Scientists have found tantalizing signs that there are four Earth-like planets orbiting Tau Ceti, a Sun-like star just 12 million light-years away. This means that liquid water could exist on the surface of the planets, making them prime candidates for exoplanets that could harbour life as it is known on Earth. The planets have masses as low as 1.7 Earth mass meaning they're some of the smallest planets ever discovered around a Sun-like star.
This is particularly interesting because it helps refine a technique to find other Earth-like planets based on how stars "wobble", tracking the minute gravitational effects of planets on the stars that orbit them.
"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".
"Tau Ceti is the nearest Sun-like star to the Sun and finding habitable planets around it is exciting", Avi Loeb of Harvard's Center for Astrophysics told Gizmodo, although he too noted that their habitability is, right now, a question mark.
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Tau Ceti is similar to our Sun in size and brightness, and like our Sun hosts a multi-planet system. A massive debris disc around the star would reduce the planets' habitability due to bombardment from asteroids and comets. But even if none of tau Ceti's planets are remotely habitable, the existence of such nearby cosmic neighbors would be a boon to search for life beyond Earth.
As a planet orbits, it causes its parent star to wobble by a tiny degree.
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". However, the researchers say that their data seemed to show four rocky planets instead of just two, so they devised a way to make their technique more sensitive to weak planetary signals.
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. That could be crucial, because while its easier to spot exoplanets around dim red dwarf stars like Trappist-1 using the transit method, astronomers are beginning to wonder if such stars can actually support life.