In a forthcoming paper in the journal Astronomy & Astrophysics, an international team led by University of Hertfordshire astronomer Mikko Tuomi outlines the evidence for what could be a five-planet system linked to Tau Ceti, which is practically next door, in interstellar terms.
Tau Ceti is a yellow G-type main-sequence star, similar to our own sun, but a bit smaller and cooler. In their research, Tuomi and colleagues combed through more than 6,000 separate observations of Tau Ceti. They found evidence suggesting that the star is being pulled upon by five planets that are anywhere from two to six times as massive as Earth. Scientists can't directly observe exoplanets, so one of the common techniques to look for them is to see how they affect the stars they orbit -- the gravitational pull, though slight, is still measurable.
"This discovery is in keeping with our emerging view that virtually every star has planets, and that the galaxy must have many such potentially habitable Earth-sized planets," coauthor and University of California Santa Cruz astronomer Steve Vogt said in an interview with Physorg.
Based on their observations, the team thinks the five planets are all relatively close to Tau Ceti -- within the distance between the Sun and Mars. The fourth planet, known now just as planet e, seems to lie in the habitable zone where it's neither too hot nor too cold to have liquid water, which supports life as we know it. Planet e likely completes a circuit of Tau Ceti once every 168 days.
In fact, according to Vogt, our solar system, with its far-flung planets and long years -- as compared with Jupiter, for example, which completes one circuit of the sun once every 12 Earth years -- is a relatively unusual one.
"We are now beginning to understand that nature seems to overwhelmingly prefer systems that have multiple planets with orbits of less than 100 days. This is quite unlike our own solar system, where there is nothing with an orbit inside that of Mercury," Vogt told Physorg.
However, there's still a possibility that the planets might not exist, and that the effects that seem like the tug of satellites might just be activity on Tau Ceti itself.
"It's certainly very tantalizing evidence for potentially a very exciting planetary system," University of New South Wales astronomer Chris Tinney told ScienceNOW. Because confirming the evidence could take as much as a decade, "we felt that the best thing to do was to put the result out there and see if somebody can either independently confirm it or shoot it down."