In early 2017, astronomers found seven rocky planets orbiting a very cool red dwarf star 40 light-years away from Earth. These planets are similar in size and mass to the rocky planets in our solar system. However, they orbit much closer to their star than any planet in our solar system orbits the Sun.
TRAPPIST-1 b, the innermost planet, was recently observed in depth by JWST in the mid-infrared, a type of light to which our eyes are not sensitive.
A new study published in Nature Astronomy reports a complete analysis of all the mid-infrared data collected on TRAPPIST-1 b to determine whether this planet has an atmosphere.
Elsa Ducrot, co-lead author of the study and assistant astronomer at the Commissariat aux Énergies Atomiques (CEA) in Paris, France, said, “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.”
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In a previous observation, Webb measured the infrared emission of TRAPPIST-1 b at 15 microns. This observation suggested that the planet is unlikely to have a thick, CO2-rich atmosphere.
This conclusion came from that CO2 strongly absorbs radiation at the 15-micron wavelength, which would have significantly reduced the observed light if such an atmosphere were present. Based on this, the study suggested that the best explanation was a “dark bare rock” scenario, where the planet has no atmosphere and a dark surface that absorbs almost all incoming starlight. However, since the measurement was only taken at one wavelength, more was needed to rule out other possible atmospheric conditions completely.
In this new study, the authors built on their previous work by measuring TRAPPIST-1 b’s flux at a different wavelength, 12.8 microns. They conducted a comprehensive analysis of all available JWST data. They compared these new observations with various surface and atmospheric models, allowing them to identify which scenario best matched the data.
The team used transit transmission spectroscopy to determine whether an exoplanet has an atmosphere. The method involves observing the planet’s transits, i.e., when it passes in front of its host star at different wavelengths, and detecting and measuring the tiny fraction of the light emitted by the star in our direction that is absorbed by its atmosphere, which is an indicator of its chemical composition.
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Professor Michaël Gillon (ULiège), the study’s author, said, “However, very low-mass red dwarfs pose a problem in this respect. Their surface is not homogeneous, and this inhomogeneity can pollute the transmission spectrum of transiting planets and mimic atmospheric characteristics.”
This phenomenon has been observed multiple times with the JWST while studying transits of planets around red dwarf stars. To overcome the issue of stellar contamination and still gather information about a planet’s atmosphere, scientists can directly measure the planet’s heat by observing a drop in flux as the planet passes behind the star, an event called occultation. By observing the star just before and during the occultation, researchers can determine how much-infrared light is coming from the planet.
Pierre Lagage, co-lead author of the study and head of the astrophysics department at the Commissariat aux Énergies Atomiques (CEA) in Paris, France said, “Emission quickly became the preferred method for studying rocky exoplanets around red dwarfs during the first two years of JWST. The first information for the TRAPPIST-1 planets comes from emission measurements because it is still difficult to disentangle the atmospheric and stellar signals in the transit.”
The study’s results do not fully support the “dark, bare surface” scenario that Greene et al. (2023) suggested. Instead, the authors found that a surface made of ultramafic rocks—volcanic rocks rich in minerals—better explained the data, suggesting the planet might have a surface that isn’t completely dark but still lacks a thick atmosphere.
Alternatively, the authors showed that an atmosphere with a large amount of CO2 and haze could also explain the observations. This was surprising because a CO2-rich atmosphere contradicted the strong emission at 15 microns.
However, haze can significantly alter the situation by absorbing starlight and warming the upper atmosphere more than the lower layers, creating a “thermal inversion” similar to Earth’s stratosphere. This inversion causes CO2 to emit light instead of absorbing it, leading to a higher flux at 15 than 12.8 microns.
Dr. Michiel Min from SRON Netherlands Institute for Space Research said, “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. Yet, the chemistry in the atmosphere of TRAPPIST-1b 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.”
Authors noted, “However, this atmospheric model, while consistent with the data, remains less likely than the bare rock scenario. Its complexity and the questions relating to haze formation and long-term climate stability on TRAPPIST-1 b make it a difficult model to implement. Future research, including advanced 3D modeling, will be needed to explore these issues.”
The team emphasizes the challenge of confidently determining a planet’s surface or atmospheric composition based solely on emission measurements at a few wavelengths. They highlight two promising scenarios— a surface of ultramafic rocks and an atmosphere with CO2 and haze— that will be explored in more detail with future observations of TRAPPIST-1 b.
Professor Michaël Gillon, who co-directs the new JWST program with Dr Elsa Ducrot, said, “Although both scenarios remain viable, 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.”
“By analyzing the efficiency with which heat is redistributed on the planet, astronomers can deduce the presence of an atmosphere. 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.”
Journal Reference:
- Ducrot, E., Lagage, PO., Min, M. et al. Combined analysis of the 12.8 and 15 μm JWST/MIRI eclipse observations of TRAPPIST-1 b. Nat Astron (2024). DOI: 10.1038/s41550-024-02428-z