The First Hubble Detection of a Secondary Eclipse from a Rocky Exoplanet: the 5.4-hour planet TOI-2431 b

@misc{zieba2026first,
  title = {{The First Hubble Detection of a Secondary Eclipse from a Rocky Exoplanet: the 5.4-hour planet TOI-2431 b}},
  author = {Sebastian Zieba and Billy Edwards and Samuel N. Quinn and Leoni J. Janssen and Nicolas B. Cowan and Laura Kreidberg and Mercedes Lopez-Morales and Lisa Dang and Luca Malavolta and Yamila Miguel and T. Giang Nguyen and Andrew Vanderburg and Thomas G. Wilson and Mantas Zilinskas and Nicholas J. Connors and Ian J. M. Crossfield and Oliver Herbort and Adam L. Kraus and Tim Lichtenberg and Mariana Sastre and Joost P. Wardenier},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2609.31966},
  eprint = {2609.31966},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2609.31966},
  note = {CC BY 4.0},
}

TY  - UNPB
TI  - The First Hubble Detection of a Secondary Eclipse from a Rocky Exoplanet: the 5.4-hour planet TOI-2431 b
AU  - Sebastian Zieba
AU  - Billy Edwards
AU  - Samuel N. Quinn
AU  - Leoni J. Janssen
AU  - Nicolas B. Cowan
AU  - Laura Kreidberg
AU  - Mercedes Lopez-Morales
AU  - Lisa Dang
AU  - Luca Malavolta
AU  - Yamila Miguel
AU  - T. Giang Nguyen
AU  - Andrew Vanderburg
AU  - Thomas G. Wilson
AU  - Mantas Zilinskas
AU  - Nicholas J. Connors
AU  - Ian J. M. Crossfield
AU  - Oliver Herbort
AU  - Adam L. Kraus
AU  - Tim Lichtenberg
AU  - Mariana Sastre
AU  - Joost P. Wardenier
PY  - 2026
DA  - 2026/09/25
PB  - arXiv
DO  - 10.48550/arxiv.2609.31966
UR  - https://arxiv.org/abs/2609.31966
AB  - We present an HST/WFC3 G141 phase-curve observation of the ultra-short-period rocky planet TOI-2431 b and detect its secondary eclipse in the near infrared. Combined with the TESS optical eclipse, these measurements constrain the planet's dayside brightness temperature, reflected light contribution, albedo, and heat redistribution. We detect the eclipse in the optical and near infrared wavelengths of TESS and HST/WFC3 G141, with a planet-to-star flux ratio of 37+/-10 ppm and 69(+14/-13) ppm, respectively. Assuming negligible reflected light, the joint TESS+HST fit yields a brightness temperature T_b,joint=2524(+77/-84) K and a brightness temperature ratio of R_joint=T_b,joint/T_p,max=0.980(+0.032/-0.034) consistent with the zero-albedo, no-redistribution limit T_p,max=2574(+36/-35) K. The HST phase curve is consistent with zero nightside emission and no measurable hotspot offset, consistent with inefficient day-night heat distribution. Under this thermal-dominated interpretation, TOI-2431 b lies at the zero-albedo, no-redistribution limit and contrasts with previously observed lava worlds. At optical and near-infrared wavelengths, however, thermal emission is degenerate with reflected light. For a geometric albedo of A_g=0.1, reflected light would contribute approximately 11 ppm in each bandpass, reducing the temperature to T_b,joint(A_g=0.1)=2407(+91/-102) K. Longer-wavelength observations are required to isolate the thermal component. Planned JWST/MIRI LRS observations are expected to substantially reduce this degeneracy and provide more direct constraints on heat redistribution and possible silicate-atmosphere spectral features. This marks the first secondary eclipse detection of a rocky exoplanet using HST and establishes TOI-2431 b as a benchmark target for studying properties of lava worlds.
ER  -