  This article relates two topics of central importance in modern astronomy -
the discovery some fifteen years ago of the first planets around other stars
(exoplanets), and the centuries-old problem of understanding the origin of our
own solar system, with its planets, planetary satellites, asteroids, and
comets. The surprising diversity of exoplanets, of which more than 500 have
already been discovered, has required new models to explain their formation and
evolution. In turn, these models explain, rather naturally, a number of
important features of our own solar system, amongst them the masses and orbits
of the terrestrial and gas giant planets, the presence and distribution of
asteroids and comets, the origin and impact cratering of the Moon, and the
existence of water on Earth.
