New findings made possible by the James Webb Space Telescope reveal that the Earth-like exoplanet Trappist-1 b may have an atmosphere after all, potentially overturning earlier findings.
Approximately forty million light years from Earth, seven planets orbit the star Trappist-1, three of which–Trappist-1e, f, and g–exist in the star’s habitable zone and could theoretically be warm enough to allow for liquid water on their surfaces. Since its launch, the James Webb Space Telescope (JWST) has logged 290 hours observing the Trappist-1 system across ten research endeavors.
Elsa Ducort of Paris’s Commissariat aux Énergies Atomiques (CEA) led the study team, which analyzed infrared thermal radiation readings collected by JWST’s Mid-Infared Imager (MIRI) that drove last year’s conclusion that Trappist-1 b is unlikely to have an atmosphere.
“However, the idea of a rocky planet with a heavily weathered surface without an atmosphere is inconsistent with the current measurement,” said Jeroen Bouwman, an astronomer at Heidelberg’s Max Planck Institute for Astronomy (MPIA). “Therefore, we think the planet is covered with relatively unchanged material.”
The Young Surface of Trappist-1 b
Typically, intense star radiation and meteorite impacts over long periods would weather any rocky planet lacking an atmosphere. Trappist-1 b, however, features a crust only about 1,000 years old, far younger than the billions of years the planet has existed. Possible explanations for the planet’s shifting crust include volcanism or plate tectonics, although there is no evidence to prove such causation yet.
Scientists have several theories on how Trappist-1 b may retain enough heat to drive such activities. Because of its size, the planet may have retained much of the heat produced during its formation, similar to how Earth once did. The gravitational pull of its star and nearby planets may generate friction inside the planet that produces heat, something that astronomers have observed on Jupiter’s moon Io. Lastly, some nearby stars could cause heat through their magnetic fields.
“The data also allow for an entirely different solution,” says Thomas Henning, emeritus director of the MPIA and one of the principal architects of the MIRI instrument. “Contrary to previous ideas, there are conditions under which the planet could have a thick atmosphere rich in carbon dioxide (CO2).”
Carbon Dioxide Smog
The missing piece of the puzzle could be a hydrocarbon smog covering the upper atmosphere. Two Trappist-1 b observation initiatives measured the planet’s brightness in wavelengths outside the thermal infrared range. One was targeted to observe if a layer of CO2 was absorbing infrared radiation, but this method’s inability to detect any dimming suggested to researchers that there was no atmosphere.
The team then developed models that better understood how such a Co2-rich atmosphere would behave. As pressure builds from the top down, lower levels of an atmosphere are usually warmer than those above.
The team’s models showed that the CO2 haze they were expecting would behave much differently. In such an atmosphere, the upper layer would be extremely warm due to the CO2-absorbing starlight, heating up to a runaway greenhouse effect. In such a scenario, the upper atmosphere’s carbon dioxide would emit infrared radiation, like the methane from Saturn’s moon, Titan.
The Jury is Still Out on Trappist-1 b
Despite what the model reveals, some researchers are unconvinced even if the new information fits the real-world data. They point to the difficulty in getting CO2 atmospheres to create hydrocarbon clouds. While the effect occurs on Titan, those clouds are methane, not hydrocarbon. Additionally, as a red dwarf star, Trappist-1 can produce volatile space weather that could decimate the atmosphere of its surrounding planets over time.
While the JWST has been a significant game changer in observing the universe, NASA’s premier space observatory still has limits. Rocky planets like Earth produce much weaker measurements than enormous gas giants. Even though the two studies observed Trappist-1 b for almost 48 hours, they could still not collect enough data to determine conclusively whether it had an atmosphere.
Observing Exoplanets
During the recent observations, Trappist-1’s planets were viewed as they passed before the star, reducing the red dwarf’s brilliance. The transit spectroscopy method measures the wavelength of the dimming, and since different gases absorb different wavelengths, the process can determine what gases are present.
Although this method is usually effective, the volatility of a red dwarf star like Trappist-1 makes it difficult to obtain precise measurements. It’s challenging to distinguish between the dimming caused by a passing object and the variations in brightness due to star spots and eruptions.
Another method that mitigates some of these issues is to look at the side of the planet heated by the star in thermal infrared. Right before a planet passes behind its star, the starlight hitting its surface is easy to view. The infrared radiation collected provides many clues about a planet’s surface and atmosphere, but collecting that data is much more time-consuming than transit spectroscopy.
Next Steps For Finding Earth-like Planets
The team’s next step will be to analyze data from their most intensive collection effort yet and finally judge whether the planet possesses an atmosphere. By recording Trappist-1 b’s complete orbit, researchers acquired data on all illumination phases of its night side and the dayside right before and after the star covers it.
Astronomers will use this data to build a “phase curve” demonstrating the brightness variation, which, in turn, will reveal surface temperature distribution. If the temperature drops drastically during the day-to-night transition, it will indicate the lack of an atmosphere to retain heat.
The Trappist-1 effort is only one of many initiatives to investigate the atmospheres of Earth-like planets. NASA recently approved a program called Rocky Worlds, which will allow 500 hours of observing the atmospheres of rocky planets.
The new paper “Combined Analysis of the 12.8 and 15 μm JWST/MIRI Eclipse Observations of TRAPPIST-1 b” appeared in Nature Astronomy on December 16, 2024.
Ryan Whalen covers science and technology for The Debrief. He holds a BA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.