Astronomers have identified four Earth-sized planets orbiting a star 12 light years from our own Sun, two of which are in the habitable zone and so could be host to liquid water.
The planets, which are thought to be around 70% larger than Earth, orbit a star known as tau Ceti, which is in the Cetus constellation, and has a similar spectral range to our sun, although only 78% of the mass.
As a result, the astronomers, led by a team at the University of Hertfordshire, not only believe that the two within the habitable zone could be host to life, but could be targets for colonisation by our descendents.
The research, which is published today in the Astronomical Journal, represents a significant step in the ongoing hunt for life on other planets, providing one of the closest discoveries outside of our own solar system.
The planets were detected not by direct imaging – that is, capturing an image of them using an instrument such as the Hubble space telescope – but by observing wobbles in the movement of the star, from which they can infer the existence, likely size and orbit distance of surrounding planets.
“We’re getting tantalisingly close to observing the correct limits required for detecting Earth-like planets,” said study lead author Dr Fabo Feng, a research fellow at the University of Hertfordshire.
“Our detection of such weak wobbles is a milestone in the search for Earth analogs and the understanding of the Earth’s habitability through comparison with these.”
This technique was first implemented by the astronomers in 2013, and has proved vital to the discovery of planetary bodies.
“We realised that we could see how the star’s activity differed at different wavelengths and use that information to separate this activity from signals of planets,” said Dr Mikko Tuomi, study co-author and pioneer of the technique.
“Since then we’ve painstakingly improved the sensitivity of our techniques and could rule out two of the signals our team identified in 2013 as planets. But no matter how we look at the star, there seems to be at least four rocky planets orbiting it.”
However, while the discovery is undoubtedly exciting, the astronomers have identified another quality to the star that could make the survival of life, either in the form of biologically native organisms or occupying humans, more challenging.
Tau Ceti also have what the university describes as a “massive debris disc” surrounding it, which would likely mean that the planets would be frequently bombarded with asteroids and comets, playing havoc with any atmosphere are providing challenging environmental conditions.
Nevertheless, the planets could well prove to be a valuable stop for future humans, and may even provide a home for hitherto-undiscovered life.