What unreliable neighbours we have. Last year astronomers announced the discovery of a planet with a similar mass to Earth’s just 4.3 light years away in the nearby Alpha Centauri system. Another look suggests the planet may not be there after all.

Xavier Dumusque of the Geneva Observatory in Switzerland and colleagues identified the planet, known as Alpha Centauri Bb, from around 450 observations of Alpha Centauri B, the smaller of the two stars in the system.

The team looked for changes in the star’s light that could be caused by an orbiting planet, whose gravitational tug would induce a slight wobble in the star. This method, known as radial velocity, is often successfully used to hunt exoplanets. But smaller planets have less of a gravitational tug, so finding such worlds in this way is more difficult. That is because noise from the star can obscure the subtle signal and so must be carefully filtered out.

Dumusque filtered the data by removing any periodic signals, such as dark sunspots, that synced up with the rotation of the star. The remaining signal was interpreted as an orbiting planet.


Bb gone?

But Artie Hatzes of the Friedrich Schiller University in Jena, Germany, begs to differ. After using two alternative filters, he says Bb might not exist after all. “I don’t say it is not there, but I cast serious doubt,” he says.

Hatzes first removed the strongest of any periodic signals, not necessarily just those tied to the star’s rotation, and found only weak evidence for Bb. Then he used another technique that focuses on short sections of the data curve to look for signs of the planet, and this failed to detect anything.

To confirm that these methods would actually be able to find Bb if it is there, Hatzes also generated a fake signal for a similar planet and inserted it into the data. Both methods could easily detect the imposter, suggesting they should also spot Bb.

Gut feeling

“My gut feeling is that with more measurements it may not hold up. I’ve found planets that have gone away with more data, and this just doesn’t smell like a solid discovery,” says Hatzes. He would be happy to be proved wrong, though.

Dumusque, now at Harvard University, says it is always worth reanalysing planetary detections. His team is gathering more data and is also trying to detect Bb using another method that looks for a dip in light as a planet moves in front of its star, as seen from Earth.

Erasing Alpha Centauri Bb from the sky may settle a recent tussle sparked by space outreach company Uwingu when it staged a contest to name Alpha Centauri Bb. That effort raised the ire of the International Astronomical Union, which insists it acts as the sole arbiter of astronomical names.

Journal reference: arxiv.org/abs/1305.4960