To Bb or not to Bb? Unfortunately, in the case of what was thought to be the closest exoplanet to Earth – Alpha Centauri Bb – it’s looking like it’s the latter.
The discovery of this planet in 2012 sent astronomers into a frenzy, as it orbited Alpha Centauri B, a star just 4.3 light-years away – a stone’s throw in cosmic terms, making it a prime candidate for follow-up observations. But new research has suggested that the initial discovery was nothing more than an illusion in the model used, and there’s actually no planet there as described.
This latest study, published in Arxiv, was carried out by Vinesh Rajpaul of the University of Oxford and his colleagues. They re-analyzed the radial velocity measurements for the star, which involved looking for a noticeable stellar “wobble” due to the quantifiable gravitational pull of a planet. And, they suggest that the 3.24-day repeating signal observed in 2012, which was believed to be the orbital period of a planet, is simply an anomaly from the observation method.
In 2012, the original team – led by Xavier Dumusque, then of the Geneva Observatory in Switzerland – was only able to observe the star on clear nights, and when the observatory they were using – La Silla in Chile – was not booked for other observations. "If you can only look at a star at a discrete set of times, this will lead to sometimes spurious signals being introduced," Rajpaul told New Scientist.
The La Silla Observatory in Chile. S. Brunier/ESO.
To prove their hypothesis, the team used simulated signals of a star that did not have any planets, and using the same model by the original team, found the same 3.24-day signal. This heavily indicates that the original signal detected was just an anomaly of the method used, and not a planet, as a result of trying to silence the signal of the star.
"We demonstrate that the 3.24 day signal observed in the Alpha Centauri B data almost certainly arises from the window function (time sampling) of the original data," the team wrote in its paper.
This is not the first study to propose that the original observation was incorrect, but it is a significant blow to a discovery that generated huge amounts of excitement. Not only was Alpha Centuari Bb the closest exoplanet to Earth, it was also the most similar in size to Earth around a Sun-like star, despite being in a tight and uninhabitable orbit.
Perhaps most importantly, the study highlights how primitive our planet-hunting methods are. Most exoplanets have been found using the transit method, when a planet passes in front of a star and blocks some of its light, but that in itself relies on the transit being visible from Earth. The radial velocity method, too, has its own limitations, as evidenced by this seemingly incorrect observation. Direct imaging of planets, meanwhile, remains a new and difficult area.
It’s not all doom and gloom, though. It is now believed that almost all stars play host to at least one planet, so even if Alpha Centauri Bb is confirmed to be an illusion, one or more real planets may be awaiting discovery around the same star.