Scientists just made a planet disappear. According to a new study, Alpha Centauri Bb, a world in the nearest star system to us, was merely a ghost in the data.
The planet, thought to be perhaps similar in mass to Earth, was hailed as a “landmark” when it was announced in 2012 in the journal Nature. The discovery got people excited about finding neighboring worlds that might harbor life in the Alpha Centauri system 4.3 light-years away—already home to science fiction characters such as the Transformers and the creatures of Avatar.
This particular alien world wouldn't have been a good place to look for life, though. It would have been roughly a tenth the distance to its star that Mercury is to the sun, with a scorchingly hot surface probably covered in molten rock.
Now it will serve as a cautionary tale for planet hunters, a reminder that planets as small as Earth are hard to find. Distinguishing subtle clues from background noise is incredibly difficult, as shown in a new paper recently posted at arXiv.org and due to appear in the Monthly Notices of the Royal Astronomical Society.
Even the team that originally reported the planet agrees. “This is really good work,” said Xavier Dumusque of the Harvard-Smithsonian Center for Astrophysics. “We are not 100 percent sure, but probably the planet is not there.”
How a Planet Vanishes
It isn't the first world to disappear. In 2005, Polish astronomer Maciej Konacki offered tantalizing evidence that HD 188753, a tightly packed three-star system, contained a gas planet similar to Jupiter. The announcement sent ripples through the astronomical community: According to planet formation theories, the three stars' gravitational fields should have prevented such a large planet from forming at all. But two years later, researchers failed to confirm Konacki's sighting, suggesting that his discovery was in fact a false alarm.
Dumusque originally found the Alpha Centauri planet by monitoring the light of the star Alpha Centauri B. The starlight shifted toward the blue end of the spectrum, then the red, at regular intervals, indicating movement much as a siren rises and falls in pitch as it moves toward or away from a listener. The star seemed to be moving back and forth about every three days, as if tugged by a small planet in orbit.
Wobbling stars have been used to infer hundreds of other planets—but larger ones. Some doubted the find, including astronomer Artie Hatzes of Friedrich Schiller University in Jena, an early trailblazer in the exoplanet community who published a skeptical analysis.
It now appears that the patchiness of the data caused the planet to materialize.
Imagine someone trying to listen to a piano concerto but hearing only one note out of every ten. They might mistake a Bach for a Beethoven. An astronomer who looks at a star only a few times a week, as the telescope that spotted Bb did, can be similarly fooled.
Vinesh Rajpaul, a graduate student in astrophysics at the University of Oxford, showed that subtle patterns of light caused by things that have nothing to do with a planet—spots on the star's surface, electronic noise in the equipment, or the pull of another star, for instance—could be confused for a planet.
Building a Fake Planet
To prove his point, Rajpaul created a computer simulation of a star with no planet and sporadic observations.
“When we generated synthetic data, the planet popped up exactly, even though there was no planet,” Rajpaul said.
This red herring, said Rajpaul, won't be a problem for the vast majority of the more than 5,600 other planet candidates found to date, most of which are much larger worlds.
The Kepler Space Telescope has also found planets smaller than Earth. But it watches one patch of sky continuously and uses an entirely different method, waiting for planets to cross in front of their stars and dim the light ever so slightly.
Well aware of the challenges that lie ahead,, Dumusque recently challenged colleagues to a small-planet-finding contest. He created simulations of stars hosting planets of different sizes—or none at all. Teams of experts looking for wobbles caused by larger planets got it right about 90 percent of the time. For smaller planets, the best team spotted only about 10 percent and made plenty of mistakes.
Correction: Artie Hatzes' name and affiliation have been corrected.
Michael Greshko contributing reporting to this story.
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