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

Abstract 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 co

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Journal
The Astrophysical Journal Letters
Published
2026-10-30
DOI
https://doi.org/10.3847/2041-8213/aeb4ab
Primary Topic
Stellar, planetary, and galactic studies
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article
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article

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

L.J.J. Janssen, Samuel N. Quinn, Mercedes López‐Morales, Luca Malavolta et al.
1 links
The Astrophysical Journal Letters
Stellar, planetary, and galactic studies
article

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

L.J.J. Janssen, Samuel N. Quinn, Mercedes López‐Morales, Luca Malavolta, Laura Kreidberg, Ian J. M. Crossfield, Nicolas B. Cowan, Lisa Dang, Yamila Miguel, Andrew M. Vanderburg, Tue Giang Nguyen, Sebastian Zieba, Tim Lichtenberg, Nicholas J. Connors, Mantas Zilinskas, Joost P. Wardenier, Adam L. Kraus, Billy Edwards, Thomas G Wilson, Mariana Sastre, Oliver Herbort
article en

Abstract

Abstract 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 co

The Astrophysical Journal LettersVol. 1011(1)
University of Bern (CH), Leiden University (NL), California Institute of Technology (US), Space Telescope Science Institute (US), University of Vienna (AT), University of Padua (IT), University of Kansas (US), University of Waterloo (CA), University of Groningen (NL), University of Warwick (GB), Max Planck Institute for Astronomy (DE), Center for Astrophysics Harvard & Smithsonian (US), Osservatorio Astronomico di Padova (IT), Leiden Observatory (NL), Space Research Organisation Netherlands (NL), McGill University (CA), Université de Montréal (CA), The University of Texas at Austin (US), National Institute for Astrophysics (IT)
Smithsonian Institution, National Aeronautics and Space Administration, California Institute of Technology, Space Telescope Science Institute, Österreichische Forschungsförderungsgesellschaft, Canadian Space Agency, Natural Sciences and Engineering Research Council of Canada
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Stellar, planetary, and galactic studies
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