A new study by an worldwide 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. '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. In 2012, astronomers announced they'd found evidence for five planets between two and seven times the mass of the Earth, using the so-called radial velocity or "wobble" method, which measures the gravitational tug a planet exerts on its star.
Previous year we learned the nearest star beyond our sun has a planet about the size of Earth, but it's very different than our solar system and not an ideal candidate for finding alien life.
Lead study author Fabio Feng of the University of Hertfordshire told Gizmodo that we should be cautious defining the habitable zone of exoplanets, but he said he regards the two outer planets as "habitable zone candidates", especially since their orbital periods are not precisely known.
It's incredibly exciting to have two new habitable world candidates in a star system so comparatively nearby.
Mikko Tuomi, another researcher at the University of Herefordshire, explained, "We came up with an ingenious way of telling the difference between signals caused by planets and those caused by star's activity". Two of them are super-Earths located in the habitable zone* of the star and thus could support liquid surface water.
Of these six, at least three of the planets have an ocean - and scientists state that anywhere on Earth you find water, you are likely to find life, which is why astronomers look for these on other planets.
Tau Ceti is the center of the nearest solar system that's similar to ours, an assumption made stronger by the discovery of four rocky super-Earths orbiting the star just 12 light years away.
It's not the first time Tau Ceti has been found to host potentially habitable worlds.
But even if none of tau Ceti's planets are remotely habitable, the existence of such nearby cosmic neighbours would be a boon to search for life beyond Earth. Unfortunately, the odds of life on the potentially habitable rocky planets in the system might not be great after all.
But, they did warn that a massive disc of debris around the star could be bombarding them with asteroids and comets, reducing their habitability.
The findings are due to appear in an upcoming edition of The Astrophysical Journal.
Most importantly, as Feng explained via an email exchange shared with Gizmodo, the research pushes the limits of the sorts of planets we can detect with the radial velocity method, boosting our prospects of discovering truly Earth-sized, rocky worlds in the future. "Our detection of such weak wobbles is a milestone in the search for [Earth-like exoplanets] and the understanding of the Earth's habitability through comparison with these". Unlike more common smaller stars, such as the red dwarf stars Proxima Centauri and Trappist-1, they are not so faint that planets would be tidally locked, showing the same side to the star at all times.