Since prehistoric times, people have watched the moon waxing and
waning through its phases. Four hundred years ago, Galileo was
the first to observe similar phases of the planet Venus. Now,
for the first time, astronomers have seen the changing phases of
a planet outside of our solar system. They expect similar
observations in the future to reveal important details about the
atmosphere, cloud cover, and climate of planets orbiting other
stars. Most extrasolar planets (or exoplanets for short) can't
be seen directly, especially if they are very close to their
parent stars; astronomers see only the combined light of the
star and the planet. Sometimes, they happen to see the orbits of
exoplanets edge-on. In those cases, the planet alternately shows
more of its dayside and then more of its nightside as it circles
its star. These phases should show up as tiny variations in the
intensity of the observed light, researchers have predicted.
Astronomers Ignas Snellen of Leiden Observatory in the
Netherlands and colleag