Could the planet TRAPPIST-1 e, located in the habitable zone of its red dwarf star, host liquid water and perhaps even life? Its ability to retain an atmosphere is a major piece of this puzzle, an element that scientists are struggling to confirm despite recent advances.
Studies published in
The Astrophysical Journal Letters present the first detailed observations of the TRAPPIST-1 system obtained with NASA's James Webb Space Telescope. This work, led by researchers such as Sukrit Ranjan from the University of Arizona, analyzes initial data and proposes several scenarios for the atmosphere and surface of this Earth-sized exoplanet.
Artist's impression of TRAPPIST-1 e transiting its red dwarf star.
Credit: NASA, ESA, CSA, J. Olmsted (STScI)
The analysis relies on a phenomenon called a transit, when TRAPPIST-1 e passes in front of its star. During this event, starlight passes through its potential atmosphere, a method that allows for the search for chemicals like methane. Four observed transits did indeed reveal hints of this molecule, but their interpretation remains tricky for astronomers.
A major complication stems from the nature of the host star, TRAPPIST-1. This so-called "ultracool" red dwarf, much smaller and cooler than our Sun, can itself generate methane in its stellar envelope. This peculiarity muddies the waters and complicates distinguishing between signals from the planet and those from the star, requiring increased caution in the analysis.
To clarify this situation, Ranjan's team simulated models where TRAPPIST-1 e possesses a methane-rich atmosphere. The most plausible scenario would then evoke a world similar to Titan, Saturn's moon. However, this option itself appears unlikely with current information. The researchers indicate that the detected signals might simply correspond to stellar noise.
Future missions could provide clearer answers. This is notably the case for Pandora, a small NASA satellite planned for 2026. Designed to characterize exoplanet atmospheres, it will observe stars before, during, and after transits, which should deliver more precise data on potentially habitable worlds.
In the meantime, scientists are refining their methods. They are focusing on a technique called double transit, simultaneously observing TRAPPIST-1 e and an atmosphere-less planet in the same system. This comparative approach should help better separate stellar effects from planetary signals, although further observations are still needed to reach a certainty (explanation at the end of the article).
The transit method for studying atmospheres
The transit method is a common technique in astronomy for detecting and analyzing exoplanets. It relies on observing the slight dip in a star's brightness when a planet passes in front of it, an event called a transit.
During a transit, the star's light passes through the planet's atmosphere, if it has one. Atmospheric molecules absorb certain wavelengths, leaving a chemical fingerprint that instruments like the NIRSpec spectrograph on the James Webb telescope can measure.
This approach allows for the identification of gases like methane, water, or carbon dioxide, providing clues about composition and surface conditions. It is particularly useful for small terrestrial planets, where other methods are less effective.
However, the method has limitations, especially with active stars like red dwarfs. Stellar variations can mask planetary signals, requiring advanced techniques and multiple observations to confirm results, as is the case for TRAPPIST-1 e.