Scientists using the James Webb Space Telescope are confronting a tantalizing possibility about TRAPPIST-1e: the rocky exoplanet may have lost an early atmosphere to its volatile star, then started over. The emerging picture is complex—and essential to understanding whether Earth-sized worlds around red dwarfs can stay habitable.
Early Webb measurements suggest TRAPPIST-1e lacks a puffy, hydrogen-rich envelope, the easiest kind of atmosphere for telescopes to detect. That finding fits with expectations for planets bathed in the intense radiation of an ultracool dwarf. Yet researchers say it does not rule out a “secondary” atmosphere built later from volcanic outgassing, cometary delivery, or chemical reactions on and within the planet.
Why scientists suspect atmospheric loss
TRAPPIST-1 is a small, active red dwarf about 40 light-years away, orbited by seven rocky worlds roughly Earth-sized. The system is a stress test for habitability theories because such stars bombard their planets with flares and high-energy radiation, especially in their youth. Studies led by teams at Cornell University and MIT, published in Astrophysical Journal Letters, indicate that TRAPPIST-1e almost certainly does not retain a primordial hydrogen-helium atmosphere.
Webb’s transmission spectroscopy—watching starlight filter through a planet’s air during transits—did not show the broad spectral imprints expected from a thick, lightweight atmosphere. That outcome echoes a recent result for neighboring TRAPPIST-1d and aligns with models from NASA and the Space Telescope Science Institute showing that intense X-ray and ultraviolet flux from M dwarfs can strip light gases from close-in planets on geologically short timescales.
There’s a catch: red dwarf surfaces are mottled with starspots and faculae, which can imprint their own signals on the spectrum and masquerade as atmospheric features. Disentangling the star from the planet is now central to the analysis.
Could TRAPPIST-1e rebuild its air?
Earth likely shed its early hydrogen, then developed a denser atmosphere from volcanic outgassing, later modified by life. TRAPPIST-1e could have followed an analogous path, especially if it maintained internal heat to power long-lived volcanism. Secondary atmospheres around rocky planets are typically richer in heavier molecules such as carbon dioxide, nitrogen, and water vapor—harder to erode and harder to detect, but potentially adequate to keep surface water stable.
Climate models from multiple groups, including NASA Goddard and the University of Chicago, show that a tidally locked planet like TRAPPIST-1e could sustain temperate conditions if it carries enough greenhouse gases to transport heat from its dayside to its nightside. In some scenarios, water could persist as a global ocean; in others, it could pool on the illuminated hemisphere, protected beneath clouds that form a reflective, stabilizing “thermal umbrella.”
On the flip side, a thin or patchy atmosphere risks nightside collapse, where gases freeze out and the climate system shuts down. That makes the presence—or absence—of robust CO₂ or N₂ a decisive factor.
A clever Webb test: pairing planet b and e
To cut through stellar noise, the Webb team plans a comparative experiment: observe TRAPPIST-1e’s transit nearly simultaneously with TRAPPIST-1b, a hotter, likely airless world closer to the star. If both planets produce the same spectral quirks at the same wavelengths, those signatures likely come from the star. Any features unique to TRAPPIST-1e are better candidates for its own atmosphere.
This “control planet” strategy, coordinated with Webb’s NIRSpec and NIRISS instruments, is an advance on standard transit spectroscopy and a response to lessons learned from Hubble and Spitzer. It could become a template for studying multi-planet systems around active stars, where stellar contamination has long muddied interpretations.
What Webb will look for next
Beyond ruling out hydrogen, the next priority is to probe for heavier gases. Carbon dioxide leaves distinct fingerprints near 4.3 microns, accessible to Webb, while carbon monoxide signals cluster near 4.6 microns. Water vapor features in the near-infrared can also be detectable if clouds aren’t too thick. Secondary-eclipse and phase-curve observations, especially with MIRI, can measure heat emission and help distinguish a bare rock from a world blanketed by air.
The campaign on TRAPPIST-1e comprises dozens of planned transits and eclipses. Publishing early results—four observations down, many to go—allows other groups to stress-test methods, refine stellar activity models, and combine datasets. That open approach is crucial, given how close current signals are to the limits of precision.
Why the stakes are high for red dwarf worlds
Red dwarfs are the most common stars in the Milky Way, and they host a large fraction of the galaxy’s Earth-sized planets. If planets like TRAPPIST-1e can regenerate atmospheres after early erosion, the odds of finding habitable environments rise dramatically. If not, the habitable zones of such stars could be more barren than their abundance suggests.
Agencies including NASA, the European Space Agency, and the Canadian Space Agency built Webb to tackle precisely these frontier questions. The results from TRAPPIST-1e will help steer future investments, from large ground-based observatories to proposed space missions like the Habitable Worlds Observatory, which aim to characterize rocky planets with far greater sensitivity.
For now, the case remains open—but not empty. A missing hydrogen blanket, a promising test design, and a plausible path to a rebuilt atmosphere put TRAPPIST-1e at the center of the most consequential habitability experiment underway.