- TRAPPIST-1 b is the innermost rocky exoplanet in the TRAPPIST-1 planetary system. Like the other six planets, it’s about the same size as Earth.
- Does TRAPPIST-1 b have an atmosphere? Previous observations by the Webb Space Telescope suggested it does not. A new study of recent Webb data, however, indicates it might have one after all.
- The planet could have a hazy carbon dioxide atmosphere. Or it might have no atmosphere, but be highly geologically active with volcanoes or even tectonic plates.
Exoplanet TRAPPIST-1 b might have a hazy atmosphere
TRAPPIST-1 is one of the most intriguing known planetary systems found so far. It has no less than seven Earth-sized planets orbiting a red dwarf star. Scientists are still studying it with the Webb Space Telescope and trying to determine which of the planets might have atmospheres. The two innermost planets, TRAPPIST-1 b and c, were the first scientists looked at. Initial indications were that TRAPPIST-1 b did not have an atmosphere. But on December 16, 2024, a team of researchers said that might not be the case after all.
The researchers published their peer-reviewed results in Nature Astronomy on December 16, 2024. A free preprint version of the paper is also available on arXiv.
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Does TRAPPIST-1 b have an atmosphere?
Previous studies of data from Webb seemed to show the planet had no atmosphere, or very little at best. That wasn’t too surprising because it orbits so close to its red dwarf star, which emits intense radiation. A similar result was also obtained for TRAPPIST-1 c.
For both planets, the star’s radiation might strip away any atmosphere the planets had. Red dwarf stars are usually very active, more so than our own sun.
Two possibilities for TRAPPIST-1 b
The new study shows how difficult it is to detect and analyze atmospheres on planets that are so far away from us. That is especially true for smaller rocky planets like TRAPPIST-1’s. TRAPPIST-1 is 40 light-years from Earth.
The researchers used the Mid-Infrared Imager (MIRI) instrument on Webb to study TRAPPIST-1 b further. The results suggested two possibilities. Either the planet is highly geologically active with volcanoes or even plate tectonics but no weathering on its surface. Or, it could have a thin, hazy atmosphere, probably composed of carbon dioxide.
Is the planet geologically active?
The first possibility is that TRAPPIST-1 b has a weathered rocky surface but no atmosphere. That seemed to fit with the analysis of previous Webb data. But it doesn’t match up as well with the newer data. Astronomer Jeroen Bouwman at the Max Planck Institute for Astronomy (MPIA) said:
The idea of a rocky planet with a heavily weathered surface without an atmosphere is inconsistent with the current measurement. Therefore, we think the planet is covered with relatively unchanged material.
The new results suggest the surface is only about 1,000 years old at most. That’s really young, geologically speaking. That may indicate the planet’s crust is active and subject to extreme changes, such as through volcanism or plate tectonics. It could be like Jupiter’s moon Io. Jupiter’s gravitational pull squeezes and stretches the moon’s interior, generating heat. This powers Io’s hundreds of volcanoes. In fact, Io is the most active volcanic body in the whole solar system.
This explanation for TRAPPIST-1 b is still hypothetical, but it’s plausible.
Or does TRAPPIST-1 b have a hazy atmosphere?
The other potential scenario is also tantalizing. The planet might have a hazy atmosphere of carbon dioxide. Thomas Henning, also at the Max Planck Institute for Astronomy, explained:
The data also allow for an entirely different solution. Contrary to previous ideas, there are conditions under which the planet could have a thick atmosphere rich in carbon dioxide (CO2).
Previously, the observations of TRAPPIST-1 b seemed to preclude any atmosphere. That’s because the scientists measured the planet’s brightness at different wavelengths. All of those were in the thermal infrared range, between 12.8 and 15 micrometers. The first observation should have detected carbon dioxide if it were present. But it didn’t. The carbon dioxide would be expected to absorb the planet’s infrared radiation. That would cause the planet to dim in brightness as seen by Webb. There might, however, be another explanation for that: haze.
How haze could explain Webb observations
The new study showed carbon dioxide could reverse the usual pattern. Instead of the lower layers of the atmosphere being warmer, haze would heat the upper layers more. The warmer carbon dioxide in the upper atmosphere would emit the infrared radiation, which Webb could detect. In other words, instead of the measured infrared radiation meaning no carbon dioxide atmosphere, the radiation would actually be produced by the atmosphere itself. This is similar to what happens on Saturn’s moon Titan. Titan has a thick atmosphere – the only moon that does – with a layer of dense smog-like haze composed of hydrocarbons.
While also plausible, and it does also fit the data, the researchers currently consider this scenario to be less likely than the first. It would be surprising if TRAPPIST-1 b still had a substantial atmosphere. Like most other red dwarfs, its star emits intense radiation that can strip nearby planets of their atmospheres.
The transit method
It is still difficult to even detect atmospheres on smaller rocky planets like those in the TRAPPIST-1 system. The researchers noted that the two observations cited in the study are not enough. Additional observations will be required to determine if TRAPPIST-1 b actually does have an atmosphere … or not.
Webb observes the seven planets of TRAPPIST-1 as they transit in front of its star, as seen from Earth. This is called the transit method, or transit spectroscopy. As a planet passes in front of its star, it dims the starlight slightly. Scientists can measure that to determine things like the planet’s size and if it has an atmosphere. Sometimes they can also determine what the atmosphere is composed of.
But red dwarf stars like TRAPPIST-1 often have large starspots – like our sun’s sunspots – and powerful solar eruptions. Those can make transit observations even more difficult. So astronomers also use thermal infrared light observations, like they did with TRAPPIST-1 b. As with the transit method, the infrared radiation also contains clues about a planet’s surface and atmosphere.
Infrared temperature clues
Astronomers have already made these infrared observations of TRAPPIST-1 b. But the data are still being fully analyzed, and, as noted, further observations will be needed. For example, if astronomers see the temperature of the planet abruptly change at the division between the day side and night side, that would be strong evidence for no atmosphere. That’s because an atmosphere would typically carry heat from the day side to the night side. But with no atmosphere, the day side would remain hot while the night side remained cold.
Bottom line: Does exoplanet TRAPPIST-1 b have an atmosphere? A new study said it either has a hazy atmosphere or is geologically active, with volcanoes or plate tectonics.
Source: Combined analysis of the 12.8 and 15 um JWST/MIRI eclipse observations of TRAPPIST-1 b
Source (preprint): Combined analysis of the 12.8 and 15 um JWST/MIRI eclipse observations of TRAPPIST-1 b
Via Max Planck Society
Read more: Some TRAPPIST-1 exoplanets could have stable atmospheres
Read more: Are the TRAPPIST-1 exoplanets habitable, or not?