  With the recent discoveries of hundreds of extrasolar planets, the search for
planets like Earth and life in the universe, is quickly gaining momentum. In
the future, large space observatories could directly detect the light scattered
from rocky planets, but they would not be able to spatially resolve a planet's
surface. Using reflectance models and real cloud data from satellite
observations, here we show that, despite Earth's dynamic weather patterns, the
light scattered by the Earth to a hypothetical distant observer as a function
of time contains sufficient information to accurately measure Earth's rotation
period. This is because ocean currents and continents result in relatively
stable averaged global cloud patterns. The accuracy of these measurements will
vary with the viewing geometry and other observational constraints. If the
rotation period can be measured with accuracy, data spanning several months
could be coherently combined to obtain spectroscopic information about
individual regions of the planetary surface. Moreover, deviations from a
periodic signal can be used to infer the presence of relatively short-live
structures in its atmosphere (i.e., clouds). This could provide a useful
technique for recognizing exoplanets that have active weather systems, changing
on a timescale comparable to their rotation. Such variability is likely to be
related to the atmospheric temperature and pressure being near a phase
transition and could support the possibility of liquid water on the planet's
surface.
