An international team of scientists, including Carnegie's Paul Butler, has discovered that Tau Ceti, one of the closest and most Sun-like stars, may have five planets. Their work is published by Astronomy and Astrophysics and is available online. At a distance of twelve light years and visible with a naked eye in the evening sky, Tau Ceti is the closest single star with the same spectral classification as our 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. 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 a Sun-like star.

The international team of astronomers, led by Mikko Tuomi from the University of Hertfordshire, combined more than six-thousand observations from three different instruments and applied intensive modeling to the data.

Using new techniques, the team found a method to find signals half the size previously thought possible, which greatly improves the sensitivity of searches for small planets and suggests that Tau Ceti is not a lone star but has a planetary system.

"We pioneered new data modeling techniques by adding artificial signals to the data and testing our recovery of the signals with a variety of different approaches," Tuomi said. "This significantly improved our noise modeling techniques and increased our sensitivity to find low mass planets."

Tau Ceti was chosen for this noise-modeling study because the team thought it had no signals and would be a good benchmark system to test their methods for planet detection.

This is particularly true because it is so bright and similar to our own Sun. It's also one of Earth's nearest cosmic neighbors, so scientists could be able to learn about the atmospheres of these planets in the not-too-distant future.

More than 800 planets have been discovered orbiting other worlds, but planets in orbit the around the nearest Sun-like stars are particularly valuable for research.

"We are now glimpsing for the first time the secrets of our nearest companion stars and their previously hidden reservoirs of potentially habitable planets," Butler said. "This work presages the time when we will be able to directly see these planets, and search them for water, carbon dioxide, methane, and other signposts of life."

This research is supported by Planets Around Cool Stars, a Marie Curie Initial Training Network funded by the European Commission's Seventh Framework Programme. Fondecyt, Centro de Astrofisica, the GEMINI-CONICYT FUND, the Comit'e Mixto ESO-GOBIERNO DE CHILE, the NSF, the Australian government.