A multinational group of astronomers has detected four possible alien worlds circling tau Ceti, which is less than 12 light-years from our Solar System.

Artists illustration of a four-planet system. Image credit: Mark A. Garlick, University of Warwick / Space-art.co.uk.

Tau Ceti is a Sun-like star located just 11.9 light-years away, in the constellation Cetus.

Also known as 52 Ceti, HD 10700 and HIP 8102, the star is very similar to our Sun in its size and brightness, but it is not as active as the Sun.

In 2012, University of Hertfordshire astronomer Dr. Mikko Tuomi and co-authors announced the discovery of five potential planets in the tau Ceti system, labeled tau Ceti b, c, d, e and f.

In a new study, the same team confirmed the existence of tau Ceti e and f and detected two completely new planets, tau Ceti g and h.

We came up with an ingenious way of telling the difference between signals caused by planets and those caused by a stars activity, Dr. Tuomi explained.

We realized that we could see how a stars activity differed at different wavelengths, then used that information to separate this activity from signals of planets.

The four planets are all relatively small, with minimum masses less than 4 times that of Earth. They orbit tau Ceti once every 20, 49, 160 and 642 days, respectively.

Two of them tau Ceti e and f are located on the inner and outer edges of the habitable zone, respectively. They are likely to be candidate habitable worlds, although a massive debris disc around their host star probably reduces their habitability due to intensive bombardment by asteroids and comets.

This illustration compares the four planets detected around tau Ceti (top) and the inner planets of our Solar System (bottom). Image credit: F. Feng, University of Hertfordshire.

Tau Ceti e, f, g and h were detected by observing the wobbles in the movement of the parent star.

This required techniques sensitive enough to detect variations in the movement of the star as small as 30 cm per second.

We are getting tantalizingly close to the 10 cm per second limit required for detecting Earth analogs, said team member Dr. Fabo Feng, also from the University of Hertfordshire.

Our detection of such weak wobbles is a milestone in the search for Earth analogs and the understanding of the Earths habitability through comparison with these analogs.

We are now finally crossing a threshold where, through very sophisticated modeling of large combined data sets from multiple independent observers, we can disentangle the noise due to stellar surface activity from the very tiny signals generated by the gravitational tugs from Earth-sized orbiting planets, added team member Professor Steven Vogt, of the University of California, Santa Cruz.

The data were obtained by using the High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph, an instrument installed on the 3.6-m telescope at ESOs La Silla Observatory in Chile, combined with the High-Resolution Echelle Spectrometer (HIRES) at the W. M. Keck Observatory on Maunakea, Hawaii.

The teams findings will be published in a paper in the Astronomical Journal. The article is also publicly available at arXiv.org.

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Fabo Feng et al. 2017. Color difference makes a difference: four planet candidates around tau Ceti. AJ, in press; arXiv: 1708.02051

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