Photo: Artist's impression: five planets orbit the star Tau Ceti, which is just 11.9 light-years from Earth. (J Pinfield/RoPACS network/University of Hertfordshire)
By Science Online's Stuart Gary and wires
Scientists have been surprised by the unexpected discovery of a nearby planetary system using a new experimental technique.
The discovery, reported in the journal Astronomy and Astrophysics, will make it easier to find smaller, more Earth-like planets orbiting other stars.
As well as being the nearest system to the Earth with a Sun-like star, it includes one planet with a mass about five times that of the Earth that is orbiting in the star's habitable zone, where liquid water could exist on the surface.
The new planetary system was discovered using the radial velocity method, which detects the wobble in a star caused by the gravitational pull of an orbiting planet.
Since 1995, more than 800 exoplanets have been discovered orbiting stars other than the Sun.
But most are either uninhabitable gas giants or big rocky worlds that swing so close to their star that they are literally roasted.
The Tau Ceti finding was made by astronomers from Australia, Britain, Chile and the United States.
One of the paper's authors, University of New South Wales astronomer Jonti Horner, says the discovery was serendipity.
"Mikko Tuomi, from the University of Hertfordshire, was looking at new ways to model data to remove unwanted interference and noise generating false signals in our equipment," Dr Horner said.
"This noise can be caused by vibrations in the star due to effects like starquakes and turbulence, as well as instabilities in the equipment itself."
Valuable targetTau Ceti was selected to calibrate the new technique because it is a very stable star, which after 14 years of study showed no signs of a planetary system.
"Because it's so close, bright and similar to the Sun, it's a particularly valuable target for study," Dr Horner said.
Once all the noise had been accounted for using the new modelling techniques, astronomers detected a signal indicating the presence of a planetary system.
University of NSW astronomer Chris Tinny, a co-author on the paper, says the new technique doubles the sensitivity of detecting planets using the radial velocity method.
Astronomers believe the proximity and brightness of Tau Ceti will allow them to eventually study the atmospheres of the planets in the system.
Gliese 667CThe radial velocity technique has also been used in the discovery of three new planets orbiting in the habitable zone of a red dwarf star called Gliese 667C.
The research by University of British Columbia astronomer Philip Gregory includes a planet just over twice the mass of the Earth.
That makes it the lowest mass planet ever detected in a habitable zone.
Gliese 667C is part of a triple star system about 22 light years from Earth in the constellation of Scorpius.
Professor Gregory discovered six planets orbiting the red dwarf, which circles about 30 billion kilometres out from a pair of larger Sun-like stars named Gliese 667A and B.