Three medium-sized planets of roughly the same mass as Neptune have been discovered around a nearby sunlike star, scientists announced Wednesday.
The finding, detailed in Thursday's issue of the journal Nature, marks a first for astronomers because previously discovered multiplanet solar systems besides our own contain at least one giant, Jupiter-sized planet.
"For the first time, we have discovered a planetary system composed of several Neptune-mass planets," said study team member Christophe Lovis of the Geneva Observatory in Switzerland.
The setup is similar to our own solar system in many ways: The outermost planet is located just within the star's habitable zone, where temperatures are moderate enough for liquid water to form, and the system also contains an asteroid belt.
The newly discovered planets have masses about 10, 12 and 18 times that of Earth, and they zip around the star in rapid orbits of about nine, 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, shrouded by a thick gas envelope.
The planets were discovered around HD 69830, a star slightly less massive than the sun, located 41 light-years away in the constellation Puppis the Stern, using the ultra-precise HARPS spectrograph on the European Southern Observatory's 3.6-meter La Silla telescope in Chile.
Recent observations by NASA's Spitzer Space Telescope last year revealed that HD 69830 also hosts an asteroid belt, making it the only other sunlike 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. However, the technique has since been refined to the point where lower-mass planets can now be detected.