In 2012, astronomers announced a tantalyzing find: the closest exoplanet to our solar system, an Earth-sized world orbiting Alpha Centauri B in a close orbit. As the closest star to the sun, That put a planet in our cosmic backyard just 4.3 light years away, around the closest star system to our own. Except ... there is no Alpha Centauri Bb.

The news has been a few years coming, as detailed in this Planetary Society blog post by Bruce Betts. The purported planet orbited the star in just 3.2 earth days, making for a very short year and a very hard detection. So hard, in fact, that follow-up observations couldn't repeat the results of the 2012 find at other observatories. Found at ESO's HARPS Observatory, that same team is now concurring with the new results: there is no planet. The findings will be published in Monthly Notices of the Royal Astronomical Society.

So how do you just mistakenly detect a planet? The astronomers were looking at subtle changes in red shift and blue shift in the star every three days, showing the smaller of the two stars in the Alpha Centauri binary pair moving toward and away from the Earth every three days in small, subtle shifts. This method, called radial velocity or the Doppler method, has been used to find very, very large planets called Hot Jupiters that orbit their stars in a few days time.

But the Earth-sized Alpha Centauri Bb was a harder object to find, owing to its small size. When an Oxford team analyzed the same results as the HARPS team, 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.

All of this means the object around 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 more distant red dwarf Proxima Centauri, which orbits the other pair. Any of those stars could harbor a planet or planets, albeit one that hasn't been detected with current methods.

This doesn't rule out planets around the stars in the Alpha Centauri system, it simply means it rules out this planet. So now, it's back to the drawing board. So congratulations, Gliese 15Ab, you are now the closest planet to Earth at 11 light years away. Just don't go squandering it by also being an observational error. And better luck next time, Alpha Centauri. Your day may still come. (But please don't make The Sparrow a prophecy.)

Source: National Geographic

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