Artist's concept of Earthlike planet.

Artist's concept of planet orbiting a star.

Artist's concept of Terrestial Planet Finder.

Looking for Life in all the Right Places

In this vast universe, we know of only one site that has rolled out the welcome mat for life -- our planet Earth. Yet we can't help but wonder whether life in some form might exist elsewhere in the cosmos -- perhaps advanced life forms like humans or maybe the slimy mold like the type that thrives in showers.

NASA scientists are trying to answer that age-old question "Are we alone?" by looking at other celestial bodies that might have life, with much of their search concentrated on finding Earthlike planets orbiting other stars. That's because scientists understand many of the conditions that led to life here on Earth, and by looking for similar planets with Earthlike conditions, they hope to increase the odds of finding life.

Location, Location, Location

Until 1995, scientists suspected that other stars might have planets in orbit around them, but they couldn't prove it. Then, in October 1995, Michel Mayor and Didier Queloz, astronomers at the Geneva Observatory in Switzerland, found an extra-solar planet orbiting the star 51 Pegasi. They knew it was there because they watched it over a long period of time and observed that the star was moving slightly toward and away from Earth every four days. That indicated that the star was being affected by the gravitational tug of an unseen planet in orbit around it.

Since then, astronomers have detected dozens of planets using similar techniques. With current technology, however, they can find only very large planets like Jupiter, which probably don't harbor life. Unlike Earth or Mars, Jupiter is composed completely of gas and has no solid surface where life can form. Astronomers are anxious to find smaller, more Earthlike planets, with conditions more favorable to life as we know it.

The Hunt is On

This quest presents an enormous challenge. NASA's Origins Program is taking on the challenge with a series of missions designed to find warm, wet, Earthlike planets around other stars -- planets that might sustain life. To do this, we'd need extremely large telescopes, much too big and expensive to be practical. In addition, a planet looks very dim compared to its parent star, which shines like our Sun. It's like looking for a firefly in front of a searchlight.

To help overcome these obstacles, NASA is using a technology called interferometry, which combines light gathered by multiple telescopes to create a much larger, "virtual telescope." This produces a much sharper, more detailed image. Several Origins missions, including the Space Interferometry Mission, with a planned launch later this decade, and the Keck Interferometer, already operating on the ground in Hawaii, are expected to help us find smaller planets than current technology allows.

A much more ambitious mission, like the Terrestrial Planet Finder, may put multiple telescopes on separate spacecraft to fly in formation. Working together, these telescopes would then be able to take "family portraits" of entire solar systems and would analyze spectra to look for the chemical fingerprint life would leave on the reflected light from the planet.

If you went far out into space and looked back at Earth, with special instruments you could detect oxygen, ozone, carbon dioxide and other chemicals in Earth's atmosphere. That would tell you that Earth has living things. Terrestrial Planet Finder will look for similar, telltale chemical signatures of life when it observes Earth-sized planets around other, nearby stars.

We know that life on Earth requires water, an energy source and certain chemicals. We also know that life is surprisingly hardy -- it survives in extremely hostile environments, such as boiling, toxic thermal vents on the ocean floor. If and when we find life elsewhere in the universe, we don't know whether it will be like anything we see on Earth. One thing we can guarantee -- the hunt for planets and the search for life take us on an exciting and profound adventure.