Position of TOI-270 b relative to the cosmic shoreline. Escape velocity and cumulative XUV irradiation, computed using the scaling of Zahnle & Catling (2017), for all known exoplanets with equilibrium temperatures below 1,000 K, radii smaller than 1.8 R⊕, and masses measured to higher than 20% precision (data taken from the NASA Exoplanet Archive, Akeson et al. 2013). The blue dashed line indicates the position of the cosmic shoreline shown in Zahnle & Catling (2017), and we add a shaded region corresponding to a factor of four uncertainty in XUV irradiation, as in Xue et al. (2024). The solar system telluric planets are shown for comparison, and we also indicate the position of TRAPPIST1 b (Greene et al. 2023), TRAPPIST-1 c (Zieba et al. 2023), GJ 1132 b (Xue et al. 2024), GJ 486 b (Mansfield et al. 2024), and LTT 1445 Ab (Wachiraphan et al. 2024), which have all been found to be consistent with a dark, bare-rock scenario from JWST thermal emission observations. However, some of these targets, such as TRAPPIST-1 c (Zieba et al. 2023) remain consistent with a variety of atmospheric scenarios. The two sub-Neptunes of the TOI-270 system, planets c and d, are also shown for comparison with planet b. — astro-ph.EP
The search for atmospheres on rocky exoplanets is a crucial step in understanding the processes driving atmosphere formation, retention, and loss.
Past studies have revealed the existence of planets interior to the radius valley with densities lower than would be expected for pure-rock compositions, indicative of the presence of large volatile inventories which could facilitate atmosphere retention.
Here we present an analysis of the JWST NIRSpec/G395H transmission spectrum of the warm (Teq,AB=0 = 569 K) super-Earth TOI-270 b (Rp = 1.306 R⊕), captured alongside the transit of TOI-270 d. The JWST white light-curve transit depth updates TOI-270 b’s density to ρp = 3.7±0.5 g/cm3 , inconsistent at 4.4σ with an Earth-like composition. Instead, the planet is best explained by a non-zero, percentlevel water mass fraction, possibly residing on the surface or stored within the interior.
The JWST transmission spectrum shows possible spectroscopic evidence for the presence of this water as part of an atmosphere on TOI-270 b, favoring a H2O-rich steam atmosphere model over a flat spectrum (ln B = 0.3−3.2, inconclusive to moderate), with the exact significance depending on whether an offset parameter between the NIRSpec detectors is included.
We leverage the transit of the twice-larger TOI270 d crossing the stellar disk almost simultaneously to rule out the alternative hypothesis that the transit-light-source effect could have caused the water feature in TOI-270 b’s observed transmission spectrum.
Planetary evolution modeling furthermore shows that TOI-270 b could sustain a significant atmosphere on Gyr timescales, despite its high stellar irradiation, if it formed with a large initial volatile inventory.
Broadband and spectroscopic light-curve fits of the NIRSpec/G395H transit of TOI-270 b, extracted with exoTEDRF. The top panel is a schematic of TOI-270 b (blue) and d (red) as they transit across the stellar disk at time t = 2460222.265 BJD, illustrating their trajectories (dashed lines) and the 1σ confidence region corresponding to their orbital inclination uncertainties. The broadband light-curve fits (white) are shown for the NRS1 and NRS2 detectors, along with eight systematics-corrected spectroscopic light curves (colored dots) and their best-fit transit models (black lines). The light curves are plotted with a relative flux offset and are binned in time in groups of 75 data points (corresponding to 13.6 minute bins). The best-fit model transit of TOI-270 d, whose position is indicated by the gray dashed lines, partially overlaps with that of TOI-270 b and is removed from the data and best-fit model for visual clarity. All light curves are well-behaved and show Gaussian distributions in their residuals (computed from the non-binned light curves, right). — astro-ph.EP
Louis-Philippe Coulombe, Björn Benneke, Joshua Krissansen-Totton, Alexandrine L’Heureux, Caroline Piaulet-Ghorayeb, Michael Radica, Pierre-Alexis Roy, Eva-Maria Ahrer, Charles Cadieux, Yamila Miguel, Hilke E. Schlichting, Elisa Delgado-Mena, Christopher Monaghan, Hanna Adamski, Eshan Raul, Ryan Cloutier, Thaddeus D. Komacek, Jake Taylor, Cyril Gapp, Romain Allart, François Bouchy, Bruno L. Canto Martins, Neil J. Cook, René Doyon, Thomas M. Evans-Soma, Pierre Larue, Alejandro Suárez Mascareño, Joost P. Wardenier
Comments: Published in The Astronomical Journal
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2509.14224 [astro-ph.EP] (or arXiv:2509.14224v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2509.14224
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Journal reference: Coulombe, L.-P., Benneke, B., Krissansen-Totton, J., et al. 2025, The Astronomical Journal, 170, 226
Related DOI:
https://doi.org/10.3847/1538-3881/adfc6a
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Submission history
From: Louis-Philippe Coulombe
[v1] Wed, 17 Sep 2025 17:55:16 UTC (5,758 KB)
https://arxiv.org/abs/2509.14224
Astrobiology,