But for now, they are the best candidates we have for habitable extrasolar planets close to home.
Tau Ceti is similar to our Sun in size and brightness, and like our Sun hosts a multi-planet system.
Two Tau Ceti signals previously identified in 2013 were now known not to have a planetary origin.
The data were obtained by using the HARPS spectrograph (European Southern Observatory, Chile) and Keck-HIRES (W. M. Keck Observatory, Mauna Kea, Hawaii). That works out well, as Tau Ceti is a bit smaller (78 percent) than the Sun, and is correspondingly less intense. The technique, they claim, is now almost precise enough to detect Earth-mass planets.
"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.
Neither of Tau Ceti's "super-Earths" lie in the center of its habitable zone.
As planets pass by their sun, they lead it to move a little - and the size of that move can be seen in the light that comes to Earth from the star.
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"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 analogs", said Fabo Feng from the University of Hertfordshire in the UK. Study co-author Guillem Anglada of Queen Mary University of London, meanwhile, urged some skepticism surrounding the existence of the planets, but he told Gizmodo the evidence "cannot be denied from a statistical sense".
The planets were detected using the "wobble" method, whereby astronomers monitor any wobbles made by a star. More may be found and confirmed later, of course. The masses of the planets found reach of 1.7 Earth masses, making them among the smallest of the heavenly bodies, found near sun-like stars. Numerous other "potentially habitable" exoplanets discovered to date orbit dim red dwarf stars, which, for a variety of reasons, could prove hostile to life.
The planets are in the Goldilocks "habitable zone" around Tau Ceti, a star very like our own sun. 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.
So what about the closest star that would actually remind us of our own beloved sun?
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.
"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.