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

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