  Over the past decade, observations of giant exoplanets (Jupiter-size) have
provided key insights into their atmospheres, but the properties of lower-mass
exoplanets (sub-Neptune) remain largely unconstrained because of the challenges
of observing small planets. Numerous efforts to observe the spectra of
super-Earths (exoplanets with masses of one to ten times that of Earth) have so
far revealed only featureless spectra. Here we report a longitudinal thermal
brightness map of the nearby transiting super-Earth 55 Cancri e revealing
highly asymmetric dayside thermal emission and a strong day-night temperature
contrast. Dedicated space-based monitoring of the planet in the infrared
revealed a modulation of the thermal flux as 55 Cancri e revolves around its
star in a tidally locked configuration. These observations reveal a hot spot
that is located 41 +- 12 degrees east of the substellar point (the point at
which incident light from the star is perpendicular to the surface of the
planet). From the orbital phase curve, we also constrain the nightside
brightness temperature of the planet to 1380 +- 400 kelvin and the temperature
of the warmest hemisphere (centred on the hot spot) to be about 1300 kelvin
hotter (2700 +- 270 kelvin) at a wavelength of 4.5 microns, which indicates
inefficient heat redistribution from the dayside to the nightside. Our
observations are consistent with either an optically thick atmosphere with heat
recirculation confined to the planetary dayside, or a planet devoid of
atmosphere with low-viscosity magma flows at the surface.
