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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.