A high geometric albedo for LTT9779b points toward a metal-rich atmosphere and silicate clouds

¹ Instituto de Estudios Astrofísicos, Facultad de Ingeniería Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
² Centro de Excelencia en Astrofísica y Tecnologías Afines (CATA), Camino El Observatorio 1515, Las Condes, Santiago, Chile
³ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, France
⁴ Aix Marseille Univ, CNRS, CNES, LAM, 38 reu Frédérik Joliot-Curie, 13388 Marseille, France
⁵ Department of Physics and Astronomy, University of Kansas, Lawrence, KS, USA
⁶ Instituto de Astronomía, Universidad Católica del Norte, Angamos 0610, 1270709 Antofagasta, Chile
⁷ Instituto de Astrofisica, Departamento de Fisica y Astronomia, Facultad Ciencias Exactas, Universidad Andres Bello, Fernandez Concha 700, Las Condes, Santiago, Chile
⁸ Departamento de Astronomía, Universidad de Chile, Camino el Observatorio 1515, Las Condes, Santiago, Chile
⁹ Las Campanas Observatory, Carnegie Institution for Science, Raul Bitrán 1200, La Serena, Chile

^* Corresponding author: suman.saha@mail.udp.cl

Received: 27 February 2025
Accepted: 12 June 2025

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.