In the end, our most definitive proof of extra-solar planets might come from space probes, sent from Earth as our species' first cosmic explorers. The first probes, space telescopes freed from the obscuring effects of the earth's atmosphere, should include:

• The Kepler Spacecraft

  Illustration: NASA's Kepler Mission Web Site

  KEPLER: This proposed orbiting telescope, named after German astronomer Johannes Kepler, could be launched within five years. Kepler would rely on "photometry," precise measurements of a star's brightness, to see if there are any changes caused by the passage of Earth-sized or larger planets in front of the star. A small portion of the sun's light, for example, changes by one part in 12,000 when the Earth passes in front of it, explains David Koch of the NASA Ames Research Center. "That's not a lot, but it's measurable." Kepler would continuously monitor 160,000 stars, according to current plans. Only a fraction of those stars are solar-like, "but we decided to broaden our horizons," Koch says. "We don't want to be too parochial in our views of where we might find planets."
  

• The Planet Finder Spacecraft

  Illustration: Exploration of Neighboring Planetary Systems Web Site

  Terrestrial Planet Finder: NASA is laying plans for an orbiting telescope that would not only take pictures of planetary systems, but also study the atmospheres of the planets. OASES, a leading design proposed by University of Arizona astronomers Roger Angel and Nick Woolf, would consist of an array of four infrared telescopes, aligned with unparalleled precision, placed near the orbit of Jupiter -- far enough from the sun that views would not be obscured by the haze of glowing dust. By studying the light emanating from extrasolar planets, scientists could look for atmospheric gases such as carbon dioxide, water vapor, oxygen, and ozone which might reveal unmistakable signs of life.
  
  
• The European Space Agency (ESA) is planning an almost identical space observatory called Darwin. Since both missions would be so complex and expensive (estimated at about $1.5 billion), "there is the probability that the NASA and ESA efforts will eventually merge into a single, joint venture," says Harley Thronson, a NASA scientist closely involved in both projects.