Not very long ago, space scientists expected that if other stars
had solar systems, they'd be much like our own - small, rocky
planets (like Mercury, Venus, Earth and Mars) orbiting close to
their host star, with gas giants (think of Jupiter, Saturn,
Uranus and Neptune) farther out. There were good reasons for
this, not the least of which was that any self-respecting star
would tear a Jupiter-sized planet apart if it were too close.
Everything made sense. So much for that. Since the 1990s,
astrophysicists report they have identified at least 600 stars
with planets circling them and found that solar systems like
ours are a rarity. Take Kepler 36, reported Friday in the
journal Science. It's a solar system about 1,200 light-years
from our own, with two very different planets right on top of
each other. One is a rocky "super Earth," about 1.5 times as
large as our world. The other is a gaseous "hot Neptune," about
3.7 times as large. "They are the closest to each other of any
planetary system we've found," said