TOI-561 b CHEOPS lightcurves

Methods. We obtained ultra-precise transit photometry of TOI-561 b with CHEOPS, and performed a joint analysis of this data along with three archival visits from CHEOPS and four TESS sectors. Our analysis of TOI-561 b transit photometry put strong constraints on its properties.

In particular, we restrict the uncertainties on the planetary radius at ~2% retrieving Rp=1.42+/-0.02R_{Earth}_. This result informs our internal structure modelling of the planet, which 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.

Additionally, we run a grid of forward models with a water-enriched atmosphere to explain the transit radius. We searched for variability in the measured Rp/R* over time, which could trace changes in the structure of the planetary envelope. No temporal variations are however recovered within the present data precision.

In addition to the transit event, we tentatively detect occultation signal in the TESS data with an eclipse depth L=27.40^+10.87^_-11.35_ppm. We use models of outgassed atmospheres from the literature to explain this eclipse signal. We find that the thermal emission from the planet can mostly explain the observation.

Based on this, we predict that near/mid-infrared observations with the James Webb Space Telescope 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.