Key Takeaways:
- JWST observations of exoplanet TRAPPIST-1d reveal a lack of a substantial, Earth-like atmosphere, as indicated by a "flat" spectral signature lacking typical atmospheric molecule fingerprints (e.g., CO2, CH4, H2O).
- The absence of a significant atmosphere diminishes the planet's potential habitability, as such an atmosphere is crucial for maintaining surface liquid water.
- This finding follows similar JWST observations of other TRAPPIST-1 planets (1b and 1c), indicating a trend of atmospheric limitations within the system, possibly linked to proximity to the star's intense activity.
- Future research will focus on the outer TRAPPIST-1 planets (e, f, g, and h), which may retain atmospheres more effectively due to increased distance from the star; however, detection at greater distances will present increased challenges.
Recent observations from the James Webb Space Telescope (JWST) delivered a blow to hopes that the exoplanet TRAPPIST-1 d could be an Earth-like world, according to a new study published in The Astrophysical Journal. The data reveal that the rocky, Earth-sized planet does not possess a thick, Earth-like atmosphere.
The finding is critical because a substantial atmosphere is necessary to maintain the right pressure and temperature for liquid water to remain stable on a planet’s surface. Without a stable and substantial reservoir of surface water, the potential for a habitable world dwindles.
A seven-world system
The TRAPPIST-1 system, located about 40 light-years away, is a planetary system that provides astronomers an excellent opportunity for studying possible habitable worlds. It consists of seven rocky, Earth-sized planets orbiting a volatile, ultracool red dwarf star — the most common type of star in our galaxy. Because red dwarfs are so numerous, confirming that planets can successfully hold on to their atmospheres in such a harsh environment would greatly increase the chances of finding other habitable worlds.
The search for life often begins with the “habitable zone,” also known as the “Goldilocks zone” — the distance from a star where a planet receives just enough energy for liquid water to theoretically exist on its surface.
For a dim, cool star like TRAPPIST-1, this habitable zone is much tighter and closer than in our solar system. The subject of the new study, TRAPPIST-1 d, is the third planet from the star. It is a rocky world similar in size to Earth that completes a full orbit, its year, in only four Earth days, orbiting at a distance that is just 2 percent of that between Earth and the Sun.
The TRAPPIST-1 system became a focal point for astronomers in 2017, when NASA announced the discovery of a record-breaking seven Earth-sized planets orbiting the single star. Subsequent research built a complex picture of these worlds.
A 2018 study provided a much clearer picture by pinning down the density of each planet with greater precision, suggesting some TRAPPIST-1 planets could harbor far more water than Earth’s oceans. According to NASA, this work made TRAPPIST-1 the “most thoroughly known planetary system apart from our own.”
A 2021 study added another layer, finding that the planets are likely made of similar materials but are about 8 percent less dense than if they shared Earth’s composition.
However, the picture began to shift as the Webb telescope started delivering its first direct observations. In March 2023, an analysis of the innermost planet, TRAPPIST-1b, found little evidence of an atmosphere. And a study from June 2023 found that the next planet out, TRAPPIST-1 c, has only a thin atmosphere — if it has any at all.
The latest study on TRAPPIST-1 d continues this trend.
No signs of Earth-like atmosphere
To probe the planet for air, the team used a technique called transmission spectroscopy. This method involves analyzing starlight after it has filtered through a planet’s atmosphere as it passes in front of its star from our point of view. Using Webb’s Near-Infrared Spectrograph (NIRSpec) instrument, scientists can split this light into a spectrum and look for specific colors that have been absorbed by gases. Each gas leaves a unique chemical fingerprint.
For TRAPPIST-1 d, the spectrum was flat. The study notes this “flat, or muted” signal showed none of the tell-tale fingerprints of molecules common to Earth’s atmosphere, such as carbon dioxide, methane, or water. The result, the paper concluded, “enables us to place stringent constraints on the presence of an atmosphere.” Given how tight these constraints are, there could well be no atmosphere at all.
There are a couple of other possibilities for TRAPPIST-1 d, however, as Caroline Piaulet-Ghorayeb, the study’s lead author, outlined in a news release. The planet “could have an extremely thin atmosphere that is difficult to detect, somewhat like Mars. Alternatively, it could have very thick, high-altitude clouds that are blocking our detection of specific atmospheric signatures — something more like Venus. Or,” she concluded, ‘it could be a barren rock, with no atmosphere at all.”
What’s next for TRAPPIST-1?
These findings have significant implications for future research. While TRAPPIST-1 d may be ruled out as an Earth twin, the investigation into the system is far from over. Next, scientists turn their attention to the outer planets — e, f, g, and h. These worlds may have a better chance of retaining their atmospheres because they are farther from the host star’s volatile flares.
However, their greater distance and colder temperatures will make detecting any atmospheric signatures more challenging. The work serves as crucial groundwork for future missions by helping scientists “define the line between planets that can hold onto an atmosphere, and those that cannot,” Björn Benneke, study co-author, noted.
While the result for TRAPPIST-1 d might seem disappointing, it underscores the unique nature of our planet. As co-author Ryan MacDonald said in a statement, the research is just beginning. “Thanks to Webb, we now know that TRAPPIST-1 d is a far cry from a hospitable world. We’re learning that the Earth is even more special in the cosmos.”s
