An international team of astronomers has discovered that one of the closest and most Sun-like stars may host five planets, including one in the star's habitable zone.

At a distance of 12 light years from Earth and visible to the naked eye in the evening sky, Tau Ceti is the closest single star that has the same spectral classification as the sun. Its five planets are estimated to have masses between two and six times the mass of the Earth, making it the lowest-mass planetary system yet detected, researchers announced Wednesday.

One of the planets lies in the habitable zone of the star and has a mass around five times that of Earth, making it the smallest planet found to be orbiting in the habitable zone of any Sun-like star.

The astronomers combined more than 6,000 observations from three different instruments and intensively modeled the data. Using new techniques, the team has found a method to detect signals half the size previously thought possible.

The team presented its findings in a paper for Astronomy & Astrophysics.

"This discovery is in keeping with our emerging view that virtually every star has planets, and that the galaxy must have many such potentially habitable Earth-sized planets," said co-author Steve Vogt, a professor of astronomy and astrophysics at UC Santa Cruz. "We are now beginning to understand that nature seems to overwhelmingly prefer systems that have multiple planets with orbits of less than

100 days. This is quite unlike our own solar system, where there is nothing with an orbit inside that of Mercury.

"So our solar system is, in some sense, a bit of a freak and not the most typical kind of system that Nature cooks up."