The First Hubble Detection of a Secondary Eclipse from a Rocky Exoplanet: The 5.4 hr Planet TOI-2431 b

@article{zieba2026first,
  title = {{The First Hubble Detection of a Secondary Eclipse from a Rocky Exoplanet: The 5.4 hr Planet TOI-2431 b}},
  author = {Sebastian Zieba and Billy Edwards and Samuel N. Quinn and Leoni Janssen and Nicolas B. Cowan and Laura Kreidberg and Mercedes López-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},
  journal = {The Astrophysical Journal Letters},
  year = {2026},
  month = oct,
  doi = {10.3847/2041-8213/aeb4ab},
  url = {https://iopscience.iop.org/article/10.3847/2041-8213/aeb4ab},
  note = {CC BY 4.0},
}

TY  - JOUR
TI  - The First Hubble Detection of a Secondary Eclipse from a Rocky Exoplanet: The 5.4 hr Planet TOI-2431 b
AU  - Sebastian Zieba
AU  - Billy Edwards
AU  - Samuel N. Quinn
AU  - Leoni Janssen
AU  - Nicolas B. Cowan
AU  - Laura Kreidberg
AU  - Mercedes López-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  - 2026/10/30
JO  - The Astrophysical Journal Letters
DO  - 10.3847/2041-8213/aeb4ab
UR  - https://iopscience.iop.org/article/10.3847/2041-8213/aeb4ab
AB  - We present a Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) G141 phase-curve observation of the ultrashort-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 6 9 − 13 + 14 ppm , respectively. Assuming negligible reflected light, the joint TESS+HST fit yields a brightness temperature T b , joint = 252 4 − 84 + 77 K and a brightness temperature ratio of R joint = T b , joint / T p , max = 0.98 0 − 0.034 + 0.032 consistent with the zero-albedo, no-redistribution limit T p , max = 257 4 − 35 + 36 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 ) = 240 7 − 102 + 91 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.
SN  - 2041-8213
SN  - 2041-8205
ER  -