There could be two Earth-like planets within cosmic spitting distance of our own. Both are likely too close to their star to host life, but the discovery opens the possibility of other planets in the system with more temperate climates.
Alpha Centauri is a binary star system just 4.3 light years away from our own. In 2012 astronomers announced that the system had a planet, which they dubbed Alpha Centauri Bb as it was apparently orbiting the smaller of the stars, Alpha Centauri B.
The team said it was a rocky world slightly more massive than Earth. But in 2013, other researchers called into question the existence of Bb, saying the evidence wasn't good enough.
"If you ask anyone working in exoplanets, they would all have a different opinion about the existence of Alpha Centauri Bb," says Brice-Oliver Demory of the University of Cambridge.
That's why he and his colleagues have been using the Hubble Space Telescope to search for planet. They weren't able to find it, but have instead seen hints of a second Earth-sized world in the system.
Star wobble
The original claim was based on the radial velocity method – a planet-hunting technique which looks at how the gravitational pull of a planet slightly wobbles its star. Demory's Hubble search took a different approach, looking for signs of a dip in the light from Alpha Centauri B caused by the planet passing in front of, or transiting, the star. These two methods are independent of each other, so seeing Bb transit would reinforce the earlier patchy radial velocity data.
The original measurements suggested that Bb, if it exists, takes three or so days to orbit its star. But not all planets make transits as seen from Earth, because it depends on how the planet and star are aligned.
Demory's team observed Alpha Centauri B in 2013 and 2014, for a total of 40 hours. The 2013 data showed signs of a transit consistent with Bb's suggested orbital parameters, but it seemed to last slightly longer than expected, and the statistical validity of the signal disappeared when combined with the 2014 data. That doesn't mean Bb isn't there, just that if it exists, it is unlikely to transit as seen from Earth.
That still leaves a puzzle over what caused the 2013 signal. The team ruled out errors with Hubble itself or spots on the surface of the star, which can sometimes be mistaken for exoplanets.
They also dismissed the possibility of interference from Alpha Centauri A, the other star in the binary system, or from an unrelated, more distant star system that could have just been passing behind.
Scorchingly close
The only explanation left was that there is another planet in the system. The observations point to an Earth-sized planet with a year lasting no more than 20.4 days, putting it slightly further out than Bb but still scorchingly close to the star.
Astronomers have confirmed nearly 2000 exoplanets so far, and the evidence suggest many stars host multiple planets, just like our own solar system. That means confirming the discovery of one planet around Alpha Centauri B – even one with a hot, close orbit – hints at other planets in the system that might be more hospitable. "If you see one planet, the chance is there are other planets in the system," says Demory.
"They work they've done holds tight; they give a very well balanced view on what this transit could be," says Paul Wilson of the Paris Institute of Astrophysics in France. He's not sure there is enough evidence yet to support a full discovery, but is keen to encourage the team. "I hope they will be able to detect an Earth-sized planet through the transit method. That would be fantastic."
Unfortunately it's going to be difficult to confirm either of these planets with our current generation of telescopes. Hubble could do it, but it would have to stare at Alpha Centauri for 20 days with no guarantee of finding anything, which would be seen as a waste of time for our most important space telescope, says Demory.
Upcoming instruments like the European Extremely Large Telescope
Reference: arxiv.org/abs/1503.07528
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