Perhaps the most intriguing exoplanets found so far are those bigger than our rocky, oceanic Earth but smaller than cold, gas-shrouded Uranus and Neptune. This mysterious class
of in-between planets —alternatively dubbed super-Earthsor mini-Neptunes — confounds scientists because nothing like them exists as a basis for comparison in our solar system.
"We don't really know what they are," said Björn Benneke, a graduate student in astronomy at the Massachusetts Institute of Technology. "They can be a scaled-down version of the giant planets in our solar system, a scaled-up version of terrestrial planets like Earth or something completely different."
Benneke is co-author of a paper accepted by the Astrophysical Journal that attempts to solve this vexing riddle. Based on numerical computer models, he developed an observational strategy that would let astronomers distinguish between two very different types of atmospheresassociated with these planets. Learning about their atmospheres will speak to the overall nature of these heretofore unknowable worlds with masses ranging up to about 10 times that of Earth. (Uranus and Neptune have 14 and 17 Earth-masses, respectively.) [The Strangest Alien Planets (Gallery)]