  Orbiting a bright, nearby star the 55 Cnc system offers a rare opportunity to
study a multiplanet system that has a wide range of planetary masses and
orbital distances. Using two decades of photometry and spectroscopy data, we
have measured the rotation of the host star and its solar-like magnetic cycle.
Accounting for this cycle in our velocimetric analysis of the system allows us
to revise the properties of the outermost giant planet and its four planetary
companions. The innermost planet 55 Cnc e is an unusually close-in super-Earth,
whose transits have allowed for detailed follow-up studies. Recent observations
favor the presence of a substantial atmosphere yet its composition, and the
nature of the planet, remain unknown. We combined our derived planet mass (Mp =
8.0+-0.3 Mearth) with refined measurement of its optical radius derived from
HST/STIS observations (Rp = 1.88+-0.03 Rearth over 530-750nm) to revise the
density of 55 Cnc e (rho = 6.7+-0.4 g cm-3). Based on these revised properties
we have characterized possible interiors of 55 Cnc e using a generalized
Bayesian model. We confirm that the planet is likely surrounded by a
heavyweight atmosphere, contributing a few percents of the planet radius. While
we cannot exclude the presence of a water layer underneath the atmosphere, this
scenario is unlikely given the observations of the planet across the entire
spectrum and its strong irradiation. Follow-up observations of the system in
photometry and in spectroscopy over different time-scales are needed to further
investigate the nature and origin of this iconic super-Earth.
