Mankind's knowledge of planets, limited for most of human
history to our own Earth and its solar system neighbors, has
exploded in the last 20 years with the discovery of hundreds of
exo-planets in the cosmos, a search with the ultimate goal of
finding an Earthly twin that could harbor some form of life. For
most of history the only planets (from the Greek for "wandering
star") known were those visible to the naked eye as they moved
-- or "wandered" -- against the background of stars. With the
invention of the telescope the number of planets in our solar
system was fixed at nine, now reduced to eight (apologies,
Pluto, for being redesignated a "dwarf"). Then in 1992, the
first "extra-solar" planets were detected around a pulsar,
followed in 1995 by the detection of the first planet orbiting a
main-sequence star, a giant planet circling 51 Pegasi. What
followed was an explosion of discovery, with the number of
confirmed exo-planets now at 760. Most have been giant planets,
so-called "hot Jupiters," since their