Two extremely irradiated volatile-rich sub-Neptunes with companions in the TOI-426 and TOI-1839 systems: Insights into arrival and survival near the lower edge of the Neptunian desert

@misc{castrogonzalez2026extremely,
  title = {{Two extremely irradiated volatile-rich sub-Neptunes with companions in the TOI-426 and TOI-1839 systems: Insights into arrival and survival near the lower edge of the Neptunian desert}},
  author = {A. Castro-González and O. Barragán and D. J. Armstrong and A. Aguichine and V. Bourrier and D. Ehrenreich and E. X. Tao and J. Lillo-Box and M. R. Standing and S. G. Sousa and E. Delgado-Mena and A. Moya and V. Adibekyan and M. Lendl and C. Hellier and S. B. Howell and E. Furlan and C. Ziegler and A. C. M. Correia and K. Cui and P. Figueira and J. M. Jenkins and M. A. Fetzner Keniger and B. Merín and A. Osborn and L. Parc and F. Pepe and P. J. Wheatley and J. N. Winn},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2609.05413},
  eprint = {2609.05413},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2609.05413},
  note = {CC BY 4.0},
}

TY  - UNPB
TI  - Two extremely irradiated volatile-rich sub-Neptunes with companions in the TOI-426 and TOI-1839 systems: Insights into arrival and survival near the lower edge of the Neptunian desert
AU  - A. Castro-González
AU  - O. Barragán
AU  - D. J. Armstrong
AU  - A. Aguichine
AU  - V. Bourrier
AU  - D. Ehrenreich
AU  - E. X. Tao
AU  - J. Lillo-Box
AU  - M. R. Standing
AU  - S. G. Sousa
AU  - E. Delgado-Mena
AU  - A. Moya
AU  - V. Adibekyan
AU  - M. Lendl
AU  - C. Hellier
AU  - S. B. Howell
AU  - E. Furlan
AU  - C. Ziegler
AU  - A. C. M. Correia
AU  - K. Cui
AU  - P. Figueira
AU  - J. M. Jenkins
AU  - M. A. Fetzner Keniger
AU  - B. Merín
AU  - A. Osborn
AU  - L. Parc
AU  - F. Pepe
AU  - P. J. Wheatley
AU  - J. N. Winn
PY  - 2026
DA  - 2026/09/10
PB  - arXiv
DO  - 10.48550/arxiv.2609.05413
UR  - https://arxiv.org/abs/2609.05413
AB  - 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.
ER  -