Sen— Inventive techniques using observations with some of the world's biggest telescopes have allowed astronomers to discover five new planets orbiting one of the closest stars to the Earth.
Tau Ceti, which lies only 12 light-years away and can easily be spotted on a clear night, is a star similar to the Sun and also single. One of the new worlds around it is in its habitable zone, so named because water could exist on it in liquid form.
The new planets all have sizes, or masses, between two and six times that of the Earth, making the new solar system one of the least massive yet found. But that is not altogether surprising because smaller worlds are bound to be easier to detect when they are closer to us.
The discoveries, which come just two months after the announcement of an Earth-sized planet in the nearest star system to us, Alpha Centauri, were made by an international team from the UK, Chile, the USA and Australia.
They examined the starlight from Tau Ceti using spectrographs on three telescopes - HARPS on the 3.6m telescope at the European Southern Observatory in La Silla, Chile, UCLES on the Anglo-Australian Telescope in Siding Spring, Australia, and HIRES on the 10m Keck telescope on Mauna Kea, Hawaii (567 data points).
After making more than 6,000 measurements, the team using computer modelling techniques and managed to improve the sensitivity of their observations so that smaller planets than normal revealed themselves in the data.
The team's leader, Mikko Tuomi from the University of Hertfordshire in the UK, explained: “We pioneered new data modelling techniques by adding artificial signals to the data and testing our recovery of the signals with a variety of different approaches. This significantly improved our noise modelling techniques and increased our ability to find low mass planets."
Hugh Jones, also from the University of Hertfordshire, said: "We chose Tau Ceti for this noise modelling study because we had thought it contained no signals. And as it is so bright and similar to our Sun it is an ideal benchmark system to test out our methods for the detection of small planets."
Where to spot Tau Ceti in the night sky. Credit: University of Hertfordshire
The new worlds add to a harvest of more than 800 exoplanets that have been discovered around other stars since 1995. Most of those found have been "hot Jupiters" - gas giants zipping round close to their host stars in just days.
Team member Steve Vogt, from University of California Santa Cruz, said "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. They are everywhere, even right next door!
"We are now beginning to understand that Nature seems to overwhelmingly prefer systems that have a multiple planets with orbits of less than one hundred 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."