A team of astronomers used the James Webb Space Telescope to study the exoplanet TRAPPIST-1 c, which is one of seven rocky planets orbiting an ultracool red dwarf star 40 light-years from Earth. The team found that the planet’s atmosphere, if it exists at all, is extremely thin.
TRAPPIST-1 c is about the same size as Venus and receives a similar amount of radiation from its host star as Venus gets from the Sun. This led scientists to believe that the planet might have a thick carbon dioxide atmosphere like Venus. However, the Webb observations suggest that this is not the case.
The team used the Mid-Infrared Instrument on the Webb space telescope to observe TRAPPIST-1 c as it moved behind its star. Webb can detect the heat that comes from the planet when it passes behind the star, which is called a secondary eclipse. By comparing the brightness of the star when the planet was behind it to the brightness when the planet was in front of it, the team was able to measure the amount of heat energy coming from the planet’s dayside.
The amount of heat energy emitted by a planet is directly related to its temperature, which in turn is influenced by its atmosphere. Carbon dioxide gas preferentially absorbs heat energy at the wavelengths that MIRI is sensitive to, so if TRAPPIST-1 c had a thick carbon dioxide atmosphere, the team would have seen a smaller decrease in brightness when the planet moved behind its star.
However, the team did not see a significant decrease in brightness, which suggests that the planet’s atmosphere, if it exists at all, is very thin. The team estimates that the atmosphere of TRAPPIST-1 c is at least 100 times thinner than Earth’s atmosphere.
The lack of a thick atmosphere around TRAPPIST-1 c suggests that the planet may have formed with relatively little water. This is because water vapor is a major component of thick atmospheres, and if TRAPPIST-1 c had a lot of water vapor, it would have been easier for the planet to retain a thick atmosphere.
The absence of a thick atmosphere also makes it less likely that TRAPPIST-1 c is habitable. Habitable planets are thought to need thick atmospheres to protect them from harmful radiation and to help regulate their temperatures.
All is not lost however with regard to the TRAPPIST-1 system. This planet is after all, very close to the star and just outside the region where liquid water could exist on the surface. The observations of TRAPPIST-1c support the idea that planets close to their star have trouble holding on to an atmosphere, but that may not be the case with planets further out and actually inside the habitable zone, notably the planets TRAPPIST-1 e,f and g.
The findings of this study are a significant step forward in our understanding of exoplanets. The Webb space telescope is still in its early stages of operation, and scientists are excited to see what other discoveries it will make in the years to come.