Title: A Case for an Atmosphere on Super-Earth 55 Cancri e,
Abstract: One of the primary questions when characterizing Earth-sized and
super-Earth-sized exoplanets is whether they have a substantial atmosphere like
Earth and Venus or a bare-rock surface like Mercury. Phase curves of the
planets in thermal emission provide clues to this question, because a
substantial atmosphere would transport heat more efficiently than a bare-rock
surface. Analyzing phase curve photometric data around secondary eclipse has
previously been used to study energy transport in the atmospheres of hot
Jupiters. Here we use phase curve, Spitzer time-series photometry to study the
thermal emission properties of the super-Earth exoplanet 55 Cancri e. We
utilize a semi-analytical framework to fit a physical model to the infrared
photometric data at 4.5 micron. The model uses parameters of planetary
properties including Bond albedo, heat redistribution efficiency (i.e., ratio
between radiative timescale and advective timescale of the atmosphere), and
atmospheric greenhouse factor. The phase curve of 55 Cancri e is dominated by
thermal emission with an eastward-shifted hot spot. We determine the heat
redistribution efficiency to be ~1.47, which implies that the advective
timescale is on the same order as the radiative timescale. This requirement
cannot be met by the bare-rock planet scenario because heat transport by
currents of molten lava would be too slow. The phase curve thus favors the
scenario with a substantial atmosphere. Our constraints on the heat
redistribution efficiency translate to an atmospheric pressure of ~1.4 bar. The
Spitzer 4.5-micron band is thus a window into the deep atmosphere of the planet
55 Cancri e.