So far, thousands of planets outside the solar system (exoplanets) have already been detected, but no moons (exomoons). However, the first evidence for an exomoon , orbiting a planet in the Kepler-1625 system, may have finally been detected by a team led by David M. Kipping of Columbia University.
The star system is about 4,000 light years distant, which means that the planet’s light is very dim to begin with.
Exoplanets are observing the star dimming that occurs when a planet passes between Earth and the star. A moon would be located in a similar way: looking for the dimming that occurs in the reflected light of a planet caused by the transit of a moon.
Kipping and his team report that they recorded three blackouts as the planet made three trips around its star. If Hubble finally confirms its existence, it would have to be much larger than our moon to be detected, perhaps as large as Neptune, and therefore the planet it orbits would be as large as Jupiter.