An international team of researchers has just published in Nature Astronomy a comprehensive analysis of all mid-infrared data collected on TRAPPIST-1 b, with the aim of determining whether this planet has an atmosphere.
New observations of TRAPPIST-1 with JWST highlight the complexity of confirming a planet’s atmosphere using only broadband thermal emission data. This point of view takes on greater importance with the “Rocky Worlds” observation program recently approved by the Space Telescope Science Institute (STScI), which plans to apply this same method to study numerous rocky exoplanets orbiting stars. cold.
The James Webb Space Telescope (JWST) is revolutionizing the study of exoplanets (planets orbiting stars other than the Sun), including enabling detailed spectroscopic studies of small, rocky planets, but only if they are orbiting ” nearby red dwarfs, the smallest, least massive and coldest stars. Top of its list of targets is the very low-mass red dwarf TRAPPIST-1, whose astonishing system of seven Earth-sized rocky planets, including three located in the star’s habitable zone, has been discovered in 2017 by an international team led by ULiège astronomer Michaël Gillon.
The innermost planet, TRAPPIST-1 b, was recently observed at depth by JWST in mid-infrared, a type of light to which our eyes are not sensitive. An international team of researchers has just published in Nature Astronomy a comprehensive analysis of all mid-infrared data collected on TRAPPIST-1 b, with the aim of determining whether this planet has an atmosphere. Planets orbiting red dwarfs are our best chance to study for the first time the atmospheres of temperate rocky planets, those that receive stellar fluxes between those of Mercury and Mars,” explains Elsa Ducrot, co-lead author of the study and assistant astronomer at the Atomic Energy Commission (CEA) in Paris, France. The TRAPPIST-1 planets constitute an ideal laboratory for this innovative research.
A previous observation with JWST measured infrared emission from TRAPPIST-1 b at 15 microns and suggested that a thick, CO2-rich atmosphere was unlikely (Greene et al., 2023). This conclusion was based on the fact that CO2 strongly absorbs radiation at this wavelength, which would have significantly reduced the observed flux if such an atmosphere were present. The study proposed that the measurement is more consistent with a “dark bare rock” scenario – a planet with no atmosphere and a dark surface that absorbs almost all incoming starlight. However, a single measurement at one wavelength is not enough to rule out all potential atmospheric scenarios.
In this new study, the authors took this work further by measuring the planet’s flux at another wavelength, 12.8 microns. They performed a global analysis of all available JWST data and compared these observations with surface and atmosphere models to identify the scenario that best fits the data.
The show to the rescue
The most widely used method of determining whether an exoplanet has an atmosphere – transit transmission spectroscopy – involves observing its “transits”, i.e. when it passes in front of its host star at different wavelengths. , and to detect and measure 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. However, very low mass red dwarfs pose a problem in this regard,” explains Professor Michaël Gillon (ULiège), author of the study. Their surface is not homogeneous and this inhomogeneity can pollute the transmission spectrum of transiting planets and mimic atmospheric characteristics.” Such a phenomenon has been observed several times with JWST when observing transits of planets around red dwarfs.
One solution to overcoming this stellar contamination while obtaining information about the presence (or absence) of an atmosphere is to directly measure the planet’s heat by observing a drop in flux as the planet passes behind the star (a event called occultation). By observing the star just before and during the occultation, we can deduce the amount of infrared light coming from the planet.
©️ CEA | Principle of measuring the emission of a planet when it passes behind its star (occultation). Outside occultation, what is measured is the sum of the star and planet fluxes, whereas during occultation, only the stellar flux is measured. The difference between the two measurements therefore provides the flux of the planet.
The emission quickly became the preferred method for studying rocky exoplanets around red dwarfs during the first two years of JWST,” explains Pierre Lagage, co-lead author of the study and head of the astrophysics department at the Commissariat des Énergies Atomiques (CEA) in Paris, France. For the TRAPPIST-1 planets, the first information comes from emission measurements, because it is still difficult to disentangle the atmospheric and stellar signals in the transit.
Reflecting this growing interest, the Space Telescope Science Institute (STScI), which manages JWST operations, recently approved a 500-hour Director’s Discretionary Time (DDT) program called “Rocky Worlds” to study the atmospheres of terrestrial exoplanets around nearby M dwarf stars using exactly the same approach as the authors, via occultation observations, but at only 15 microns.
The study results are not very consistent with the “dark, bare surface” scenario suggested by Greene et al. 2023. The authors found that a not-so-gray bare surface composed of ultramafic rocks (mineral-enriched volcanic rocks) best explained the data.
©️ CEA | Illustration of the emitted flux at 12.8 microns and 15 microns for different bare rock and atmospheric scenarios, indicating which are consistent with current data and which are not.
Furthermore, they were able to show that an atmosphere containing a large quantity of CO2 and mist could also explain the observations. This result is surprising, because an atmosphere rich in CO2 seems incompatible with the strong emission at 15 microns. However, haze can radically change the situation: it can absorb starlight and make the upper atmosphere warmer than the lower layers, creating what is called a “thermal inversion”, like the Earth’s stratosphere. This inversion causes the CO₂ to emit light instead of absorbing it, resulting in a higher flux at 15 microns than at 12.8 microns.
“Such 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, the chemistry of TRAPPIST-1b’s atmosphere is expected to be very different from that of Titan or any other rocky body in the Solar System, and it is fascinating to think that we might observe a type of atmosphere that we have never seen before. seen before,” says Dr. Michiel Min of the Netherlands Space Research Institute SRON.
The authors note, however, that this atmospheric model, although it matches the data, remains less likely than the bare rock scenario. Its complexity and 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 questions. More generally, the team highlights the difficulty of determining with certainty a planet’s surface or its atmospheric composition using only emission measurements in a few wavelengths, while highlighting two compelling scenarios that will be explored further. in detail during the next observations of TRAPPIST-1 b.
What are the next steps?
“While both scenarios remain viable, our recent observations of TRAPPIST-1 b’s phase curve – which tracks the planet’s flow throughout its orbit – will help solve the mystery,” says Professor Michaël Gillon , who co-directs the new JWST program with Dr. Elsa Ducrot. She adds: “By analyzing how efficiently heat is redistributed across the planet, astronomers can infer the presence of an atmosphere. If an atmosphere exists, heat should be distributed from the day side of the planet to its night side; without an atmosphere, heat redistribution would be minimal. »
We should therefore soon know more about the presence or absence of an atmosphere around the inner planet of TRAPPIST-1.
Source
Combined analysis of the 12.8 and 15 μm JWST/MIRI eclipse observations of TRAPPIST-1 bNature Astronomy.
Contacts
Michaël Gillon (ULiege)
Elsa Ducrot (CEA, Paris)
