They often follow highly elliptical orbital paths, subjecting them to enormous temperature swings. Some are more dense than lead; others would float like foam on water, notes William Borucki, a scientist at National Aeronautics and Space Administration’s Ames Research Center in Sunnyvale, Calif.
Some orbit so close to their host stars that one “year” in those planets is less than one Earth day.
The ultimate goal, however, is “to someday take a picture of a pale blue dot orbiting a nearby star,” says Debra Fischer, an astronomer at San Francisco State University. To decide how to do that in the most efficient and cost-effective way, it’s vital to know how many of these planets are likely to be out there for a given number of stars. The frequency with which Earth-like planets are found will be a key driver for future planet-hunting missions, she says.
And if Kepler finds only a tiny handful of such planets in habitable zones?
“That would be another profound discovery,” says Dr. Borucki, the mission’s lead scientist. “It will mean that Earths must be very rare. We may be the only extant life,” at least in our galaxy.
No aliens? Alien chemistries will do
Even if Kepler detects few Earth-like worlds in Earth-like orbits, the number of other planets it finds is likely to explode. Some solar-system experts would be content to give up the prospect of alien life forms for alien geology and chemistry.