Webb rules out that TRAPPIST-1 c is really a Venus analogue

This artist’s concept shows what the hot, rocky exoplanet TRAPPIST-1 c might look like based on this work. – NASA, ESA, CSA, JOSEPH OLMSTED (STSCI

The Webb Space Telescope has made it possible to calculate the amount of thermal energy coming from the rocky exoplanet TRAPPIST-1 c. The result suggests that its atmosphere, if it exists, is very thin.

“TRAPPIST-1c is interesting because it is basically a twin of Venus: it is about the same size as Venus and receives a similar amount of radiation from its host star that Venus receives from the Sun,” Laura Kreidberg of the Max Planck Institute explained in a statement. of Astronomy and co-author of the study. “We think it could have a thick carbon dioxide atmosphere like Venus.” But the data collected by Webb shows that the planet is unlikely to be a true analogue of Venus with a thick atmosphere of CO2 and clouds of sulfuric acid. With a daytime temperature of about 380 kelvins (about 107 degrees Celsius), TRAPPIST-1 c is now the coldest rocky exoplanet ever characterized based on thermal output. The precision required for these measurements further demonstrates Webb’s usefulness in characterizing rocky exoplanets similar in size and temperature to those in our own solar system.

The result marks another step in determining whether planets orbiting small red dwarfs like TRAPPIST-1, the most common type of star in the galaxy, can sustain the atmospheres necessary to support life as we know it. “We want to know whether rocky planets have atmospheres or not,” said Sebastian Zieba, a graduate student at the Max Planck Institute for Astronomy in Germany and first author of the results published in Nature. “In the past, we could only study planets with dense, hydrogen-rich atmospheres. With Webb we can finally start looking for atmospheres dominated by oxygen, nitrogen, and carbon dioxide.” TRAPPIST-1 c is one of seven rocky planets orbiting an ultracool red dwarf (or M dwarf) star 40 light-years from Earth. Although the planets are similar in size and mass to the rocky inner planets of our own solar system, it is not clear whether they in fact have similar atmospheres. During the first billion years of their lives, M dwarfs emit bright X-rays and ultraviolet radiation that can easily wash away a young planetary atmosphere. Also, there may or may not be enough water, carbon dioxide, and other volatiles available to create substantial atmospheres when the planets formed.

To address these questions, the team used MIRI (Webb’s Mid-Infrared Instrument) to observe the TRAPPIST-1 system on four separate occasions as the planet moved behind the star, a phenomenon known as a secondary eclipse. By comparing the brightness when the planet is behind the star (starlight only) to the brightness when the planet is next to the star (starlight and planet combined), the team was able to calculate the amount of mid-infrared light with wavelengths of 15 microns emitted from the day side of the planet. This method is the same as another research team used to determine that TRAPPIST-1 b, the innermost planet in the system, probably has no atmosphere. The amount of mid-infrared light emitted by a planet is directly related to its temperature, which in turn is influenced by the atmosphere. Carbon dioxide gas preferentially absorbs 15-micron light, making the planet appear dimmer at that wavelength. However, clouds can reflect light, making the planet appear brighter and masking the presence of carbon dioxide.

Also, a substantial atmosphere of any composition will redistribute heat from the dayside to the nightside, making the dayside temperature lower than it would be without an atmosphere. (Because TRAPPIST-1 c orbits so close to its star, about 1/50th of the distance between Venus and the Sun, it is thought to be tidally locked, with one side in perpetual daylight and the other in infinite darkness.) . Although these initial measurements do not provide definitive information about the nature of TRAPPIST-1 c, they do help narrow down the potential possibilities. “Our results are consistent with either the planet being a bare rock with no atmosphere, or the planet having a really thin (thinner than Earth or even Mars) CO2 atmosphere with no clouds,” Zieba said. “If the planet had a thick CO2 atmosphere, we would have observed a very shallow secondary eclipse, or none at all. This is because the CO2 would absorb all 15-micron light, so we wouldn’t detect any coming from the planet.” .

source: No thick carbon dioxide atmosphere on the rocky exoplanet TRAPPIST-1 c