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.