Rocky extrasolar planets thought to be half frozen and half
scorched might instead rock back and forth, creating large
swaths of twilight with temperatures suitable for life. Because
of gravitational tugs with the objects they orbit, rocky bodies
often settle into trajectories in which they always show the
same face to their hosts. Such 'tidally locked' exoplanets would
thus seem like bad candidates for life, since the hemisphere
facing their host stars would roast and the dark side would
freeze. But a new computer model by Anthony Dobrovolskis of NASA
Ames Research Center in California, US, suggests this is not
always so. He finds that such planets can rock to and fro if
they travel on elongated, or eccentric orbits, creating a
'twilight zone' that could be hospitable to life. The Moon
experiences a similar rocking motion. It always shows the same
face to Earth, taking the same amount of time to rotate around
its axis as it does to circle our planet once. However, because
the Moon's path around the Earth is