Using the James Webb Space Telescope (JWST) and its Mid-Infrared Imager (MIRI), researchers - including those from the Max Planck Institute for Astronomy (MPIA) - observed Trappist-1 b to measure its thermal radiation. The study, led by Elsa Ducrot from the Commissariat aux Energies Atomiques (CEA) in Paris, builds on prior findings that described the planet as a dark, rocky body with no atmosphere. However, new data challenges this earlier conclusion.
"However, the idea of a rocky planet with a heavily weathered surface without an atmosphere is inconsistent with the current measurement," says MPIA astronomer Jeroen Bouwman, who was jointly responsible for the observation programme. Instead, the planet's surface material appears relatively young - potentially less than 1,000 years old - suggesting geologic activity such as extreme volcanism or tectonics. Such activity could be fueled by tidal forces exerted by Trappist-1 and the system's other planets, similar to the volcanic processes seen on Jupiter's moon Io.
An alternative interpretation proposed by Thomas Henning, emeritus director at MPIA and key architect of the MIRI instrument, suggests Trappist-1 b could possess a thick carbon dioxide (CO2) atmosphere, despite prior findings to the contrary. "Contrary to previous ideas, there are conditions under which the planet could have a thick atmosphere rich in carbon dioxide (CO2)," he adds. This scenario depends on haze, possibly hydrocarbon compounds, forming in the planet's upper atmosphere.
The data stems from two complementary JWST observations, which measured the planet's infrared brightness at 12.8 and 15 micrometres - wavelengths sensitive to CO2 absorption. While no clear absorption was detected, haze could alter the thermal structure of a CO2-rich atmosphere, causing it to radiate infrared light at measurable levels. A similar process occurs on Saturn's moon Titan, where ultraviolet radiation creates smog-like haze in its methane-rich atmosphere.
However, astronomers caution that this scenario remains unlikely for Trappist-1 b. Red dwarf stars like Trappist-1 emit strong stellar winds and radiation that can erode planetary atmospheres over billions of years. Additionally, CO2 atmospheres are less likely to produce the kind of hydrocarbon haze observed on Titan, which has a methane-dominated atmosphere.
Detecting atmospheres on rocky planets remains challenging, even for the JWST. Unlike gas giants, rocky planets produce faint signals, requiring extended observations. The JWST team relied on "secondary eclipse" observations - tracking the planet's infrared emissions as it passes behind its star. These methods offer insights into surface temperatures and atmospheric composition but demand significant observation time.
NASA recently approved an extensive JWST programme, "Rocky Worlds," dedicating 500 hours to studying rocky planets around nearby stars like Trappist-1. This programme aims to clarify whether planets like Trappist-1 b retain atmospheres and the role of stellar environments in their evolution.
The team plans to refine their conclusions by analyzing Trappist-1 b's full orbital phases. By tracking temperature changes across the planet's surface - from its dayside to the shadowed nightside - astronomers can determine if an atmosphere exists. "If the temperature changes abruptly at the transition between the two sides, this indicates the absence of an atmosphere."
With continued JWST observations, scientists hope to resolve the debate over Trappist-1 b's atmosphere and better understand the complex processes shaping planets in red dwarf systems.
Research Report:Combined analysis of the 12.8 and 15 um JWST/MIRI eclipse observations of TRAPPIST-1 b
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