Alpha Centauri Bb may be an ex-exoplanet. This long-sought world was announced with great excitement in 2012 as the first Earth-mass planet in the nearest star system to our own, but a new statistical analysis has revealed it to be nothing more than an apparition.

The original discovery was the work of Xavier Dumusque, then at the Geneva Observatory in Switzerland, and his colleagues. Using four years of data from the La Silla Observatory in Chile, they looked for a periodic wobble in the light from Alpha Centauri B, a star just 4.3 light years away.

After stripping out noisy contributions to the starlight, including star spots that can mimic transiting planets and the star’s own rotation, the team spotted a signal that seemed to repeat every 3.24 days. They attributed this to the gravity of a planet of that orbital period pulling on the star.


Since then, other astronomers had tried other methods of filtering out the star’s noisy signal, and found that the evidence for planet Bb was inconclusive. But they couldn’t rule it out.

“Most people have been focusing their attention on trying to model the stellar activity,” says Vinesh Rajpaul of the University of Oxford. But now he and his colleagues have discovered that the signal could have been introduced by the way the measurements were made.

A matter of timing

The culprit was the timing. La Silla could only look at this star on clear nights and when it was not booked for other observations. This could have created a rhythm in the signal that had nothing to do with the presence of exoplanets.

“If you can only look at a star at a discrete set of times, this will lead to sometimes spurious signals being introduced,” says Rajpaul.

To check whether the 3.24-day signal could have been real, he and his colleagues simulated signals that looked very close to the one observed by the original team, but deliberately didn’t include any planets. Then they used the original team’s model to analyse this fake data, and were still able to see a spurious 3.24-day signal – even though there were no planets to be found. “Given what we’ve shown, it seems very, very implausible that the planet is real,” he says.

It’s a bit like listening to a distant orchestra play, but only being able to hear the occasional note. If you try to guess the composition, your answer depends on the particular notes that make it through. You might get lucky and hear the distinctive opening tones of Beethoven’s fifth symphony, but another set of notes could call to mind an entirely different tune.

This timing problem is not normally an issue for exoplanet hunters, but the efforts to strip out stellar activity boosted other periodic signals in the data, leading to a false detection, says Rajpaul – something the original team couldn’t have anticipated.

“They really did try to do a very thorough job,” he says. “The source of this bogus signal is totally unexpected.” In future astronomers will need to keep the timings of their observations in mind when searching for hard-to-spot planets, he says.

Another exoplanet

“The trap here was the stellar rotation signal,” agrees Brice-Olivier Demory of the University of Cambridge. Earlier this year he led an inconclusive attempt to use the Hubble Space Telescope to spot Bb passing in front of its star.

Instead the team spotted another possible Earth-mass planet with a 20-day orbit, but the evidence wasn’t strong enough to claim a discovery. Rajpaul’s work doesn’t rule that planet out, he says. “We are still hoping there are other planets in the system. We just have to find better techniques to detect them.”

So if this possible new planet becomes the first official exoplanet found around this star, will it assume the name Alpha Centauri Bb, in line with convention?

“Common sense would tell us that now this [original] planet is dismissed, we should put the name back into the pool of discovery,” says Demory. But that could make the scientific literature confusing, says Rajpaul, so any new exoplanet might have to be called Alpha Centauri Bc. In that case Bb would live on as a ghost in the astronomical annals.

Reference: arxiv.org/abs/1510.05598

(Image credit: Mark Garlick/SPL)

Correction, 21 October: When this article was first published, it incorrectly described some of the details of the team’s analysis. This has now been corrected.