[astro-ph.EP] Using TESS photometry and 147 HARPS radial velocities, we confirm two extremely irradiated, volatile-rich sub-Neptunes near the lower edge of the Neptunian desert: TOI-426 b and TOI-1839 b, orbiting the solar-type stars HD 34390 and TYC 304-865-1.
We modelled stellar activity with shared-timescale and multidimensional Gaussian processes. The planets have orbital periods of approximately 1.32 and 1.42 days, radii of 2.19±0.09 and 2.27±0.10 Earth radii, and masses of 6.7±1.8 and 7.10±0.78 Earth masses, respectively.
They receive approximately 1700 and 1050 times Earth’s irradiation, yet their densities require substantial volatile content. We also detect a transiting sub-Neptune, TOI-1839 c, at approximately 4.02 days and a giant companion, TOI-426 c, with a period of 235.8+9.2−8.5 days and a minimum mass of 444+18−17 Earth masses, assuming a circular orbit.
Interior-structure and atmospheric-escape models yield mass-loss timescales of 1-100 Myr for hydrogen/helium envelopes, compared with 100-1000 Gyr for water-dominated envelopes, favouring a steam-world interpretation for all three transiting planets.
A population analysis reveals a striking excess of detected outer giant companions to low-mass sub-Neptunes near the desert edge, where high-eccentricity tidal migration is expected to circularize surviving planets: 36% (9/25) within the tidal survival band versus 6% (9/148) outside it (Fisher exact test p=1.4×10−4). This excess is robust to changes in sample selection and companion definitions.
The survival of these volatile-rich planets challenges a boundary set solely by evaporation of primordial hydrogen/helium envelopes, while the companion excess supports an important role for high-eccentricity tidal migration in shaping the lower desert edge.
A. Castro-González, O. Barragán, D. J. Armstrong, A. Aguichine, V. Bourrier, D. Ehrenreich, E. X. Tao, J. Lillo-Box, M. R. Standing, S. G. Sousa, E. Delgado-Mena, A. Moya, V. Adibekyan, M. Lendl, C. Hellier, S. B. Howell, E. Furlan, C. Ziegler, A. C. M. Correia, K. Cui, P. Figueira, J. M. Jenkins, M. A. Fetzner Keniger, B. Merín, A. Osborn, L. Parc, F. Pepe, P. J. Wheatley, J. N. Winn
Comments: Accepted for publication in A&A. 24 pages, 16 figures (including appendices)
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2609.05413 [astro-ph.EP] (or arXiv:2609.05413v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2609.05413
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Submission history
From: Amadeo Castro-González
[v1] Fri, 4 Sep 2026 17:58:47 UTC (9,647 KB)
https://arxiv.org/abs/2609.05413
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