TRAPPIST-1 c’s infrared data suggest that it is probably not as resembling Venus as formerly thought. The heat emitted by TRAPPIST-1 c, an exoplanet circling a red dwarf star 40 light-years from Earth, has been successfully measured by NASA’s James Webb Space Telescope. It is the coolest rocky planet yet discovered using this method, with a dayside temperature of around 225 degrees Fahrenheit.
The results are quite underwhelming, which is unfortunate for those who hoped that the TRAPPIST-1 system would truly be an exact replica of our own. Although TRAPPIST-1 c has a mass and size that are similar to Venus and receives the same amount of radiation from its star, it doesn’t seem possible that it has the same quantity of carbon dioxide in its atmosphere.
This suggests that there may not have been much water present during planet’s formation, or even across the entire solar system. The finding is the most recent in the ongoing investigation into whether planetary atmospheres can withstand the harsh conditions around a red dwarf star.
An international team of researchers has used NASA’s James Webb Space Telescope to calculate the amount of heat energy coming from the rocky exoplanet TRAPPIST-1 c. The result suggests that the planet’s atmosphere if it exists at all is extremely thin.
With a dayside temperature of roughly 380 Kelvin (about 225 degrees Fahrenheit), TRAPPIST-1 c is now the coolest rocky exoplanet ever characterized based on thermal emission. The accuracy required for these measurements further exemplifies Webb’s usefulness in describing rocky exoplanets with temperatures and sizes similar to those in our own solar system.
The finding is another step in the process of figuring out if planets orbiting small red dwarf stars, like TRAPPIST-1, the most prevalent kind of star in the galaxy, can maintain the kinds of atmospheres necessary to support life as we know it.
Our results are consistent with the planet being a bare rock with no atmosphere, or the planet having a really thin CO2 atmosphere (thinner than on Earth or even Mars) with no clouds. 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.
Sebastian Zieba
“We want to know if rocky planets have atmospheres or not,” said Sebastian Zieba, a graduate student at the Max Planck Institute for Astronomy in Germany and first author on results being published today in Nature. “In the past, we could only really study planets with thick, hydrogen-rich atmospheres. With Webb we can finally start to search for atmospheres dominated by oxygen, nitrogen, and carbon dioxide.”
“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,” explained co-author Laura Kreidberg, also from Max Planck. “We thought it could have a thick carbon dioxide atmosphere like Venus.”
TRAPPIST-1 c is one of seven rocky planets orbiting an ultracool red dwarf star (or M dwarf) 40 light-years from Earth. It is unclear whether the planets’ atmospheres are comparable to those of the inner, rocky planets in our own solar system, despite the fact that they are comparable in size and mass.
Bright X-ray and ultraviolet radiation from M dwarfs during the first billion years of their existence can readily take away a newborn planetary atmosphere. Additionally, when the planets formed, there may or may not have been sufficient amounts of water, carbon dioxide, and other volatiles to produce significant atmospheres.
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.
The team was able to determine the amount of mid-infrared light with wavelengths of 15 microns emitted by the dayside of the planet by comparing the brightness when the planet is behind the star (starlight only) to the brightness when the planet is beside the star (light from the star and planet combined).
This method is the same as that used by another research team to determine that TRAPPIST-1 b, the innermost planet in the system, is probably devoid of any atmosphere.
The amount of mid-infrared light emitted by a planet is directly related to its temperature, which is in turn influenced by atmosphere. The planet appears darker at 15-micron wavelengths because carbon dioxide gas preferentially absorbs that wavelength. Clouds, however, can reflect light, giving the impression that the planet is brighter and hiding the existence of carbon dioxide.
A significant atmosphere of any composition will also shift heat from the dayside to the nightside, resulting in a lower dayside temperature than would otherwise be the case. (Because TRAPPIST-1 c orbits so close to its star about 1/50th the distance between Venus and the sun it is thought to be tidally locked, with one side in perpetual daylight and the other in endless darkness.)
Although these initial measurements do not provide definitive information about the nature of TRAPPIST-1 c, they do help narrow down the likely possibilities.
“Our results are consistent with the planet being a bare rock with no atmosphere, or the planet having a really thin CO2 atmosphere (thinner than on Earth or even Mars) with no clouds,” said Zieba. “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.”
The data also show that it is unlikely the planet is a true Venus analog with a thick CO2 atmosphere and sulfuric acid clouds.
The lack of a dense atmosphere means that there may not have been much water present during planet formation. If the cooler, more temperate TRAPPIST-1 planets developed under comparable circumstances, they might have also had a dearth of the water and other elements required to make a planet livable at the beginning.
On such a little planet so far away, the sensitivity needed to discern between different atmospheric situations is simply amazing. The decrease in brightness that Webb detected during the secondary eclipse was just 0.04 percent: equivalent to looking at a display of 10,000 tiny light bulbs and noticing that just four have gone out.
“It is extraordinary that we can measure this,” said Kreidberg. “There have been questions for decades now about whether rocky planets can keep atmospheres. Webb’s ability really brings us into a regime where we can start to compare exoplanet systems to our solar system in a way that we never have before.”
This research was conducted as part of Webb’s General Observers (GO) program 2304, which is one of eight programs from Webb’s first year of science designed to help fully characterize the TRAPPIST-1 system.
This coming year, researchers will conduct a follow-up investigation to observe the full orbits of TRAPPIST-1 b and TRAPPIST-1 c. This will allow researchers to observe how the two planets’ temperatures vary from day to night and put additional restrictions on whether or not they have atmospheres.
