Aliens could be watching aliens watching aliens. That's a realistic prospect now that three potentially habitable planets – a record – have been glimpsed orbiting the same star.
Earlier studies had suggested that a nearby star, Gliese 667C, had three planets, only one of which might support life. But the very presence of multiple planets made their precise number hard to tease out.
Now Guillem Anglada-Escudé of the University of Göttingen in Germany and his colleagues have reanalysed the original data and added some new observations. They found evidence for up to seven worlds, including three rocky planets in the star's habitable zone, where temperatures should suit life.
"Five are very solidly detected by any standard," says Anglada-Escudé. "This includes all three habitable zone candidates." Located about 22 light years away, Gliese 667C is itself part of a triple-star system, making this one of the most crowded planetary neighbourhoods yet.
Aliens next door
The team used more than 200 data points from three different spectrographs, which can detect how a star is tugged back and forth by the gravity of an orbiting planet. The five strongest signals were from planets between 1.94 and 5.94 times the mass of Earth, making them all likely to be rocky. But only three are in the habitable zone.
These three worlds are close enough to each other that any intelligent life there with the ability to build rockets could easily visit the neighbours. "Larger rockets would take you pretty quickly from one planet to the other – one to two months at most," says Anglada-Escudé.
"This discovery adds more targets to the many exciting worlds we are discovering out there," says Lisa Kaltenegger of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. She reported the first pair of neighbouring habitable worlds
That's assuming the new trio of habitable planets is real. In 2010, two of the paper's co-authors were acclaimed and then criticised when they claimed to have found the first potentially habitable rocky planet around the star Gliese 581 – a discovery others were unable to confirm. Anglada-Escudé is not worried: "We made sure to be very careful this time."
Journal reference: Astronomy & Astrophysics, in press
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