Anybody out there? Astronomers announce recent discoveries of two Earth-like planets.
These are heady times for planet hunters. During the past decade, astronomers have uncovered at least 125 confirmed planets in 98 solar systems in our corner of the Milky Way. But generally these planets have been huge - up to 17 times more massive than Jupiter. Earth-like planets - the "holy grail" of planet hunting - are too small to detect.
Tuesday, however, two teams of astronomers in the US reported breaking the size barrier with the discovery of two of the smallest planets yet, orbiting two nearby stars. The announcement follows a similar report last week from European astronomers of another small planet orbiting a third star. Where past discoveries have detected planets the size of Jupiter and larger, these new finds are closer in scale to Neptune and Uranus.
In addition, a fourth team announced last week that they had found a Jupiter-class planet using a new network of telescopes.
The discovery of the three smaller planets in particular - combined with a new, exquisitely sensitive camera bolted to the back end of one of the world's most powerful telescopes at Keck Observatory in Hawaii - appears to boost the prospects for discovering Earth-size planets within the next few years.
Such finds not only would be groundbreaking in their own right, astronomers say. They also would provide a set of early targets for a suite of space-based planet-hunting telescopes, the first of which is scheduled for launch in 2007. And the new finds are presenting a fresh set of challenges to theories about how solar systems form.
The pace of discovery for smaller planets "is picking up," says Johns Hopkins University astronomer Zlatan Tsvetanov, who is the program scientist for NASA's Terrestrial Planet Finder project. "It's really an exciting time."
Underpinning these efforts is a question that has captivated humanity since it first understood the true nature of the night sky: Are we alone in the galaxy?
"It's easy to get carried away and ask questions you have no way of answering," says Timothy Brown, a researcher at the National Center for Atmospheric Research in Boulder, Colo., and co-leader of one of the four planet-hunting teams. But, he adds, humans do have the capability to begin grappling with the question of how common Earth-like planets may be. "The only way to answer that is to go out and look," he says.