Sunday, November 15, 2015

Nearest Exoplanet Alpha Centauri Bb May Not Exist

The bright star Alpha Centauri and its surroundings. Credit: ESO/Digitized Sky Survey 2. Acknowledgement: Davide De Martin

The existence of Alpha Centauri Bb, the closest exoplanet to Earth, located just 4.37 light-years away, was announced in 2012. However, the announcement was met with skepticism by some astronomers and now a new research, led by Vinesh Rajpaul of the University of Oxford (UK), shows that this planet doesn't actually exist. “Based on our new analysis, we believe that it is very, very unlikely that Alpha Centauri Bb exists,” Rajpaul told astrowatch.net.

The planet was discovered by Xavier Dumusque from the Geneva Observatory in Switzerland, and his colleagues. Using data from the La Silla Observatory in Chile, they looked for a periodic wobble in the light from the neighboring star Alpha Centauri B. The starlight shifted toward the blue end of the spectrum, then the red, at regular intervals, indicating movement. The star seemed to be moving back and forth about every three days, as if tugged by a small planet in orbit. The research team was convinced that Alpha Centauri Bb, with a mass of a little more than that of the Earth, is orbiting about six million kilometers away from the star, much closer than Mercury is to the sun in the solar system.

Now, when the Oxford team analyzed the same results, they found that all sorts of things could cause such a result: solar weather, instrument problems, the tug from another star. All of them could account for the discrepancy in observational data. Taking into account the new research, even the planet’s original discoverer, puts the existence of Alpha Centauri Bb in doubt.

“Rajpaul and collaborators have done a good job. Their analysis shows that the signal detected at the time is present in the data. However it seems to be due to the sampling of the data and not a planet. I am not convinced about their explanation on how the sampling can create such a signal, but I agree that the analysis put a doubt on the existence of the planet,” Dumusque told astrowatch.net.

According to Rajpaul, the real issue in the case of the initial Alpha Centauri Bb "detection", appears to have been the times the observations were made. Because the star Alpha Centauri B was observed only once every few days, on average, using a ground-based telescope, inherently limited by clouds, daylight, busy observing schedules. This introduced spurious but apparently significant periodicities in the observations, which were mistaken for a planetary signal.

“Additionally, there are many other sources of variations in the apparent motion of Alpha Centauri B that could be mistaken for planetary signals like stellar magnetic activity cycles and the effects of binary companion star Alpha Centauri A. These need to be modelled very carefully before any conclusions can be drawn about the existence of a planet,” Rajpaul noted.

Even if Alpha Centauri Bb doesn't exist, it does not preclude other planets around one of the three stars in the system, Alpha Centauri A, Alpha Centauri B, and the red dwarf Proxima Centauri. Any of those stars could harbor a planet or planets.

“It'd be unusual if the Alpha Centauri system did not host other planets,” Rajpaul concluded.

The research by Rajpaul’s team is published online in the arXiv journal.

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