Over the past decade about 130 extrasolar planets have been discovered, and the number is steadily rising.
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None of the distant planets are visible by modern telescopes, however. Scientists rely on indirect methods to detect the planets. For example, some are identified by the "wobble" their gravity induces in the stars they orbit.
Many of these exoplanets are large planets that resemble the gas giants of our solar system: Jupiter, Saturn, Uranus, and Neptune.
"We just don't have the technology at the moment to detect Earth-size planets," Jones said. He added that it may be ten years until astronomers detect such and analyze their atmospheres.
Only then can scientists determine if they "are potentially habitable or even inhabited," Jones said.
Geoscientist James F. Kasting hopes to be among the first researchers to directly observe such planets. The Pennsylvania State University professor is part of a NASA Jet Propulsion Lab project dubbed Terrestrial Planet Finder C (TPF-C).
It will likely be a decade before TPF-C can spy Earthlike planets. When it does, Kasting suspects they might be located around stars that currently reveal no planets at all.
"I personally think that the systems we've seen so far are not the best candidates for having Earthlike planets," Kasting said. "There are a lot of stars out there which could also have Earthlike planets and may be even more likely candidates."
"With the observations we've used so far, we wouldn't detect planets looking at our own solar system. We just don't have the observations yet."
Perhaps 150 subgiant or red giant stars lie within a hundred light-years of Earth. (Subgiants and red giants are stars in the later stages of their evolution.)
Some say those 150 stars may lie close enough to Earth that we may one day launch planet-finding space missions to their systems.
But long before any such efforts take place, new imaging technology will probably be available. Most likely it could add to the tally of known exoplanetswithout a space mission. The number already seems to grow on a monthly basis.
"It's still philosophical at this point, but I'm an optimist," Kasting said. "I think we'll end up finding a lot of terrestrial planets. Over the next five years or so, I think people using Doppler techniques [observing shifts in electromagnetic waves] will find systems that look much more like ours in terms of giant planets."
For now, the possibility that life inhabits any such distant worlds remains purely theoretical.
Jones, the Open University astronomer, said, "We do believe that if you form Earth-mass planets in the Goldilocks zone, there is no reason we know [of] why those planets couldn't be habitable."
"We've offered a tantalizing possibility: We've shown that 'Earths' could indeed exist in the Goldilocks zone of many of the systems we already know of," he said. "The next job is to see if they are really there."
Jones and his colleagues described their study in the April 1 issue of The Astrophysical Journal.
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