This rocky planet lacks thick carbon dioxide atmosphere like Venus

Despite being similar to Venus, both in size and amounts of radiation from its star, TRAPPIST-1 c is unlikely a true Venus analog as the rocky exoplanet lacks a thick carbon dioxide atmosphere. And if it exists at all - is extremely thin, according to a new study that used the James Webb Space Telescope to calculate the amount of heat energy coming from this world.

TRAPPIST-1 c is the second of seven known planets orbiting an ultracool red dwarf star 40 light-years from Earth.

The team, led by Sebastian Zieba, a graduate student at the Max Planck Institute for Astronomy in Germany, used Webb's Mid-Infrared Instrument (MIRI) 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 during the secondary eclipse (starlight only) with the brightness when the planet is beside the star (combined light from the star and planet), the researchers measured the amount of mid-infrared light with 15-micron wavelengths emitted by the planet's dayside.

Carbon dioxide gas can absorb 15-micron light, resulting in a dimmer appearance at that specific wavelength. However, the presence of clouds can reflect light, making the planet appear brighter and potentially masking the existence of carbon dioxide.

Furthermore, any substantial atmosphere, regardless of its composition, would redistribute heat from the dayside to the nightside, leading to a lower temperature on the dayside compared to what it would be in the absence of an atmosphere. Given TRAPPIST-1 c's close proximity to its star (approximately 1/50th the distance between Venus and the Sun), it is believed to be tidally locked, with one side in perpetual daylight and the other in continuous darkness.

"If the planet had a thick CO2 atmosphere, we would have observed a really shallow secondary eclipse or none at all. This is because the CO2 would be absorbing all of the 15-micron light, so we wouldn't detect any coming from the planet," Zieba said.

The absence of a thick atmosphere suggests that TRAPPIST-1 c may have formed with relatively little water, the researchers said.

"TRAPPIST-1 c is interesting because it's basically a Venus twin: It's about the same size as Venus and receives a similar amount of radiation from its host star as Venus gets from the Sun. We thought it could have a thick carbon dioxide atmosphere like Venus," stated co-author Laura Kreidberg, also from Max Planck."

TRAPPIST-1 b lacked atmosphere. TRAPPIST-1 c? Much the same. Though thought to be similar to Venus (both in size and amounts of radiation from its star), Webb found it lacks Venus’s thick CO2-rich atmosphere. If there is one, it’s very thin. https://t.co/41DctdtCNy pic.twitter.com/fZ0Hh68a99

— NASA Webb Telescope (@NASAWebb) June 19, 2023