How to reveal the atmosphere of an exoplanet: the case of TRAPPIST-1 b – Paris Observatory – PSL

The most intuitive method used by astronomers to determine whether an exoplanet has an atmosphere is to observe its transit – passing in front of its host star (partially blocking the star’s light for about 30 minutes) – at different lengths. waveform to detect which parts of the light are transmitted or not, indicating the presence of molecules.

However, stellar contamination problems arise for planets orbiting very cold stars. This is because these stars are not homogeneous and have hot and cold spots on their surfaces. These spots have their own spectra, which can pollute the transmission spectrum of transiting planets and mimic atmospheric features. Such a phenomenon has been observed several times with the JWST when it observed transits of planets around cool stars.

The show to the rescue

One solution to overcome this stellar contamination and obtain 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 (an 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.

JWST is particularly effective at performing these types of detailed spectroscopic studies of small, rocky planets orbiting red dwarf stars. In this context, the red dwarf star TRAPPIST-1, which is home to seven Earth-sized rocky planets, including three located in the star’s habitable zone (Gillon et al., 2017), appears to be a target ideal. In particular, its closest planet, TRAPPIST-1 b, was widely observed in the mid-infrared, at 15 microns wavelength, by JWST (October 2022, November 2022, July 2023, November 2023).

The unresolved case of TRAPPIST-1 b

From this first cohort of data, a study conducted in 2023 by Greene et al. had suggested that a thick atmosphere, rich in CO_2, was unlikely on TRAPPIST-1 b.

But these conclusions are qualified by the same team, in light of new data now available at 12.8 microns on the planet’s flow. On December 16, 2024, she reported in the journal Nature Astronomy a complete analysis of all infrared data collected on TRAPPIST-1 b. In this new study led by Elsa Ducrot, then a postdoctoral student at the Paris-PSL Observatory (and currently an astronomer at the CEA), the authors carried out a global analysis of all available JWST data and compared these observations with models of surfaces and atmospheres in order to identify the scenario that best matches the data.

In the “bare dark rock” scenario proposed by Greene et al. (2023), the expected temperature at 12.8 microns was around 227°C. However, the actual measurement showed a lower temperature of 150°C.

To explain such a discrepancy, the authors explored various surface and atmosphere models. They found that a bare surface composed of ultramafic rocks (mineral-rich volcanic rocks) could explain the observations. Indeed, ultramafic rocks emit less thermal radiation at 12.8 microns than a classic dark surface.

The authors also found that an atmosphere rich in CO₂ and in mists could explain the observations. Mists are tiny particles or droplets suspended in a planet’s atmosphere, often created by chemical reactions, volcanic activity, or solar radiation. These particles can scatter and absorb light, which affects the appearance of the atmosphere and its temperature. For example, mists are present in the atmosphere of Titan, Saturn’s famous moon.

It is surprising that a foggy, CO-rich atmosphere₂ matches the data, because it was thought that CO₂ was inconsistent with the high emission observed at 15 microns. However, mists can be a game changer. They reflect a lot of light and can make the upper atmosphere warmer than the lower layers, creating a thermal inversion similar to that in Earth’s stratosphere. This brings the CO₂ to emit radiation instead of absorbing it, resulting in a higher flux at 15 microns compared to 12.8 microns — an unexpected result compared to the behavior of CO₂ on Earth or Venus.

The authors note, however, that this atmospheric model, although compatible with the data, remains less likely than the bare rock scenario. Its complexity and questions about haze formation and long-term climate stability on TRAPPIST-1 b make it a difficult fit.

Future research, including advanced 3D modeling, will be needed to explore these questions. More generally, the team highlights the difficulty of definitively determining the composition of a planet’s surface or atmosphere using only measurements with broadband filters, while putting forward two compelling scenarios that will be explored in more detail with future observations of the TRAPPIST-1 b phase curve, which represents the variation in the brightness of an exoplanet during its orbit, caused by changes in the illuminated portion visible from Earth. This provides information about the planet’s atmosphere, surface properties and temperature distribution.

And now ?

Although both scenarios remain viable, the authors explain that recent observations of TRAPPIST-1 b’s phase curve—which track the planet’s flow throughout its orbit—could help solve the mystery. By analyzing how efficiently heat is redistributed across the planet, astronomers can infer whether an atmosphere is present. 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. Investigations are therefore expected to continue.

Last modified on December 16, 2024