. "The TRAPPIST-1 planets provide an ideal laboratory for this important research."
Previous data at 15 microns indicated that a thick CO2-rich atmosphere was unlikely, favoring a "bare dark rock" model where the planet lacks an atmosphere and has a highly absorptive surface. However, a single wavelength was insufficient to rule out all atmospheric possibilities. Expanding on this, the new study measured TRAPPIST-1 b's flux at 12.8 microns and combined these results with comprehensive atmospheric and surface models.
Pierre Lagage, co-lead author and astrophysics department head at CEA Paris, emphasized the value of emission measurements in avoiding stellar contamination, a challenge for transit spectroscopy around red dwarfs. "Emission quickly became the preferred method for studying rocky exoplanets around red dwarfs during the first two years of JWST," Lagage said.
The results challenge the initial bare rock model, suggesting two viable interpretations: a surface composed of ultramafic rocks (volcanic rocks rich in minerals) or an atmosphere containing CO2 and haze. While haze introduces the possibility of thermal inversion - a warmer upper atmosphere absorbing starlight - this model raises questions about haze formation and climate stability.
"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," explained Dr. Michiel Min of SRON Netherlands Institute for Space Research. "Yet, the chemistry in the atmosphere of TRAPPIST-1 b is expected to be very different from Titan or any of the Solar System's rocky bodies, and it is fascinating to think we might be looking at a type of atmosphere we have never seen before."
Despite the potential for an atmospheric explanation, the team maintains that the bare rock scenario remains more likely given the current data. However, future observations, including phase curve analyses that track heat distribution across the planet, will help resolve the debate.
"By analyzing the efficiency with which heat is redistributed on the planet, astronomers can deduce the presence of an atmosphere," said Professor Michael Gillon. "If an atmosphere exists, the heat should be distributed from the day side of the planet to its night side; without an atmosphere, the redistribution of heat would be minimal."
With JWST's ongoing Rocky Worlds program, which dedicates 500 hours to observing terrestrial exoplanets around red dwarfs, additional data will further refine our understanding of TRAPPIST-1 b. The results will offer critical insights into whether these distant rocky worlds harbor atmospheres - or remain barren, silent rocks in space.
Research Report:Combined analysis of the 12.8 and 15 um JWST/MIRI eclipse observations of TRAPPIST-1 b
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