The young mini-Neptune HD 207496b that is either a naked core or on the verge of becoming one

S. C.C. Barros, O. D.S. Demangeon, D. J. Armstrong, E. Delgado Mena, L. Acuña, J. Fernández Fernández, M. Deleuil, K. A. Collins, S. B. Howell, C. Ziegler, V. Adibekyan, S. G. Sousa, K. G. Stassun, N. Grieves, J. Lillo-Box, C. Hellier, P. J. Wheatley, C. Briceño, K. I. Collins, F. HawthornS. Hoyer, J. Jenkins, N. Law, A. W. Mann, R. A. Matson, O. Mousis, L. D. Nielsen, A. Osborn, H. Osborn, M. Paegert, R. Papini, G. R. Ricker, A. A. Rudat, N. C. Santos, S. Seager, C. Stockdale, P. A. Strøm, J. D. Twicken, S. Udry, G. Wang, R. Vanderspek, J. N. Winn

Research output: Contribution to journalArticlepeer-review

2 Scopus citations


Aims. We report the discovery and characterisation of the transiting mini-Neptune HD 207496 b (TOI-1099) as part of a large programme that aims to characterise naked core planets. Methods. We obtained HARPS spectroscopic observations, one ground-based transit, and high-resolution imaging which we combined with the TESS photometry to confirm and characterise the TESS candidate and its host star. Results. The host star is an active early K dwarf with a mass of 0.80 ± 0.04 M⊙, a radius of 0.769 ± 0.026 R⊙, and a G magnitude of 8. We found that the host star is young, ~0.52 Gyr, allowing us to gain insight into planetary evolution. We derived a planetary mass of 6.1 ± 1.6 M⊕, a planetary radius of 2.25 ± 0.12 R⊕, and a planetary density of ρp = 3.27-0.91+0.97 g cm-3. Conclusions. From internal structure modelling of the planet, we conclude that the planet has either a water-rich envelope, a gas-rich envelope, or a mixture of both. We have performed evaporation modelling of the planet. If we assume the planet has a gas-rich envelope, we find that the planet has lost a significant fraction of its envelope and its radius has shrunk. Furthermore, we estimate it will lose all its remaining gaseous envelope in ~0.52 Gyr. Otherwise, the planet could have already lost all its primordial gas and is now a bare ocean planet. Further observations of its possible atmosphere and/or mass-loss rate would allow us to distinguish between these two hypotheses. Such observations would determine if the planet remains above the radius gap or if it will shrink and be below the gap.

Original languageEnglish (US)
Article numberA4
JournalAstronomy and Astrophysics
StatePublished - May 1 2023

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Methods: data analysis
  • Planets and satellites: composition
  • Planets and satellites: detection
  • Planets and satellites: fundamental parameters
  • Techniques: photometric
  • Techniques: radial velocities


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