But will there be life?

While an Earth-like planet might be the top prize for exoplanet hunters, the ultimate goal for many researchers is to find extraterrestrial life. For that, an exoplanet must be found in a so-called "habitable zone."

Habitability depends on a number of other factors, Deeg explains, including the size and temperature of the host star. And the planet must have a stable long-term climate with the right chemical ingredients, and there must be a magnetic field to protect against deadly radiation coming from the star and elsewhere in the cosmos.

"By virtue of being the largest planet, Jupiter acts as a gravitational vacuum cleaner," Butler explains. "Jupiter ended the period of 'heavy bombardment' in the early solar system in the same way that it dealt with comet Shoemaker-Levy." What Butler means is that Jupiter swallowed space rocks, sparing Earth the bulk of the potentially life-destroying collisions. "It is only after the period of heavy bombardment that life could gain a toehold on Earth," he says.

Jupiter's circular orbit also keeps Earth in a calm, circular orbit, Butler says. If Jupiter's orbit were not circular, it would throw Earth into an eccentric orbit, which would cause extreme climate variations as the planet moved close to the Sun and then farther away.

In spite of tremendous cost and technical hurdles, "the pressure on NASA and ESA to launch Darwin/TPF will be overwhelming," said Andrew Collier Cameron, a planet hunter at University of St. Andrews.

Cameron also thinks a potentially habitable planet could be discovered within a decade, but said it might be another five to 10 years before we know if that planet is actually capable of supporting life.

But Doyle, of the SETI Institute, said the discovery of an Earth-like planet would change everything. He wouldn't be surprised if his SETI colleagues decided to "sit on it for awhile," pointing their radio telescopes in the direction of the planet to listen for possible alien broadcasts.

"I think it might spark a lot of ingenuity to see if it is habitable or not," Doyle said.

Morris Aizenman, senior science associate in the National Science Foundation's Mathematical and Physical Science Directorate, is not a planet hunter. But he knows all about the current searches, and while his expectations for a discovery of a habitable planet anytime soon are reserved, he too expects they exist.

"It would be very unusual if there were no other habitable planets in our galaxy," Aizenman said. "But we're only looking at a small part of our galaxy with the instruments we currently have."

Aizenman agrees with Doyle and others that finding an Earth-like planet would be the ultimate impetus for developing better instruments, so a newly found sister world could be studied in detail and the search for other Earths could expand.

"The bottom line is that we just don't know what the probabilities are for finding a planet about the size of Earth in an essentially circular orbit at a distance from the star that such conditions would be habitable," Aizenman says. "Nevertheless, the hunt is a great adventure."