To have or not to have an atmosphere? The question puzzling astronomers! 🪐

The TRAPPIST-1 system, fascinating with its seven Earth-sized rocky planets, three of which are located in the habitable zone, represents a unique opportunity to study the atmospheres of exoplanets.

The James Webb Space Telescope (JWST) plays a key role by enabling the measurement of the thermal emission of these temperate planets. An initial observation campaign at λ=15 µm revealed a temperature of 503 K on the day side of the planet TRAPPIST-1 b, suggesting the absence of an atmosphere and a very dark surface.

However, based on observations from a second campaign at λ=12.8 µm, this new study conducted by the Astrophysics Department of IRFU at CEA Paris-Saclay measured a temperature much lower than expected by the previous scenario, thus forcing researchers to explore new avenues.



Among the hypotheses considered, an atmosphere rich in CO? and hazes is a possibility, although an ultramafic volcanic surface scenario seems more likely. To solve this mystery, a new phase of observations has been launched, aiming to track the planet's luminous flux throughout its orbit.

This result was published in the prestigious journal Nature Astronomy: "Combined analysis of the 12.8 and 15 μm JWST/MIRI eclipse observations of TRAPPIST-1 b"

TRAPPIST-1: an ideal laboratory for studying rocky planet atmospheres

The TRAPPIST-1 system stands out with its ultra-cool dwarf star surrounded by seven Earth-sized rocky planets, three of which are located in the habitable zone, offering an exceptional scientific opportunity for the study of exoplanets and atmospheres. This system is therefore a prime target for the James Webb Space Telescope (JWST), whose infrared spectroscopic capabilities allow for detailed study of this type of planet.

In particular, the JWST is capable of directly measuring the heat emitted by a planet by comparing the star's luminous flux during an occultation - when the planet passes behind the star - to the flux observed just before and after this event (see Figure 2). This method allows for the deduction of the infrared light emitted by the illuminated side (day side) of the planet while avoiding stellar contamination that can complicate measurements in other configurations, such as transits.


"Thermal emission quickly became the preferred method for studying rocky exoplanets around M dwarf stars during the first two years of JWST", explains Pierre-Olivier Lagage, co-lead author of the study and director of the Astrophysics Department at CEA. "Regarding the planets of TRAPPIST-1, the first information comes from emission measurements, as it remains difficult to distinguish atmospheric and stellar signals in transit."


Two observation campaigns with the JWST were conducted to study the planet TRAPPIST-1 b because, being the closest to the host star, it emits more infrared than the other planets in the system. These observations were made with the MIRIm imager, developed at CEA Paris-Saclay, using carefully chosen filters to detect the presence of carbon dioxide (CO?) and measure its content.

The first campaign, conducted in 2023 by a NASA team in collaboration with the Astrophysics Department of IRFU at CEA Paris-Saclay, used a filter centered at λ=15 µm. These observations determined that the day side of TRAPPIST-1 b has a temperature of about 503 K (+/- 26 K), marking the very first direct measurement of the temperature of a temperate rocky planet in the history of exoplanet studies.

With such a temperature, scientists suggested that TRAPPIST-1 b would rather have a "bare and dark surface", where the planet would not have an atmosphere, and its surface would absorb almost all incident stellar light (Greene et al., 2023). This hypothesis is based on the fact that CO? strongly absorbs at this wavelength; an atmosphere rich in CO? would therefore have significantly reduced the observed flux. However, a single measurement at one wavelength is not enough to exclude all possible atmospheric scenarios.

Bare surface or complex atmosphere?

This new study, conducted by a team from CEA Paris-Saclay, complements previous observations by this time measuring the flux of TRAPPIST-1 b at 12.8 microns, a second absorption band characteristic of CO?. While the initial "bare dark surface" scenario proposed by Greene et al. (2023) predicted a temperature of about 227 °C at this wavelength, researchers measured a significantly lower temperature of 150 °C. This result invalidates the previous scenario, based on observations at 15 microns, forcing researchers to explore other surface and atmosphere models. Two new scenarios seem to emerge (see figure 3):

- "Ultramafic bare surface" scenario: TRAPPIST-1 b would lack an atmosphere, but its surface would be composed of ultramafic rocks, volcanic rocks rich in minerals that emit less light at 12.8 microns than a classic dark surface. This result suggests the possible existence of volcanism, as without this process creating new rocks, the rocks would quickly be altered and darkened by the star's activity.

- "CO?-rich haze atmosphere" scenario: TRAPPIST-1 b would have a CO?-rich atmosphere with thick hazes, formed of tiny particles or droplets resulting from chemical reactions related to volcanic activity or solar radiation. These hazes would absorb stellar light and cause warming of the upper layers of the atmosphere, creating a thermal inversion where temperature increases with altitude. This phenomenon, similar to that of Earth's stratosphere - although here related to CO? and not ozone - would explain a higher emission at 15 microns compared to 12.8 microns, an unexpected behavior compared to CO? observed on Earth or Venus.

Although hazes are already known to influence temperature and atmospheric appearance, as on Titan, their impact on TRAPPIST-1 b remains surprising. However, the authors believe that the "ultramafic bare surface" scenario is more likely, due to the complexity and uncertainties associated with the formation of such hazes.

"We were surprised to measure a significantly lower temperature than expected. We thought the case of TRAPPIST-1 b was closed, but this new wavelength reminds us of all the ambiguities that exist in describing a planet from discrete observations." emphasizes Elsa Ducrot, researcher at the Astrophysics Department of CEA and lead author of this study. "Moreover, this measurement has stimulated our curiosity and allowed us to propose an atmospheric scenario with unprecedented hazes consistent with the data. Although it seems less likely, it is very interesting that the scientific community can take it into account in the interpretation of future observations of rocky exoplanets."

How to solve the mystery?

This new study highlights the challenges posed by definitively determining the presence of an atmosphere on a planet based solely on thermal emission measurements during occultations. To definitively solve the mystery of the presence of an atmosphere on TRAPPIST-1 b, researchers have initiated a new observation campaign with the JWST aiming to measure the planet's flux throughout its complete orbit, and not just its day side (see figure 4).


Complemented by complex 3D atmospheric simulations, this method, although costly in observation time, is essential to determine the existence or absence of an atmosphere around TRAPPIST-1 b.

"If an atmosphere is present, heat will be redistributed from the day side to the night side of the planet. Without an atmosphere, this redistribution will be minimal", explains Michaël Gillon from the University of Liège, co-author of this study.

These answers could usher in a new era in the study of rocky exoplanet atmospheres.