A "hot Neptune" passes close to a "super Earth" orbiting a distant star. Artist's conception by David A. Aguilar (CfA)

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 today 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 Eric Agol of the University of Washington, one of the researchers. He and his colleagues spotted the planets using NASA's Kepler probe, which has been planet-hunting since 2009.

Both worlds are probably infernos, so close to their host star that they zip around it in 14 and 16 Earth-days, respectively. That means they pass very close to each other - less than five times as far apart as our moon is from us. If you've ever been struck by moonrise on a summer night, imagine a world about 12 times as large.

David Aguilar, a space artist at the Harvard-Smithsonian Center for Astrophysics, has done us the favor of imagining the scene himself. The result is the conception above.

Which is good, because the two planets are so close to their sun (11-12 million miles, compared to our 93 million) that you wouldn't want to go there yourself.