Abstract

Aims. In this work, our aim is to confirm the high albedo of the benchmark ultra-hot Neptune LTT9779b using 20 secondary eclipse measurements of the planet observed with CHEOPS. In addition, we performed a search for variability in the reflected light intensity of the planet as a function of time. Methods. First, we used the TESS follow-up data of LTT9779b from three sectors (2, 29, and 69) to remodel the transit signature and estimate an updated set of transit and ephemeris parameters, which were directly used in the modeling of the secondary eclipse light curves. This involved a critical noise-treatment algorithm, including sophisticated techniques such as wavelet denoising and Gaussian process (GP) regression, to constrain noise levels from various sources. In addition to using the officially released reduced aperture photometry data from CHEOPS DRP, we also reduced the raw data using an independent PSF photometry pipeline, known as PIPE, to verify the robustness of our analysis. The extracted secondary eclipse light curves were modeled using the PYCHEOPS package, where we detrended the background noise correlated with the spacecraft roll angle, originating from the inhomogeneous and asymmetric shape of the CHEOPS point spread function, using an N-order glint function. Results. Our independent light curve analyses have resulted in consistent estimations of the eclipse depths, with values of 89.9 ± 13.7 ppm for the DRP analysis and 85.2 ± 13.1 ppm from PIPE, indicating a high degree of statistical agreement. Adopting the DRP value yields a highly constrained geometric albedo of 0.73 ± 0.11. No significant eclipse depth variability is detected down to a level of -37 ppm. Conclusions. Our results confirm that LTT9779b exhibits a strikingly high optical albedo, which substantially reduces the internal energy budget of the planet compared to more opaque atmospheres. By modeling our new and precise eclipse measurements, we find that the planet's atmosphere is likely highly metal-rich, with silicate clouds probably present. Our models find it difficult to explain the high geometric albedo since fitting these high optical bands leads to a decrease in the near-IR planetary emission, well below the current observations. However, we discuss additional physical processes that could circumvent this problem, such as the introduction of strong particle backscattering. © The Authors 2025.