Exoplanets, -moons, -comets

Stringent Upper Bounds on Atmospheric Mass Loss from Three Neptune-Sized Planets in the TOI-4010 System

By Keith Cowing

Status Report

astro-ph.EP

September 4, 2025

Filed under astro-ph.EP, atmosphere, exoplanet, Neptune Desert, Neptune savanna, photoevaporation, TOI-4010, TOI-4010 b, TOI-4010 c, TOI-4010 d

Stringent Upper Bounds on Atmospheric Mass Loss from Three Neptune-Sized Planets in the TOI-4010 System — Top row: Keck/NIRSPEC excess absorption in percent of TOI-4010 b (first panel), TOI-4010 c (second panel) and TOI-4010 d (third panel), in each planet’s rest frame as a function of time and wavelength (air wavelengths in planetary rest frame). Horizontal white lines mark the beginning (top) and end (bottom) of transit. Dashed vertical red lines denote the positions of the three helium lines. Red shaded regions mark the location of a telluric OH line that we masked in our analysis (see Fig. 2). Bottom row: Average excess absorption spectra for TOI-4010 b (first panel), TOI-4010 c (second panel) and TOI-4010 d (third panel) are shown as black points. The pyTPCI solar metallicity model predictions of the excess absorptions are shown as a blue line (nominal XUV flux), purple line (XUV flux reduced by factor of five), pink line (XUV flux reduced by factor of 10), and orange line (XUV flux reduced by factor of 15). — astro-ph.EP

Photoevaporative models predict that the lower edge of the Neptune desert is sculpted by atmospheric mass loss.

However, the stellar high energy fluxes that power hydrodynamic escape and set predicted mass loss rates can be uncertain by multiple orders of magnitude. These uncertainties can be bypassed by studying mass loss for planets within the same system, as they have effectively undergone scaled versions of the same irradiation history.

The TOI-4010 system is an ideal test case for mass loss models, as it contains three Neptune-sized planets with planet b located in the Neptune Desert, planet c in the Neptune ridge’, and planet d in the `Neptune savanna’. Using Keck/NIRSPEC, we measured the metastable helium transit depths of all three planets in order to search for evidence of atmospheric escape.

We place upper bounds on the excess helium absorption of 1.23%, 0.81%, and 0.8% at 95% confidence for TOI-4010~b, c and d respectively. We fit our transmission spectra with Parker wind models and find that this corresponds to 95th-percentile upper limits of 1010.17g~s⁻¹, 1010.53g~s⁻¹, and 1010.50g~s⁻¹ on the mass loss rates of TOI-4010~b, c, and d respectively. Our non-detections are inconsistent with expectations from one-dimensional hydrodynamic models for solar composition atmospheres.

We consider potential reductions in signal from a decreased host star XUV luminosity, planetary magnetic fields, enhanced atmospheric metallicities, and fractionation, and explore the implications of our measurements for the past evaporation histories of all three planets.

Transiting planet radii, periods, and densities in the vicinity of the Neptune desert, drawn from the NASA Exoplanet Archive on April 4, 2025 (Akeson et al. 2013; NASA Exoplanet Archive 2024). The dashed lines indicate the Neptune desert, ridge, and savanna boundaries from CastroGonz´alez et al. (2024). Planets in the TOI-4010 system are indicated with a star and a darker opacity. — astro-ph.EP

Morgan Saidel, Shreyas Vissapragada, Michael Zhang, Heather A. Knutson, Matthäus Schulik, Jorge Fernández Fernández, Michelle Kunimoto, Peter J. Wheatley, Jessica Spake

Comments: 16 pages, 8 figures; Accepted for publication in the Astronomical Journal
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2508.21166 [astro-ph.EP] (or arXiv:2508.21166v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2508.21166
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Submission history
From: Morgan Saidel
[v1] Thu, 28 Aug 2025 19:02:23 UTC (1,284 KB)
https://arxiv.org/abs/2508.21166
Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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