The team of researchers built on research they had done in 2013 and detected the planets by examining the wobbles of the sun-like star, tau Ceti, that the planets orbit. One has an estimated mass of just 1.7 times that of the Earth, and two of the planets are super-Earths - exoplanets with masses between 1.5 and roughly 10 times Earth's mass - in the tau Ceti system's habitable zone. The gravitational tug of nearby planets causes the parent star to wobble around the shared center of mass, and the resulting doppler shift in the starlight can be measured.
If so, that means they're far less likely to be habitable - and there's also the issue of the super Earths hanging uncomfortably close to the outskirts of Tau Ceti's habitable zone - although it's premature to abandon all hope. The technique, they claim, is now almost precise enough to detect Earth-mass planets.
Tau Ceti is particularly prevalent in science fiction that imagines future civilizations visiting the relatively nearby star system.
Earth-sized planets, due to their similarity to the Sun.
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. We now know that it probably hosts a handful of Earth-like planets, including a pair that might be able to support life.More news: Beware of government-run health care
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Still, the researchers say we shouldn't get carried away with ourselves here, because a huge ring of space debris surrounds the Tau Ceti system, meaning these alien worlds - lifeless or not - could be threatened by intensive bombardment from asteroids and comets. "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", Feng said.
He said: "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". Additionally, the planets may have different mineral compositions that give them very different geological properties compared to earth. '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 even when using this technique the researchers found that there are definitely at least four rocky planets orbiting the star. "This enabled us to essentially verify the existence of the two outer, potentially habitable planets in the system", said Mikko Tuomi from the University of Hertfordshire.
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). 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.