The closest ‘known’ exoplanet doesn’t actually exist

The universe is a big, big place. When you’re dealing with distances that stretch into many light years, finding something that’s just a few light years away counts as “close.” Astronomers from the ESO’s HARPS Observatory were excited in 2012 when an exoplanet was found “close by” in orbit of Alpha Centauri B. At just 4.3 light years distant, it was the nearest exoplanet to Earth and a likely target for future study.
There’s just one problem — it doesn’t exist.

The non-existent planet was known as Alpha Centauri Bb. The characteristics for the reported planet were strange to begin with. It was roughly Earth-sized and orbited Alpha Centauri every 3.2 Earth days, making it hard to detect in the first place. The first attempts to confirm the existence of Alpha Centauri Bb failed, but that was chalked up to its size and velocity. However, a new comprehensive review of the published data has led the original discoverers to conclude there is no Alpha Centauri Bb.

The original detection was made with a technique called radial velocity (RV) measurement (sometimes called the Doppler method). This approach consists of watching for subtle red and blue shifting in the spectra of a star as the gravity of a planet tugs on it. In this case, astronomers were looking for these aberrations every three days to account for Alpha Centauri Bb. This approach works fine when it comes to larger planets in the range of Jupiter or Neptune, or even a super-Earth several times the sicze of our planet, but something the size of Earth would produce very small Doppler signals.

Re-analysis of the data by a team at Oxford pointed to a number of factors that could completely explain the blip in the planet, which was previously seen as evidence of Alpha Centauri Bb. Factors like solar weather, instrument problems, and the tug from another star could all account for the phantom planet. With no additional data to prove Alpha Centauri Bb is out there, the original HARPS team has penned a paper to be published next month that concurs with the new analysis. There is no Alpha Centauri Bb.

Just because this one planet turned out to be a ghost in the data doesn’t mean there can’t be a planet in orbit of Alpha Centauri B. Astronomers are sure to go hunting for another planet in orbit of Alpha Centauri B or one of the other stars in the system (Alpha Centauri A and Proxima Centauri). For now, the closest confirmed exoplanet is Gliese 15Ab, which is 11 light years away. It’s about five times as massive as Earth and orbits close to its parent star, a small red dwarf.