Taking dim view of stars to find more Earths

Proposed infrared interferometer with multiple observatories anchored to one structure.

(CNN) -- The U.S. space agency has selected two promising designs for an advanced space telescope that could detect Earth-sized planets and life around other stars.

NASA chose the finalists from more than 60 candidates after 2.5 years of deliberation. The agency hopes to launch the Terrestrial Planet Finder, or TPF, mission within a decade or so.

Observatories based on the ground and in orbit have discovered dozens of gas giant planets in other star systems.

But to detect ones as small as our own -- the equivalent of finding a needle in a field of haystacks -- would require a major step forward in technology, according to space scientists.

The two proposals, NASA announced Friday, "were determined to be sufficiently realistic to warrant further study and technological development."

They would use extremely different methods to achieve the same goal. The infrared interferometer would rely on a fleet of small telescopes attached to one structure or flying in a precise formation, thereby simulating a much bigger observatory.

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When looking at distant stars, the contraption would use a technique to cancel out light waves and give astronomers a chance to pick up the infrared emissions of planets, according to NASA.

If done correctly, the procedure would work sort of like a comb, blocking out the starlight with the "teeth" and allowing the light of much fainter companion objects to slip between the cracks, according to Dan Coulter, TPF scientist at NASA's Jet Propulsion Laboratory in Pasadena, California.

The other finalist, the visible light coronagraph, would be an optical telescope with a mirror several times larger and more precise than that on the Hubble Space Telescope, outfitted with special optics to block starlight.

"The coronagraph approach is a fancy way of putting a thumb over the starlight, looking at the edge and seeing what's there," said Charles "Chaz" Beichman, another TPF researcher.

Making the search more formidable, NASA hopes the orbiting observatory will offer hints whether any spotted terrestrial-sized planets exhibited signs of biologic activity.

"The brightest dozen or two dozen will be bright enough to see water vapor, carbon dioxide, which tell you a lot about the state of the planet," Beichman said.

"Further down the line, you could see oxygen, ozone and methane, chemicals that could mean they have life."

No one has come up with a price tag for the mission, but preliminary estimates place it between $1 billion and $2 billion.