Imma Perfetto
Cosmos science journalist
The question of whether an atmosphere exists around the exoplanet TRAPPIST-1 b has continued to challenge astronomers with the publication of a new study in the journal Nature Astronomy.
The analysis of data on thermal infrared radiation (heat) emitted by the exoplanet, which was collected with MIRI (Mid-Infrared Imager) on the James Webb Space Telescope (JWST), suggests 2 scenarios.
Either TRAPPIST-1 b has a bare surface composed of mineral-enriched volcanic rocks, or it has an atmosphere with a large amount of carbon dioxide and haze.
TRAPPIST-1 b orbits a red dwarf – the smallest, least massive and coldest stars – located about 40 light-years away. There are 7 rocky planets in the system, all roughly Earth-sized. Of these, 3 (excluding TRAPPIST-1 b) are found in the “habitable zone” where they could have liquid water on their surface.
Red dwarfs, or “M dwarfs”, are the smallest, least massive and coldest of the main sequence stars – those that fuse hydrogen atoms to form helium atoms in their cores. TRAPPIST-1 is estimated to have a surface temperature of about 2,290°C, compared to our Sun’s 5,500°C. Its radius is only slightly larger than Jupiter and it has a mass of about 9% of the Sun.
“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,” says Elsa Ducrot of the Commissariat aux Énergies Atomiques (CEA) in France.
“The TRAPPIST-1 planets provide an ideal laboratory for this ground-breaking research.”
Previous research, which measured TRAPPIST-1 b’s heat emission at a wavelength of 15 microns, found that a thick, CO2-rich atmosphere around it was unlikely. Instead, the study proposed the planet is atmosphere-less and has a dark surface that absorbs nearly all incoming light from its star.
The new study, which expanded on the work by measuring the IR emissions at another wavelength (12.8 microns), contradicts this finding.
“A rocky planet with a heavily weathered surface without an atmosphere is inconsistent with the current measurement,’ says Jeroen Bouwman from the Max Planck Institute for Astronomy (MPIA), Germany.
“Therefore, we think the planet is covered with relatively unchanged material.”
The results suggest that the rock on the surface is about 1,000 years old at most, which could be explained by extreme volcanism or plate tectonics.
“The data also allow for an entirely different solution,’ says Thomas Henning, also from the MPIA.
“Contrary to previous ideas, there are conditions under which the planet could have a thick atmosphere rich in CO2,” he says.
Haze from hydrocarbon compounds (smog) in the upper atmosphere plays a key role in this scenario.
“Although both scenarios remain viable,” says Michaël Gillon of the University of Liège in Belgium. “Our recent observations of TRAPPIST-1 b’s phase curve – which tracks the flow of the planet throughout its orbit – will help to solve the mystery”
Ducrot says astronomers can deduce the presence of an atmosphere by analysing the efficiency with which heat is redistributed on the planet.
“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,” she says.
