Using the method of analyzing how gravity affects and intensifies otherwise undetectable starlight (gravitational microlensing), the team found a planet-like object — too small to be a planet — that’s orbiting what appears to be a larger object. The larger object does not appear to be a star, which would likely make it a planet, thus making the smaller orbiting object a moon.
It’s not a surprise that astronomers may have found the first exomoon ever identified by humans — it’d be crazy if our solar system was the only one where planets had moons — but it is a little surprising it took this long, even though gravitational microlensing is a relatively recent technique. The planet and moon, located 1,800 light years from Earth, are around four times the mass of Jupiter, and half the mass of Earth, respectively.
However, the astronomers aren’t yet completely sure that the two objects are a planet and its moon. The other theory is that the celestial bodies are actually located much farther away than 1,800 light years, and the planet is actually a very small failed star, with the orbiting moon being a Neptune-sized planet. Adding to the intrigue, if the two bodies are indeed a planet and its moon, then they would be without orbit, floating in space without a star.
Unfortunately, there isn’t yet a way to tell whether or not the two bodies are a planet and its moon, or a failed star and a planet. So, we won’t know for some time if the human race just saw its first exomoon, but there is a decent chance.