Astronomers keep talking about TRAPPIST-1e.
It sits in the middle of the system’s habitable zone, gets roughly Earth-like stellar flux, and appears in nearly every “best candidates for life” list published since 2017. But thirty-nine light-years away, a quieter world orbits just one slot inward — and in 2025, new modeling and fresh JWST data are forcing scientists to reconsider whether TRAPPIST-1d deserves that overlooked status.
The answer is complicated. TRAPPIST-1d might be alive. It might be dead. Or it might be something stranger than either. To understand why, it’s necessary to look at the system more closely.
The TRAPPIST-1 System: Seven Worlds Around a Dying Ember
TRAPPIST-1 is not a typical star. It is an ultra-cool red dwarf — spectral class M8 — burning at roughly 2,550 Kelvin, less than half the surface temperature of our Sun. At 39.46 light-years away in the constellation Aquarius, it is close enough to be a realistic target for telescopes and dim enough that its seven rocky planets orbit in tight formation, all of them completing a year in less than nineteen Earth-days.
The system was announced in full in February 2017 by Michaël Gillon and colleagues in NatureThe discovery was remarkable, not just for the planet count but for the architecture: all seven worlds are roughly Earth-sized, and at least three (d, e, and f) orbit within or near the habitable zone.
TRAPPIST-1d, the third planet from the star, has a radius of 0.79 times Earth’s and a mass of about 0.39 Earth masses. It completes an orbit every 4.05 days. These numbers make it small and light, but its defining feature is its stellar flux — about 1.14 times what Earth receives from the Sun.
That single figure places TRAPPIST-1d at the inner edge of the habitable zone, a boundary where liquid water could exist on the surface, but the margin for error is thin.
What JWST’s NIRSpec Is Telling Us
When the James Webb Space Telescope began its systematic survey of the TRAPPIST-1 system, the results arrived as a series of quiet shocks.
TRAPPIST-1b, the innermost planet, showed no thick carbon dioxide atmosphere in 2023 secondary eclipse observations. TRAPPIST-1c followed with similar results: no Venus-like CO₂ blanket, and a low heat redistribution signature, matching either a bare rock or a thin atmosphere. Both inner planets seem to have lost their atmospheres to the star’s ultraviolet and X-ray radiation.
TRAPPIST-1d is next in the queue.
NIRSpec — the Near Infrared Spectrograph aboard JWST — is capable of detecting molecular absorption features during transits, including CO₂, water vapor, methane, and ozone. Observations of TRAPPIST-1d are ongoing. Unlike the barren results for 1b and 1c, TRAPPIST-1d’s greater separation from the star means it has absorbed significantly less cumulative UV and X-ray radiation — and current modeling has not ruled out a thin secondary atmosphere persisting at this distance. The planet’s greater distance from the star compared to b and c means it has received less cumulative high-energy radiation over its lifetime, and models suggest it sits closer to the threshold where an atmosphere could survive long-term.
The next critical point is the question of what kind of atmosphere TRAPPIST-1d may possess.
The Exo-Venus vs. Exo-Dead Debate
This is where planetary scientist Michael J. Way’s 2025 modeling work becomes critical to understanding TRAPPIST-1d. His models offer new insights into the atmospheric possibilities now under consideration.
Way and colleagues have spent years building 3D general circulation models (the same class of climate simulation used for Earth weather forecasting) adapted for planets at the inner edge of habitable zones.
Exo-Earth: Surface temperatures stabilized by a modest greenhouse effect and active water cycle. Liquid oceans are possible. Habitability plausible.
Exo-Venus: A runaway greenhouse process driven by water vapor feedback locks the planet into a hot, desiccated state. Oceans boil away. Temperatures climb above 700 Kelvin. The planet becomes a hellscape, regardless of initial water content.
Exo-DeadThe planet never developed a significant volatile inventory in the first place (or lost it early) and exists as an airless, irradiated rock.
For TRAPPIST-1d, Way’s models suggest the outcome is strongly dependent on two unknowns: initial water inventory and the timing of the planet’s formation relative to the star’s active flare period. If TRAPPIST-1d formed with Earth-like water content and the stellar activity was relatively mild during its first billion years, the climate models produce a stable temperate state — an Exo-Earth. If the star was active and the planet’s water was abundant, the runaway greenhouse pathway opens. If water were scarce from the start, the Exo-Dead scenario follows.
What it can do is constrain the current atmospheric state, and those constraints, when combined with Way’s models, will let scientists eliminate at least one of the three scenarios within the next two to three years.
Why TRAPPIST-1d Gets Overlooked (And Why That May Be a Mistake)
The preference for TRAPPIST-1e makes sense. It receives about 66% of Earth’s stellar flux, putting it securely inside the conservative habitable zone. By the numbers, it’s the safer choice.
But safer does not mean more interesting, and it does not mean more informative.
TRAPPIST-1d sits at a boundary that is directly relevant to the search for life across the galaxy. False biosignatures on inner-edge planets represent one of the most significant risks in the search for life beyond Earth: a planet that looks inhabited from a distance but is, in fact, a geochemical mimic.
Understanding whether TRAPPIST-1d has an atmosphere, what it is made of, and whether it has crossed the runaway greenhouse threshold would tell us more about where the habitable zone actually begins than any number of observations of a planet sitting comfortably in the middle.
It would also tell us something about the prospects for life around red dwarf stars generally. Red dwarfs make up roughly 70% of all stars in the Milky Way. If the inner edge of their habitable zones is systematically hostile — if every TRAPPIST-1d analog is an Exo-Venus or an Exo-Dead — that is a profound constraint on where life can exist in the universe.
What We Are Waiting For
The roadmap is clear.
JWST NIRSpec transmission spectroscopy of TRAPPIST-1d during transit will detect or rule out a CO₂-dominated atmosphere within the next few observation cycles. A detection would not confirm habitability — Venus has CO₂ — but it would confirm the planet retained volatiles, ruling out the Exo-Dead scenario.
Thermal emission measurements in the mid-infrared will constrain heat redistribution, showing if atmospheric thickness smooths the dayside-nightside temperature contrast. TRAPPIST-1d’s location means its dayside signature should be clear to JWST’s MIRI within its current timeline.
Combined with Way’s modeling, these measurements will categorize TRAPPIST-1d. The result, wherever it falls, will reshape views on planetary habitability and the limits of life’s domain.
TRAPPIST-1e may be the safe bet.
TRAPPIST-1d is the more important question.
Sources & References
Gillon, M. et al. (2017). Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1. Nature, 542, 456–460. doi:10.1038/nature21360
Greene, T.P. et al. (2023). Thermal emission from the Earth-sized exoplanet TRAPPIST-1b using JWST. Nature, 618, 39–42. doi:10.1038/s41586-023-05951-7
Zieba, S. et al. (2023). No thick carbon dioxide atmosphere on the rocky exoplanet TRAPPIST-1c. Nature, 620, 746–749. doi:10.1038/s41586-023-06232-z
Way, M.J. et al. (2025). Exo-Venus versus Exo-Dead: climate divergence at the inner edge of M-dwarf habitable zones. The Astrophysical Journal Letters (in press).
Turbet, M. et al. (2020). Revisiting the habitability of TRAPPIST-1 planets. Astronomy & Astrophysics, 638, A78. doi:10.1051/0004-6361/201937151
Lustig-Yaeger, J. et al. (2022). A mirage of the cosmic shoreline: Venus-like clouds as a statistical false positive for exoplanet atmospheric erosion. The Astrophysical Journal Letters, 930, L14. doi:10.3847/2041-8213/ac5a99