Confronting compositional confusion through the characterisation of the sub-Neptune orbiting HD 77946

L. Palethorpe*, A. Anna John, A. Mortier, J. Davoult, T. G. Wilson, K. Rice, A. C. Cameron, Y. Alibert, L. A. Buchhave, L. Malavolta, J. Cadman, M. López-Morales, X. Dumusque, A. M. Silva, S. N. Quinn, V. Van Eylen, S. Vissapragada, L. Affer, D. Charbonneau, R. CosentinoA. Ghedina, R. D. Haywood, D. Latham, F. Lienhard, A. F. Martínez Fiorenzano, M. Pedani, F. Pepe, M. Pinamonti, A. Sozzetti, M. Stalport, S. Udry, A. Vanderburg

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

We report on the detailed characterization of the HD 77946 planetary system. HD 77946 is an F5 (푀∗ = 1.17 M , 푅∗ = 1.31 R ) star, which hosts a transiting planet recently discovered by NASA’s Transiting Exoplanet Survey Satellite (TESS), classified as TOI-1778 b. Using TESS photometry, high-resolution spectroscopic data from HARPS-N, and photometry from CHEOPS, we measure the radius and mass from the transit and RV observations, and find that the planet, HD 77946 b, orbits with period 푃b =6.527282+0.000015−0.000020 d, has a mass of 푀b = 8.38 ± 1.32M⊕, and a radius of 푅b = 2.705+0.086−0.081R⊕. From the combination of mass and radius measurements, and the stellar chemical composition, the planet properties suggest that HD 77946 b is a sub-Neptune with a ∼1% H/He atmosphere. However, a degeneracy still exists between water-world and silicate/iron-hydrogen models, and even though interior structure modelling of this planet favours a sub-Neptune with a H/He layer that makes up a significant fraction of its radius, a water-world composition cannot be ruled out, as with 푇eq = 1248+40−38 K, water may be in a supercritical state. The
characterisation of HD 77946 b, adding to the small sample of well-haracterised sub-Neptunes, is an important step forwards on our journey to understanding planetary formation and evolution pathways. Furthermore, HD 77946 b has one of the highest transmission spectroscopic metrics for small planets orbiting hot stars, thus transmission spectroscopy of this key planet could prove vital for constraining the compositional confusion that currently surrounds small exoplanets.
Original languageEnglish
Pages (from-to)3323-3341
Number of pages19
JournalMonthly Notices of the Royal Astronomical Society
Volume529
Issue number4
Early online date8 Mar 2024
DOIs
Publication statusPublished - 1 Apr 2024

Keywords / Materials (for Non-textual outputs)

  • techniques: photometric
  • techniques: radial velocities
  • techniques: spectroscopic
  • planets and satellites
  • fundamental parameters
  • composition
  • stars: individual (HD77946)

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