An international team of astronomers led by the University of Hertfordshire has discovered that Tau Ceti, 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 and visible with the naked eye, Tau Ceti is the closest single star that has 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 within the habitable zone of its 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 international team of astronomers, from the UK, Chile, the USA, and Australia, combined more than 6,000 observations from three different instruments and intensively modelled the data. Using new techniques, they found a method to detect signals half the size previously possible. This greatly improves the sensitivity of searches for small planets and suggests that Tau Ceti is not a lone star, but hosts a rich planetary system.
One of our nearest cosmic neighbours
Mikko Tuomi, from the University of Hertfordshire and the first author of the paper, said: "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: "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."
James Jenkins, Universidad de Chile and Visiting Fellow at the University of Hertfordshire, explained: "Tau Ceti is one of our nearest cosmic neighbours and so bright that we may be able to study the atmospheres of these planets in the not-too-distant future. Planetary systems found around nearby stars close to our Sun indicate that these systems are common in our Milky Way galaxy."
Size comparison of our Sun (left) and Tau Ceti (right). Credit: RJ Hall
Virtually every star has planets
So far, over 800 planets have been discovered, but those in orbit around the nearest Sun-like stars are especially valuable. Steve Vogt from University of California Santa Cruz: "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 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."
Chris Tinney from the University of New South Wales: "As we stare at the night sky, it is worth contemplating that there may well be more planets out there than there are stars ... some fraction of which may well be habitable."
In October 2012, an Earth-sized planet was found orbitting Alpha Centauri, our nearest stellar neighbour. However, Alpha Centauri comprises a trinary (triple) star system, whereas Tau Ceti is a single star like our own. Given its stability, similarity and relative proximity to us, Tau Ceti is consistently listed as a target for the Search for Extra-Terrestrial Intelligence (SETI).