Good news, everyone! We may have detected our first exomoon outside a solar system. It orbits a gas giant 1800 light years away from us, and it's half the size of Earth — just like the famous Yavin IV from which the Rebel Alliance launched its attack against the Death Star. It's a really weird moon too.
The moon seems to be orbiting at about 20 million kilometers from the gas giant, which is really surprising. For comparison: our moon is only 238,900 miles (384,400 km) from Earth. Ganymede — the largest moon in the solar system, which is still only a fraction of Earth's mass — orbits at 665,116 miles (1,070,000 km) from Jupiter. The distance is comparable to S/2003 J 23, which orbits at 14,950,200 miles (24,060,000 kilometers) from the Jovian planet. That, however, is just a tiny four-kilometer-wide rock, neither a real moon nor a space station.
According to the paper titled "A Sub-Earth-Mass Moon Orbiting a Gas Giant Primary or a High Velocity Planetary System in the Galactic Bulge," that's the most likely explanation for this object. David Bennett — the main author and a research professor on astrophysics and cosmology at University of Notre Dame — wrote that "the data are well fit by this exomoon model, but an alternate star+planet model fits the data almost as well. The argument for an exomoon hinges on the system being relatively close to the Sun."
And it gets even weirder: the moon and its planet are rogue objects — they seem to have abandoned the orbit of the star that the scientists used to detect it. Bennett actually thinks that the moon may have not started its life as a moon, but as another tiny planet of that solar system. When the gas giant flung out of the system, it may have trapped the smaller planet on its gravitational field, bringing it out on its interstellar trip.
Can you imagine that? One morning you are having a coffee and the next Jupiter takes Earth out of its orbit. Seems like a good start for an apocalypse survival movie.