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The researchers observed the planets TRAPPIST-1 b and c, two worlds receiving respectively four and two times more radiation than Earth, by monitoring the evolution of their infrared emission throughout their orbit.
Credit: NASA/JPL-Caltech/R. Hurt (Caltech-IPAC)
This technique, called "thermal phase curve," allows for the first time a direct comparison of the temperature between the day side and the night side of temperate rocky planets outside our Solar System. These planets have the particularity of always showing the same face to their star, just like the Moon does to Earth.
The results show that TRAPPIST-1 b likely has no significant atmosphere. Its day side reaches nearly 500 K (227 °C), while its night side remains extremely cold, a behavior expected for a rocky planet without an atmosphere and covered in dark rocks.
TRAPPIST-1 c presents a less extreme situation: its day side (~370 K) is still significantly hotter than its night side (less than 260 K). The thermal contrast remains significant, which rules out the presence of dense atmospheres like those of Venus and Earth. However, the data remain compatible with the existence of a thin atmosphere or with different surface properties (such as its reflectivity) compared to TRAPPIST-1b.
Complementary observations with JWST are underway to refine this interpretation.
Top, thermal phase curve of exoplanet TRAPPIST-1b extracted from JWST observations. These observations reveal that the planet exhibits an extremely marked day-night thermal contrast, explained by the absence of an atmosphere and a very dark surface.
Bottom, surface temperature map of TRAPPIST-1b deduced from the thermal phase curve obtained with JWST.
These results significantly tighten the plausible scenarios regarding the nature of these two worlds and teach us more about the ability of small planets around very low-mass stars to retain an atmosphere in the face of intense radiation.
The results are published in Nature Astronomy under the title: “No thick atmosphere around TRAPPIST-1 b and c from JWST thermal phase curves”
Study led by Michaël Gillon (University of Liège, FNRS) and Elsa Ducrot (CEA Paris-Saclay/Observatory of Paris, ULiège)