Infrared measurements from the James Webb Space Telescope rule out a thick carbon dioxide atmosphere for Trappist-1 c. This rocky exoplanet therefore probably doesn’t resemble Venus as much as one might imagine.
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One by one, the planets of the Trappist 1 system are studied by the James Webb Space Telescope. The atmosphere of Trappist-1b, the planet closest to the star, had already proved insignificant. It is now Trappist-1 c, the spatially closest planet whose gas envelope was examined. This finding is the latest in a search to determine whether planetary atmospheres can withstand the harsh environment of a red dwarf star.
Trappist 1c under the gaze of James Webb
Trappist-1 is an Earth-sized planet (about 1.1 times the diameter of Earth and 1.3 times its mass) orbiting its star Trappist-1 in just 2.4 days. This planet is only 2.4 million kilometers from the red dwarf (just over six times the Earth-Moon distance and less than one-twentieth the Sun-Mercury distance) and 40 years from Earth’s light.
An international team of researchers used the James Webb Space Telescope’s Miri instrument to measure the amount of heat emitted by Trappist-1c, just as another team had done to determine that Trappist-1b is unlikely to have any heat. With a daily temperature of 107 (±31) degrees Celsius, Trappist-1c is the coldest rocky planet ever characterized by this method.
“We want to know whether the rocky planets have an atmosphere or not,” explains Sebastian Zieba, a student at the Max Planck Institute for Astronomy in Germany and first author of the study published in Nature. So far we have only been able to really study planets with thick, hydrogen-rich atmospheres. With Webb, we can finally begin searching for atmospheres dominated by oxygen, nitrogen, and carbon dioxide. »
A rocky planet with little or no atmosphere
The result suggests that the planet’s atmosphere, if it exists, is extremely thin. Although Trappist-1c is about the same size and mass as Venus, and receives the same amount of radiation from its star, it seems unlikely that it has the same dense carbon dioxide atmosphere.
While these first measurements don’t provide definitive information about the nature of Trappist-1c, they do help narrow down the likely possibilities. “Our results are consistent with a planet consisting of bare rock with no atmosphere, or a planet with a very thin CO2 atmosphere (thinner than Earth or even Mars) with no clouds,” Zieba said. The data also shows that the planet is unlikely to be a true analogue of Venus, with a thick CO2 atmosphere and sulfuric acid clouds.
The lack of a dense atmosphere suggests the planet may have formed with relatively little water. If the colder planets of Trappist-1 had formed under similar conditions, they too might have started out with little water and few other components needed to make a planet habitable.
In the coming year, researchers will use Webb to observe the full orbits of Trappist-1b and Trappist-1c. This will show how temperatures change between the day and night sides of the two planets and provide further constraints on whether or not they have atmospheres.