Three medium-sized planets of
roughly the same mass as Neptune have been discovered around a nearby Sun-like star,
scientists announced today.
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.
The finding, detailed in the May
18 issue of the journal Nature,
marks a first for astronomers because previously discovered multi-planet 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 planets 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 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. However, the
technique has since been refined to the point where lower-mass planets can now be
detected.
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