Scientists have just made a big discovery about 4,000 light years from Earth, literally. They’ve found a huge moon the size of Neptune orbiting a planet the size of Jupiter, albeit with 10 times the mass, and it is the first time astronomers have ever discovered a moon outside of our solar system.

The so-called “exomoon” has been dubbed Kepler-1625b after the telescope that spotted it. Data from the Hubble Space Telescope also assisted in the search. Scientists have had trouble finding exomoons in the past because many of them are much smaller than planets, and it’s hard enough to find a planet outside of our solar system. To find a planet, astronomers measure the dips in light from the stars they orbit, which happens when the planet crosses in front of the star.

A total of 3,000 exoplanets have been identified to date, but this is the first exomoon positively identified by scientists.

“Exomoons represent an outstanding challenge in modern astronomy, with the potential to provide rich insights into planet formation theory and habitability,” reads ean xcerpt from the paper, titled On the Dearth of Galilean Analogs in Kepler, and the Exomoon Candidate Kepler-1625B and published by A. Teachey, D.M. Kipping, and A.R. Schmitt. “In this work, we stack the phase-folded transits of 284 viable moon hosting Kepler planetary candidates, in order to search for satellites. These planets range from Earth-to-Jupiter sized and from 0.1-to-1.0AU in separation – so-called \warm” planets. Our data processing includes two-pass harmonic detrending, transit timing variations, model selection and careful data quality vetting to produce a grand light curve with a r.m.s. of 5.1 ppm.”