A team of astronomers says they may have just spotted the first ever moon identified outside of our solar system. This so-called “exomoon,” situated 4,000 light years from us here on Earth, was identified as Kepler-1625b and identified by examining data from the Hubble Space Telescope.

Researchers think that this moon is huge, about the size and mass of Neptune, and is circling a planet that is about the size of Jupiter but with 10 times the mass. Exomoons are tough to identify outside of our solar system because most of them are quite small, and it’s hard enough just to identify exoplanets. In order to spot an exoplanet, we have to measure subtle light shifts of stars caused by planets passing in front of them.

So far, scientists have identified 3,000 exoplanets, or planets that orbit stars other than the sun. This is the first exomoon discovered.

“Exomoons represent an outstanding challenge in modern astronomy, with the potential to provide rich insights into planet formation theory and habitability,” according to the paper, published by A. Teachey, D.M. Kipping, and A.R. Schmitt and titled, On the Dearth of Galilean Analogs in Kepler, and the Exomoon Candidate Kepler-1625B. “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.”