Of the four planets discovered, two are super-Earths located in the habitable zone of the star, tau Ceti, meaning they could support liquid surface water, said researchers at University of California (UC), Santa Cruz in the US.
A year ago 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. 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.
"We're getting tantalizingly close to observing the correct limits required for detecting Earth-like planets", lead researcher Fabo Feng said in the press release.
In the search for potentially habitable Earth-like exoplanets, those found orbiting Sun-like stars are good candidates.
Download the News Nation Mobile App and stay connected with top stories from India and around the world. And at just 1.7 Earth masses, they're also the smallest nearby Earth-like planets.
It is now known that two signals of Tau Ceti previously identified in 2013, you do not have a planetary origin.More news: Toshiba Wins 'Qualified' Auditors' Approval to Avoid Tokyo Stock Exchange Delisting
An worldwide team of scientists devised a new, more accurate and sensitive method of detecting planets by looking for "wobbles" in the star's movement caused by the minute gravitational tug of orbiting planets, while also better ruling out false positives coming from activity on the star itself. Tau Ceti is bright enough to be seen from Earth, forming part of the constellation Cetus, which rises in the northerly winter skies a couple of hours before the better-known constellation Orion.
As far as habitability goes, the researchers themselves point out another problem: Due to the massive debris disk surrounding tau Ceti, the outer two planets are likely subject to intense bombardment via comets and asteroids, which would pose obvious issues for life. One in orbit at the internal border and the other on the outside.
Tau Ceti is very similar to the Sun in its size and brightness, and both stars host multi-planet systems. "We are slowly learning to tell the difference between wobbles caused by planets and those caused by stellar active surface".
"This allowed us to conrm the presence of two external, potentially habitable planets in the system", he said. 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.
The team made the discovery by combining more than 6,000 observations from the UCLES spectrograph on the Anglo-Australian Telescope, the HIRES spectrograph on the Keck Telescope, and reanalyzing spectra taken with the HARPS spectrograph available through the European Southern Observatory public archive.