Earths beyond Earth

Article excerpt

Twenty years from now, a quartet of space-based telescopes may revolutionize the way people think about the universe and their place within it. From a vantage point just inside the orbit of Jupiter, the four telescopes, known as the Terrestrial Planet Finder, will cast their infrared eyes on the nearest 1,000 stars similar in mass to the sun. The images they record will be combined in such a way as to cancel out the light from each star and amplify the elusive reddish glow of any planets that orbit it.

The result will be a set of pale red dots, one or more of which could be just like Earth.

Because these blobs, which show up in infrared light, have low resolution, astronomers won't be able to discern oceans, clouds, or mountains. Such features give Earth the appearance of a pale blue dot in visible light. Nonetheless, two criteria may indicate whether any of these fuzzy blobs, tens of light-years from our solar system, resembles our planet. The body must obey the Goldilocks rule: It must reside within a certain distance of its parent star so that water is neither too hot nor too cold to exist as a liquid on the planet's surface. Moreover, its atmosphere must contain three molecules-water, carbon dioxide, and ozone-in abundances deemed necessary for life as we know it.

The same set of infrared telescopes-1.5-meter detectors spaced evenly along a boom longer than a football field-will enable astronomers to determine whether these orbs pass muster as terrestrial twins. Whatever faint light planets emit or reflect, they radiate more in the infrared than in visible light, and the three molecules associated with life have clear signatures in the infrared.

"For the first time in human history, we are pretty confident that we know how to determine whether or not we are alone in the universe," says Harley A. Thronson, senior scientist with NASA's year-old Origins Program, which is devoted to exploring the origins of structure and life in the universe. "We're the first generation to say, 'Yes, we know how to do it,' and now we're taking the steps to do it."

On Earth, some of the first steps have already been taken. The search for other worlds got a major shot in the arm 18 months ago, when astronomers announced the first indirect detection of a planet orbiting a sunlike star. At last count, researchers have found evidence suggestive of eight giant planets by tracking the motion of stars. Telltale wobbles betray the gravitational tug of an unseen planet.

A spectrograph recently connected to the world's largest visible-light telescope, the 10-meter W.M. Keck atop Mauna Kea in Hawaii, and small, test versions of the telescopes to be launched into space may indirectly detect planets well below the mass of Jupiter. They may even spot planets as small as Uranus, which is about 15 times as massive as Earth. …