  55 Cnc e is an ultra-short period super-Earth transiting a Sun-like star.
Previous observations in the optical range detected a time-variable flux
modulation that is phased with the planetary orbital period, whose amplitude is
too large to be explained by reflected light and thermal emission alone. The
goal of the study is to investigate the origin of the variability and timescale
of the phase-curve modulation in 55 Cnc e. To this end, we used the
CHaracterising ExOPlanet Satellite (CHEOPS), whose exquisite photometric
precision provides an opportunity to characterise minute changes in the phase
curve from one orbit to the next. CHEOPS observed 29 individual visits of 55
Cnc e between March 2020 and February 2022. Based on these observations, we
investigated the different processes that could be at the origin of the
observed modulation. In particular, we built a toy model to assess whether a
circumstellar torus of dust driven by radiation pressure and gravity might
match the observed flux variability timescale. We find that the phase-curve
amplitude and peak offset of 55 Cnc e do vary between visits. The sublimation
timescales of selected dust species reveal that silicates expected in an
Earth-like mantle would not survive long enough to explain the observed
phase-curve modulation. We find that silicon carbide, quartz, and graphite are
plausible candidates for the circumstellar torus composition because their
sublimation timescales are long. The extensive CHEOPS observations confirm that
the phase-curve amplitude and offset vary in time.We find that dust could
provide the grey opacity source required to match the observations. However,
the data at hand do not provide evidence that circumstellar material with a
variable grain mass per unit area causes the observed variability. Future
observations with the James Webb Space Telescope promise exciting insights into
this iconic super-Earth.
