CHEOPS and TESS view of the ultra-short period super-Earth TOI-561 b

TL;DR

This study uses CHEOPS and TESS data to analyze the ultra-short period super-Earth TOI-561b, constraining its radius, internal structure, and potential atmosphere, and predicting future JWST observations to detect atmospheric features.

Contribution

The paper provides the first detailed characterization of TOI-561b's bulk properties and atmosphere using combined CHEOPS and TESS data, including constraints on its radius, density, and atmospheric composition.

Findings

TOI-561b has a radius of 1.42 R_Earth with ~2% error.

The planet's density suggests a negligible H/He atmosphere but possible water layer.

Tentative detection of occultation signal indicating thermal emission.

Abstract

Ultra-short period planets (USPs) are a unique class of super-Earths with an orbital period of less than a day and hence subject to intense radiation from their host star. While most of them are consistent with bare rocks, some show evidence of a heavyweight envelope, which could be a water layer or a secondary metal-rich atmosphere sustained by an outgassing surface. Much remains to be learned about the nature of USPs. The prime goal of the present work is to study the bulk planetary properties and atmosphere of TOI-561b, through the study of its transits and occultations. We obtained ultra-precise transit photometry of TOI-561b with CHEOPS and performed a joint analysis of this data with four TESS sectors. Our analysis of TOI-561b transit photometry put strong constraints on its properties, especially on its radius, Rp=1.42 +/- 0.02 R_Earth (at ~2% error). The internal structure modelling of the planet shows that the observations are consistent with negligible H/He atmosphere, however requiring other lighter materials, in addition to pure iron core and silicate mantle to explain the observed density. We find that this can be explained by the inclusion of a water layer in our model. We searched for variability in the measured Rp/R* over time to trace changes in the structure of the planetary envelope but none found within the data precision. In addition to the transit event, we tentatively detect occultation signal in the TESS data with an eclipse depth of ~27 +/- 11 ppm. Using the models of outgassed atmospheres from the literature we find that the thermal emission from the planet can mostly explain the observation. Based on this, we predict that NIR/MIR observations with JWST should be able to detect silicate species in the atmosphere of the planet. This could also reveal important clues about the planetary interior and help disentangle planet formation and evolution models.