What the experts want to find are planets where the temperature is right and the orbit is not too far or too close to the central star. There needs to be liquid water and oxygen. It also has to be in a reasonably quiet neighborhood without frequent asteroids or comets collisions.
A place, in other words, kind of like Earth.
"The questions are how many stars have planets and how many of those planets are habitable," said Charles Beichman, a scientists at the Jet Propulsion Laboratory in Pasadena, Calif. "Within the next decade we'll get some very good answers."
Researchers have found extrasolar planets by using a technique that measures the very, very slight wobble motion that an orbiting planet causes in its central star. But all of the discovered planets are large, most many times the size of Jupiter — which is 318 times more massive than Earth — and unfriendly to life.
Beichman said Wednesday at a symposium at the Space Telescope Science Institute that in 2007, the National Aeronautics and Space Administration hopes to start launching specialized spacecraft to conduct a systematic search for Earth-like planets, and then to probe those planets for the chemical evidence of life.
The first step is the launch of a spacecraft called Kepler. Beichman said the Earth-orbiting craft will spend two years to three years observing light from stars and will be looking for a slight dimming, evidence of a shadow cast by an orbiting planet.
Kepler's instruments will be keen enough to detect Earth-sized objects orbiting stars up to 4,000 light-years away. It will take measurements every 10 minutes and could study up to 100,000 stars.
"This will give us a statistical sample" of how common are Earth-sized planets, Beichman said.
In 2009, another spacecraft, the Space Interferometer Mission, will be launched to analyze stars out to about 50 light-years. This craft will analyze the wobble of a central star and should be able to detect planets the size of Earth orbiting within a distance of 0.5 to 10 astronomical units. An astronomical unit is the distance between the Earth and sun, about 93 million miles.
Beichman said the Kepler and SIM spacecraft will locate targets for the third spacecraft, the Terrestrial Planet Finder, scheduled for launch about 2015.
This craft, he said, will analyze the atmospheres of the candidate planets, searching for the chemical signature of life.
The Planet Finder will be able to shield its instruments from the light of the central star and gather light that has been reflected from the target planet. Each molecule in the planet's atmosphere will leave a characteristic signature in the collected light. As a result, researchers will be able to tell if the atmosphere contains water, carbon dioxide, oxygen or methane, the telltale chemicals of living systems.
Beichman said that if these missions are successful, it will provide a statistical clue about how common are habitable planets in the whole universe, which contains trillions of stars like the sun and could, conceivably, contain trillions of Earths.
The conditions required for the formation of life continues to be a source of speculation by scientists, but there is a consensus on some general principals.
For life to form, there has to be a planet or another body orbiting near enough to a star to be in the "habitable zone" — an orbit warm enough to hold liquid water, but not too hot.
The planet's central star must be one that will last for at least 10 billion years. This gives time for the planet to cool from its formation and to establish a stable surface. This could take a billion years.
After that, it is believed that life forms rather quickly. On Earth, for instance, there is evidence that life started before 3.5 billion years ago.
For that primitive life to evolve to higher forms may depend on how severely the planet is scoured by colliding asteroids and comets, experts believe.
Intelligent, technically advanced life takes about 5 billion years to evolve, and so may be very, very rare even in a universe with trillions of planets, the experts say.