What kind of atmosphere could TRAPPIST-1 e have?
TRAPPIST-1 e is a rocky planet about 8% smaller and 31% less massive than Earth. It receives about 65% as much energy as Earth receives from the Sun and lies in the habitable zone, but neither water nor an atmosphere has been confirmed. Four Webb observations of the planet crossing its star made a thick atmosphere composed mostly of hydrogen very unlikely. They could not distinguish an atmosphere of heavier gases such as nitrogen, oxygen or carbon dioxide from no atmosphere at all, because starspots and flares keep changing the starlight.
- About 6.10 days Time required to orbit the star
- About 0.92 Earth radii Radius derived from starlight blocked in transit
- About 0.69 Earth masses Mass derived from gravity exchanged with neighboring planets
- About 0.65 of Earth’s Amount of starlight the planet receives
How can we detect a hydrogen-rich atmosphere?
When a planet passes in front of its star, some starlight travels through the atmosphere around the planet’s edge. Gases absorb particular kinds of light, so measuring how the blocked amount changes with wavelength can reveal their marks. A light gas such as hydrogen spreads to high altitudes and makes large marks. Heavy gases such as nitrogen or carbon dioxide remain lower and make much smaller ones.
Webb’s NIRSpec instrument analyzed four transits recorded in 2023, covering light from the red edge of the visible range through the near infrared. Even after allowing for clouds and low pressure, an atmosphere containing more than about 80% hydrogen did not fit the data. This means that a hydrogen-and-helium atmosphere captured when the planet formed is unlikely to have survived until today. It does not rule out atmospheres rich in nitrogen, oxygen or carbon dioxide.
Why did the results differ from one observation to the next?
TRAPPIST-1’s surface does not shine with the same brightness everywhere. Darker starspots, bright regions and brief outbursts change position and strength over time. When the color of the surface not covered by the planet changes, the amount blocked can vary with wavelength even if the planet has no atmosphere, creating something that looks like an atmospheric signal.
All four observations were affected by the star’s surface, and a large flare occurred during the third transit. Researchers treated the broad changes that differed at each visit as effects made by the star, then looked for smaller marks that repeated at the same wavelengths as possible signs of the planet’s atmosphere. Yet simple estimates of starspot temperature and size did not explain all changes in the starlight. That is why a small mark remaining after the stellar effect is removed cannot immediately be called a gas in the planet’s atmosphere.
Current data show that there is no thick hydrogen atmosphere. Researchers still do not know whether a thin atmosphere of heavier gases exists or whether there is no atmosphere at all.
Is a faint methane-like trace enough to claim a discovery?
After the changes made by the star were removed, an atmosphere of heavy gases such as nitrogen with a small amount of methane could explain some of the remaining marks. But a flat result with no atmospheric marks also fit the observations and, with some analysis methods, fit them better. The small marks seen today are therefore not strong enough to announce the discovery of methane or an atmosphere.
Models in which carbon dioxide makes up most of the atmosphere, as on Venus or Mars, also tended not to fit the data well. A heavy atmosphere containing a smaller amount of carbon dioxide remains possible. If liquid water exists, calculations show that this amount could produce a greenhouse effect, but the current observations have not directly found water, oceans or clouds.
Why observe b and e one after the other?
Researchers are making 15 additional observations at times when TRAPPIST-1 b crosses the star just before or after e. Planet b appears to be a rocky world without a thick atmosphere, so marks in b’s data can be treated as effects made by the star at nearly the same time. Those marks can then be removed from e’s data to look for changes that repeat only for e.
The first three visits reported in 2025 showed both the strength and the limit of this method. When the star was quiet, its effects could be reduced. But if a strong flare occurred between the two transits, the star could no longer be treated as unchanged. Even after the full data set is collected, observations affected by flares must be identified and the team must check how much the answer changes with the analysis method.
- e was observed four times The data ruled out a thick hydrogen-dominated atmosphere but showed large changes made by the star.
- Several atmospheres were compared Both no atmosphere and several atmospheres of heavy gases remained possible.
- b and e are observed in succession Planet b reveals marks made by the star at nearly the same time.
- Researchers look for marks left only in e The full set will test whether the same gas marks repeat.
What has been confirmed about e in the habitable zone?
The starlight e receives lies in a range that could allow liquid water at the surface. But the actual temperature depends on atmospheric pressure and composition, clouds, surface reflectivity and heat moving between day and night. If the same side always faces the star, models allow both an ocean spread around the planet and water remaining only near the region that faces the star. Neither landscape has been observed.
The first question to answer is not whether life exists, but whether there is an atmosphere. Only after repeated changes made by the star are separated out, and the same kinds of light show marks found only in e several times, can researchers begin to say which gases are present.
The ocean, clouds and surface shown on this page are not an observed photograph. Webb observations have made a thick hydrogen-dominated atmosphere very unlikely. Atmospheres of heavier gases, liquid water and life remain unconfirmed.
Sources
- NASA Exoplanet Archive — TRAPPIST-1 e
- NASA Science — Webb Looks at TRAPPIST-1 e
- The Astrophysical Journal Letters — Four NIRSpec Transmission Spectra
- The Astrophysical Journal Letters — Secondary Atmosphere Constraints
- Allen et al. — TRAPPIST-1 e/b Program and First Observations
- NASA Science — Webb Results for the TRAPPIST-1 System
- NASA Science — TRAPPIST-1 e Catalog