Researchers publishing in Nature Astronomy and led by David Kipping report that a roughly Neptune-sized moon may have been found orbiting a Jupiter-sized planet, both of which are going around a Sunlike star. Named Kepler-1708 b-i, this system periodically shows two dips in light when it passes in front of its star. The pattern of how the double dips appear (and don’t appear) over the past four years of observing are consistent with just one thing: a planet and a giant moon.

I have to admit, I now want to figure out if this is really more of a binary planet than a planet-moon situation, but that is maths for another day. What is key is that this system probably didn’t form like this. According to co-author Jessie Christiansen and reported in Scientific American: One scenario is this moon got captured by the planet as the planetary system was forming. Early planetary systems are quite violent, chaotic places. We see examples of capture in our own solar system: for instance, Triton, one of Neptune’s moons. We think that was captured. So we know that this can happen, we just hadn’t scaled it up to the idea that a Jupiter-sized planet could capture a Neptune-sized moon.

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Astronomers Have Found Another Possible ‘Exomoon’ beyond Our Solar System (Scientific American)

“An exomoon survey of 70 cool giant exoplanets and the new candidate Kepler-1708 b-i,” David Kipping et al., 2022 January 13, Nature Astronomy