By Jonathan O’Callaghan
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. 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 sun-like star—an unusual but not wholly unprecedented planet-moon configuration.
There are more than 200 moons in our solar system, and they have an impressive array of variations. Saturn’s moon Titan possesses a thick atmosphere and frigid hydrocarbon seas on its surface, possibly an analog of early Earth. Icy moons such as Jupiter’s Europa are frozen balls that hide subsurface oceans, and they may be prime habitats for life to arise.
An important shared trait among these worlds, however, is their mere existence: six of the eight major planets of our solar system have moons. Logic would suggest the same should be true elsewhere.
“Moons are common,” says Jessie Christiansen of the California Institute of Technology. “In our solar system, almost everything has a moon. I am very confident that moons are everywhere in the galaxy.”
“They’re just so small,” Christiansen says. To date, only one truly plausible candidate has been found: Kepler-1625 b-i, a supposed Neptune-sized world orbiting a Jupiter-sized exoplanet about 8,000 light-years from Earth that was reported in October 2018. But even the existence of this behemoth world has been called into question by subsequent analysis.
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.
(Courtesy: Scientific American)