Astronomers Have Found Another Possible 'Exomoon' beyond Our Solar System

Astronomers say they have found a second plausible candidate for a moon beyond our solar system, an exomoon, orbiting a world nearly 6,000 light-years from Earth. Scientific American reports: Called Kepler-1708 b-i, the moon appears to be a gas-dominated object, slightly smaller than Neptune, orbiting a Jupiter-sized planet around a sunlike star -- an unusual but not wholly unprecedented planet-moon configuration. The findings appear in Nature Astronomy. Confirming or refuting the result may not be immediately possible, but given the expected abundance of moons in our galaxy and beyond, it could further herald the tentative beginnings of an exciting new era of extrasolar astronomy -- one focused not on alien planets but on the natural satellites that orbit them and the possibilities of life therein. Kepler-1708 b-i's existence was first hinted at in 2018, during an examination of archival data by David Kipping of Columbia University, one of the discoverers of Kepler-1625 b-i, and his colleagues. The team analyzed transit data from NASA's Kepler space telescope of 70 so-called cool giants -- gas giants, such as Jupiter and Saturn, that orbit relatively far from their stars, with years consisting of more than 400 Earth days. The team looked for signs of transiting exomoons orbiting these worlds, seeking additional dips in light from any shadowy lunar companions. Then the researchers spent the next few years killing their darlings, vetting one potential exomoon candidate after another and finding each better explained by other phenomena -- with a single exception: Kepler-1708 b-i. "It's a moon candidate we can't kill," Kipping says. "For four years we've tried to prove this thing was bogus. It passed every test we can imagine." The magnitude of the relevant smaller, additional dip in light points to the existence of a moon about 2.6 times the size of Earth. The nature of the transit method means that only the radius of worlds can be directly gleaned, not their mass. But this one's size suggests a gas giant of some sort. "It's probably in the 'mini Neptune' category," Kipping says, referring to a type of world that, despite not existing in our solar system, is present in abundance around other stars. The planet this putative mini Neptune moon orbits, the Jupiter-sized Kepler-1708 b, completes an orbit of its star every 737 days at a distance 1.6 times that between Earth and the sun. Presuming the candidate is genuinely a moon, it would orbit the planet once every 4.6 Earth days, at a distance of more than 740,000 kilometers -- nearly twice the distance our own moon's orbit around Earth. The fact that only this single candidate emerged from the analysis of 70 cool giants could suggest that large gaseous moons are "not super common" in the cosmos [...].

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