TRAPPIST-1 Revisited

JWST has given us the power to get up close and personal with some nearby exoplanets for the first time in history.
NASA, ESA, CSA, J. Olmsted (STScI), T. P. Greene (NASA Ames), T. Bell (BAERI), E. Ducrot (CEA), P. Lagage (CEA) (CC BY 4.0)

There was a lot of hubbub about TRAPPIST-1 when it was first discovered. Seven exoplanets Planets orbiting stars other than the sun circling a dim red dwarf Small stars less than half the mass of the sun, with red colors and low temperatures star, all of which could have liquid water under the right conditions. If conditions in the system are just right, TRAPPIST-1 could be an ark of life in the void: seven worlds, teeming with aliens we could hardly imagine.

But our tools have advanced a lot since 2017, and now we can actually look at the atmospheres and thermal properties of some of these planets. So, how do the initial results hold up?

A hopeful vision of TRAPPIST-1e.
NASA/JPL-Caltech (Public Domain)

The answer is not all that well. JWST observations of the inner two planets TRAPPIST-1b and TRAPPIST-1c revealed that they did not possess much of an atmosphere at all^{1, 2}, with surfaces made mostly of bare volcanic rock¹. But these were not dead worlds like the Moon and Mercury—their surfaces are just a few thousand years old and they display thermal signatures of volcanic eruptions³. The reason they are airless is instead because the powerful flares Outbursts of energy that occur on the surface of stars of the host star TRAPPIST-1 rip away all but the most tenuous wisps of gas from their surfaces³.

With a few more years of observation we have since investigated the next planet out, TRAPPIST-1d. This is the first of the TRAPPIST-1 planets located inside the habitable zone, so our hopes were pretty high for it. Unfortunately, it also probably doesn't have an atmosphere⁴. If our models are correct, this probably means the TRAPPIST-1 system formed with relatively little water—four times Earth's ocean or less per planet⁴.

What do these findings mean for the other possibly-habitable planets, TRAPPIST-1e, f, and g? Well, it's not great. We already know they are rocky planets without thick hydrogen-based atmospheres⁵, but we don't know any more than that and it's possible they have no atmosphere at all. With their lower temperatures and greater distances from their star it's possible that these planets will hold on to enough air to let liquid water exist on their surfaces, but it's not guaranteed. Only further studies will tell.

References

^{1 🔒}Greene, T. P., Bell, T. J., Ducrot, E., Dyrek, A., Lagage, P.-O., & Fortney, J. J. (2023). Thermal emission from the Earth-sized exoplanet TRAPPIST-1 b using JWST. Nature, 618, 39-42. https://doi.org/10.1038/s41586-023-05951-7

² Zieba, S., Kreidberg, L., Ducrot, E., Gillon, M., Morley, C., Schaefer, L., Tamburo, P., Koll, D. D. B., Lyu, X., Acuña, L., Algol, E., Iyer, A. R., Hu, R., Lincowski, A. P., Meadows, V. S., Selsis, F., Bolmont, E., Mandell, A. M., & Suissa, G. (2023). No thick carbon dioxide atmosphere on the rocky exoplanet TRAPPIST-1 c. Nature, 620, 746–749. https://doi.org/10.1038/s41586-023-06232-z

^{3 🔒}Ducrot, E., Lagage, P.-O., Min, M., Gillon, M., Bell, T. J., Tremblin, P., Greene, T., Dyrek, A., Bouwman, J., Waters, R., Güdel, M., Henning, T., Vandenbussche, B., Absil, O., Barrado, D., Boccaletti, A., Coulais, A., Decin, L., Edwards, B., ..., & Wright, G. (2025). Combined analysis of the 12.8 and 15 μm JWST/MIRI eclipse observations of TRAPPIST-1 b. Nature Astronomy, 9, 358–369. https://doi.org/10.1038/s41550-024-02428-z

⁴ Piaulet-Ghorayeb, C., Benneke, B., Turbet, M., Moore, K., Roy, P.-A., Lim, O., Doyon, R., Fauchez, T. J., Albert, L., Radica, M., Coulombe, L.-P., Lafrenière, D., Cowan, N. B., Belzile, D., Musfirat, K., Kaur, M., L’Heureux, A., Johnstone, D., MacDonald, R. J., ..., & Turner, J. D.(2025). Strict Limits on Potential Secondary Atmospheres on the Temperate Rocky Exo-Earth TRAPPIST-1 d. The Astrophysical Journal, 989(2), 181. https://doi.org/10.3847/1538-4357/adf207

^{5 🔒}de Wit, J., Wakeford, H. R., Lewis, N. K., Delrez, L., Gillon, M., Selsis, F., Leconte, J., Demory, B.-O., Bolmont, E., Bourrier, V., Burgasser, A. J., Grimm, S., Jehin, E., Lerderer, S. M., Owen, J. E., Stamenković , V., & Triaud, A. H. M. J. (2018). Atmospheric reconnaissance of the habitable-zone Earth-sized planets orbiting TRAPPIST-1. Nature Astronomy, 2, 214–219. https://doi.org/10.1038/s41550-017-0374-z

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