Why astronomers reluctantly announced a possible exomoon discovery : Nature News & Comment [1707.08563] HEK VI: On the Dearth of Galilean Analogs in Kepler and the Exomoon Candidate Kepler-1625b I [1710.06209] The nature of the giant exomoon candidate Kepler-1625 b-i
Kepler-1625 Kepler-1625b
They are being cautious because previous proposed exomoons have turned out to be false alarms. Now word on this one yet, however.Astronomers on Saturday pointed the Hubble Space Telescope in the direction of a star system 4,000 light-years away, where they hope to confirm the presence of an exomoon orbiting a distant exoplanet for the first time. Confirming the existence of such satellites beyond our own solar system opens a new door in the search for life elsewhere in the universe.
Kepler-1625 is a rather Sunlike star, and Kepler-1625b is about as big as Jupiter. Its mass is unknown, being anything from a Saturn-ish 0.4 Jupiter masses to being a brown-dwarf 75 Jupiter masses -- or even a very low-mass star (112 Jupiter masses ~ 0.11 solar masses). This big range of masses is because this planet is close to the maximum size for a cold object.
Kepler-1625b-i has a size roughly 4 times the Earth's, about as big as Uranus and Neptune. It could be a gassy body the mass of the Earth, a rock-and-water one about 180 Earth masses, or somewhere in between, like Uranus and Neptune themselves.
It orbits at roughly 20 times its planet's radius, or something like the orbit radii of Ganymede, Callisto, and Titan.
Whatever its mass is, its formation is a theoretical challenge, since its mass is relatively high compared to its planet's mass. Did some other planet collide with one of its moons? Something like a scenario proposed for Neptune's moon Triton.
But if it is real, it would indicate that Earthlike exomoons might possibly exist.