  Most known terrestrial planets orbit small stars with radii less than 60%
that of the Sun. Theoretical models predict that these planets are more
vulnerable to atmospheric loss than their counterparts orbiting Sun-like stars.
To determine whether a thick atmosphere has survived on a small planet, one
approach is to search for signatures of atmospheric heat redistribution in its
thermal phase curve. Previous phase curve observations of the super-Earth 55
Cancri e (1.9 Earth radii) showed that its peak brightness is offset from the
substellar point $-$ possibly indicative of atmospheric circulation. Here we
report a phase curve measurement for the smaller, cooler planet LHS 3844b, a
1.3 Earth radius world in an 11-hour orbit around a small, nearby star. The
observed phase variation is symmetric and has a large amplitude, implying a
dayside brightness temperature of $1040\pm40$ kelvin and a nightside
temperature consistent with zero kelvin (at one standard deviation). Thick
atmospheres with surface pressures above 10 bar are ruled out by the data (at
three standard deviations), and less-massive atmospheres are unstable to
erosion by stellar wind. The data are well fitted by a bare rock model with a
low Bond albedo (lower than 0.2 at two standard deviations). These results
support theoretical predictions that hot terrestrial planets orbiting small
stars may not retain substantial atmospheres.
