Constraining the Reflective Properties of WASP-178b Using Cheops Photometry

Context. Multiwavelength photometry of the secondary eclipses of extrasolar planets is able to disentangle the reflected and thermally emitted light radiated from the planetary dayside. Based on this, we can measure the planetary geometric albedo A(g), which is an indicator of the presence of clouds in the atmosphere, and the recirculation efficiency epsilon, which quantifies the energy transport within the atmosphere. Aims. We measure A(g) and epsilon for the planet WASP-178 b, a highly irradiated giant planet with an estimated equilibrium temperature of 2450 K. Methods. We analyzed archival spectra and the light curves collected by CHEOPS and TESS to characterize the host WASP-178, refine the ephemeris of the system, and measure the eclipse depth in the passbands of the two telescopes. Results. We measured a marginally significant eclipse depth of 70 +/- 40 ppm in the TESS passband, and a statistically significant depth of 70 +/- 20 ppm in the CHEOPS passband. Conclusions. Combining the eclipse-depth measurement in the CHEOPS (lambda(eff) = 6300 angstrom) and TESS (lambda(eff) = 8000 angstrom) passbands, we constrained the dayside brightness temperature of WASP-178 b in the 2250-2800 K interval. The geometric albedo 0.1< A(g)<0.35 generally supports the picture that giant planets are poorly reflective, while the recirculation efficiency epsilon >0.7 makes WASP-178 b an interesting laboratory for testing the current heat-recirculation models.