9 August 2017

A new study by an international team of astronomers, led by the University of Hertfordshire, reveals that tau Ceti, the nearest Sun-like star (about 12 light years away from the Sun), has four Earth-sized planets orbiting it – two of which could be habitable.

These four planets have masses as low as 1.7 Earth mass, making them among the smallest planets ever detected around the nearest Sun-like stars. Two of them are super-Earths located in the habitable zone* of the star and thus could support liquid surface water.

These planets are detected by observing the wobbles in the movement of tau Ceti. The newly discovered planets correspond to detecting variations in the movement of the star as low as 30 cm/s, whilst 10 cm/s is the upper limit required for detecting an Earth analog - a planet or moon with environmental conditions similar to those found on the planet Earth.

Milestone in the search for Earth analogs

Dr Fabo Feng, research fellow at the University of Hertfordshire and lead researcher on the study, said: ‘We’re getting tantalisingly close to observing the correct limits required for detecting Earth-like planets. 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.’

Sun-like stars are thought to be the best targets for searching for habitable Earth-sized planets, due to their similarity to the Sun. Tau Ceti is very similar to the Sun in its size and brightness, and they both host multi-planet systems. If the outer two planets around tau Ceti are found to be rocky and habitable, through methods such as direct imaging, then the star could be an optimal target for interstellar colonisation, as alluded to in science fiction. Researchers concede, however, that a massive debris disc around the star probably reduces the current habitability of the planets, due to intensive bombardment by asteroids and comets.

Previous research

The same team of researchers also investigated tau Ceti four years ago in 2013, when Dr Mikko Tuomi led a study to develop data analysis techniques using the star as a benchmark case. They came up with an ingenious way of telling the difference between signals caused by planets and those caused by a star’s activity.

‘We realized 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 Tuomi. ‘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.’

Dr Tuomi continued, ‘We’re slowly learning to tell the difference between wobbles caused by planets and those caused by stellar active surface. This enabled us to verify the existence of the two outer, potentially habitable, planets in the system.’

This work is funded by grants from STFC and Leverhulme and the data were obtained by using the HARPS spectrograph (European Southern Observatory, Chile) and Keck-HIRES (Mauna Kea, Hawaii, USA).  A paper summarising the results of the study will be published in the peer reviewed Astronomical Journal and is currently available online.