Updated 5/17/2006 8:03 PM ET
By Dan Vergano, USA TODAY
European astronomers reported the smallest planet yet spotted in the "habitable zone" of a nearby sun-like star on Wednesday.
About the size of Neptune, the planet circles the star HD 69830, some 41 light-years away in the southern sky (one light-year equals about 5.9 trillion miles.)
Two other slightly smaller planets orbit closer to the star, reports the discovery team led by Christophe Lovis of Switzerland's Geneva Observatory.
Detections of the planets "suggest that the search for habitable planets might be easier than assumed," says Harvard astronomer David Charbonneau, in a commentary accompanying the report in Thursday's Nature magazine. The habitable zone planet is not Earth-like, says Lovis, likely cloaked in a high-pressure hydrogen atmosphere.
NASA probes planned for coming decades, including the Space Interferometer Mission and the Terrestrial Planet Finder, would likely be able to measure planets like those orbiting HD 69830, says astronomer Wesley Traub of NASA's Jet Propulsion Laboratory.
"The bottom line is that the discovery of this system really does give a big boost to our chances of finding other habitable planets around nearby stars," says planetary scientist Alan Boss of the Carnegie Institution of Washington.
The finding marks a first for astronomers because previously discovered multi-planet solar systems besides our own contain at least one giant, Jupiter-sized planet.
The newly discovered planets have masses of about 10, 12 and 18 times that of Earth and they zip around the star in rapid orbits of about 9, 32 and 197 days, respectively.
Based on their distances from the star, two inner worlds nearest the star are rocky planets similar to Mercury, the scientists suspect. The outermost planet is thought to have a solid core of rock and ice and shrouded by a thick gas envelope.
Recent observations by NASA's Spitzer Space Telescope last year revealed that HD 69830 also hosts an asteroid belt, making it the only other sun-like star known to have one.
When the asteroid belt was found, it was suspected that there might be an unseen planet that was shepherding the asteroids; it now seems that there is more than one shepherd. The researchers think the asteroid belt could lie between the two outermost planets or beyond the third planet.
The planets have not been photographed. They were found using the Doppler, or "wobble," technique, in which astronomers infer the presence of a planet by measuring the gravitational influence it exerts on its parent star. This technique was used to find most of the more than 180 planets so far discovered.
In the early years of planet hunting, the wobble technique was sensitive enough to spot only large, massive planets because they produce more significant stellar wobbles.
The technique has since been refined to the point where lower-mass planets can now be detected.