First Thermal Mapping of Temperate Earth-sized Exoplanets
An international team led by Michaël Gillon (University of Liège, FNRS) and Elsa Ducrot (CEA Paris-Saclay/Observatoire de Paris, ULiège) has achieved a world first: mapping the surface temperature of two Earth-sized rocky planets in the TRAPPIST-1 system, using the James Webb Space Telescope (JWST).
The researchers observed TRAPPIST-1 b and c, two worlds receiving respectively four and two times more radiation than Earth, by tracking the evolution of their infrared emission throughout their orbit. This technique, known as a “thermal phase curve,” makes it possible for the first time to directly compare the temperatures of the day and night sides of temperate rocky planets beyond the Solar System.
The results show that TRAPPIST-1 b very likely lacks any significant atmosphere. Its dayside reaches nearly 500 K (227 °C), while its nightside remains extremely cold, behavior expected for a bare rocky planet with a dark surface.
TRAPPIST-1 c presents a more nuanced situation: its dayside (~370 K) is significantly hotter than its nightside (<260 K). A thin atmosphere, poor in greenhouse gases, remains possible, but the planet could also be entirely airless if its surface is more reflective than that of TRAPPIST-1 b.
“Our observations rule out thick, greenhouse gas-rich atmospheres, but also show that the two planets have followed divergent evolutionary paths despite their similar compositions,” explains Elsa Ducrot.
These results significantly narrow down the range of plausible scenarios for the nature of these two worlds and shed light on the ability of small planets orbiting very low-mass stars to retain an atmosphere under intense irradiation.
An iconic system: TRAPPIST-1
Discovered in 2015 by the team of Michaël Gillon at the University of Liège using the TRAPPIST telescope, and later studied with other instruments including NASA’s Spitzer Space Telescope (now retired), TRAPPIST-1 has become an emblematic planetary system.
It consists of seven temperate rocky planets with sizes comparable to Earth, three of which lie within the habitable zone. It fascinates both scientists and the general public and is now one of the best-characterized planetary systems after our own, as well as one of the most observed by JWST.
Its discovery was made within the framework of the SPECULOOS prototype, an ambitious project dedicated to the search for rocky planets orbiting nearby ultra-low-mass stars.
Atmospheres or not?
TRAPPIST-1 b and c receive approximately four and two times more radiation than Earth, respectively, and are tidally locked, always showing the same face to their star.
In such conditions, the presence of an atmosphere is crucial. A dense atmosphere would redistribute heat toward the nightside, reducing the temperature contrast. Conversely, an airless planet would exhibit a scorching dayside and a frigid nightside.
However, these planets are exposed to an extreme environment. Their host star emits intense X-ray and ultraviolet radiation, especially during its early evolution. These high-energy photons can heat and ionize the upper layers of an atmosphere, driving atmospheric escape. For close-in planets like TRAPPIST-1 b and c, this process can lead to the complete loss of an atmosphere on relatively short astronomical timescales.
Disentangling a combined signal
Thermal phase curves have so far only been measured for isolated planets, such as hot Jupiters or ultra-hot rocky exoplanets.
In the TRAPPIST-1 system, the situation is far more complex. The researchers had to extract simultaneously the signal of two small rocky planets, TRAPPIST-1 b and c, orbiting very close to their star, whose infrared emissions partially overlap.
Disentangling these intertwined signals required a highly sophisticated analysis capable of isolating the contribution of each planet from a single global signal. This achievement is based on nearly 60 hours of continuous observations with JWST, one of the most ambitious continuous observing programs ever conducted with the telescope
Worlds shaped by atmospheric erosion
The results indicate that atmospheric erosion processes are highly efficient for rocky planets in close orbits around ultracool dwarf stars. The intense X-ray and ultraviolet radiation from these stars appears capable of completely stripping such planets of their atmospheres, as strongly suggested by TRAPPIST-1 b.
However, TRAPPIST-1 c emerges as a different world. The observations remain consistent with the presence of a very thin atmosphere, suggesting that atmospheric evolution can diverge even between nearby planets with similar compositions.
These findings open an important perspective: some of the five other planets in the TRAPPIST-1 system, particularly the three located in the habitable zone, may have retained a thicker secondary atmosphere. Such an atmosphere is a key ingredient for maintaining surface conditions compatible with liquid water, and thus for potential habitability.
A key step for the search for life beyond earth
This first thermal mapping of temperate rocky planets marks a major breakthrough. It provides unprecedented constraints on the ability of these worlds to retain or lose their atmospheres, a fundamental parameter for assessing their habitability.
“With JWST, we are moving from an era of detection to an era of detailed characterization. We can now explore not only the size and mass of temperate Earth-sized exoplanets, but also their surface, climate, and evolution. This is a major step toward understanding their potential habitability,” summarizes Michaël Gillon.
This study demonstrates the power of JWST to directly probe the composition and evolution of temperate rocky planets and paves the way for detailed investigations of the three TRAPPIST-1 planets located in the habitable zone.
Reference
Study published in Nature Astronomy on 03 Avril 2026 under the title: “No thick atmosphere around TRAPPIST-1 b and c from JWST thermal phase curves”. Link to the article: https://www.nature.com/articles/s41550-026-02806-9
Contact
Michaël Gillon – Research Director – Fund for Scientific Research (FNRS) / University of Liège – Astrobiology Research Unit - ExoTIC Research Group – michael.gillon@uliege.be – +32 473 34 64 02
Elsa Ducrot – Associate Astronomer at the Observatory of the Sciences of the Universe Paris-Sud/CEA Paris-Saclay – elsa.ducrot@cea.fr – +33 624 291 668
