Have We Found Our First Exomoon?

I didn't think this would be happening for a while. Finding exoplanets is hard enough, but finding a small little moon orbiting an exoplanet? That's even harder. But the potential moon orbiting Kepler-1625b has one very important thing going for it that made it much easier to spot: It's gigantic. According to the observations, it's as big as Neptune and has about 1.5% of the mass of its host planet. Kepler-1625b is about three times as massive as Jupiter, so that puts its moon at 10-15 Earth masses, which is about the mass of Uranus.

Now, were this moon in any other location, it would be a planet in its own right, and there's a strong argument to be made that this represents a double planet rather than a planet and a moon. But the mass relationship of Kepler-1625b and its moon are almost identical to the Earth-Moon system, and I don't think anyone considers this to be a double planet. It's just a bit strange to think about. Imagine if Jupiter had Neptune orbiting around it.

It's probably safe to say though that this is an outlier and that most exomoons won't be literal gas giants. Remember, in exoplanet hunting, it's a lot easier to spot the weird things than the normal ones. Hot Jupiters are uncommon, but they make up a lot of our exoplanet catalog because they're easy to spot. We found this moon because, once again, it was really (relatively speaking) easy to spot. But now that we've done it once, we can do it again, and hopefully the next exomoon we find will be a bit smaller.