As of January 2018, well over 3,500 planets have been confirmed from outside our Solar System, and that number is constantly growing. All of these have been spotted within our neighborhood of the Milky Way, but with numbers like that it was basically a given that planets were pretty common across the universe. Now astrophysicists from the University of Oklahoma (OU) have confirmed that assumption with the first detection of extragalactic exoplanets.

A couple of candidates for planets beyond the Milky Way have been detected before, but so far none have been confirmed. There's a suspicious-looking anomaly in our galactic neighbor Andromeda, but studies are ongoing. In 2010 a planet dubbed HIP 13044 b was "discovered" about 2,000 light-years from Earth, and while it was initially believed to have originated in another galaxy before being swallowed up by the Milky Way, a later study found no evidence that the planet existed at all.

Detecting exoplanets anywhere is no easy feat, thanks to the vast distances involved. Even within our home galaxy, the farthest planets found so far are SWEEPS-04 and SWEEPS-11, which are about 27,000 light-years from Earth – that's only about a quarter of the width of the Milky Way. Most exoplanets are found through the "transit method", where telescopes like Kepler watch for dips in the brightness of stars that indicate planets have passed between them and us. Unfortunately, our most advanced telescopes have a limit on how far they can see, so to peer deeper into the universe astronomers need to pull out some other tricks.

Gravitational lensing is one such technique. Massive objects like black holes and galaxies can curve the fabric of spacetime so much that light bends as it passes by, and astronomers use these objects as "lenses" to study extremely distant stars, galaxies and supernovae. Microlensing applies this phenomenon in reverse, using a bright background light source to study the characteristics of the "lens" object itself.

In this case, the light source was a distant quasar, and the OU team used it to spot exoplanets in the lensing galaxy. Objects can have distinct signatures in the way they bend light, allowing scientists to characterize their size and mass. Functionally, it works in a similar way to the transit method, except it can be applied at much greater distances – this galaxy, for example, lies over 3.8 billion light-years from Earth, making these exoplanets the most distant ever detected by a gigantic margin.

The researchers examined data gathered by NASA's Chandra X-ray Observatory, and after calculating the models on supercomputers, they were able to identify the signatures of around 2,000 objects, ranging in mass from that of the Moon to that of Jupiter.

"We are very excited about this discovery," says Xinyu Dai, lead researcher on the study. "This is the first time anyone has discovered planets outside our galaxy. These small planets are the best candidate for the signature we observed in this study using the microlensing technique. We analyzed the high frequency of the signature by modeling the data to determine the mass."

The research was published in Astrophysical Journal Letters.

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