Researchers from Columbia University in the US analysed the dips in light from exoplanets passing in front of their stars in data from the Kepler space telescope.
A second, smaller dip that appears ahead of or behind the planet could reveal a moon, they said.
Researchers David Kipping and Alex Teachey of Columbia University have now found the first evidence for an exomoon candidate named Kepler 1625b i.
They analysed 284 planets that seemed like good candidates for hosting detectable moons, 'New Scientist' reported.
Researchers said if the object exists, it orbits a planet slightly larger than Jupiter around a star about 4,000 light-years away.
Since the potential moon is probably about the size of Neptune, the team nicknamed it 'Neptmoon'.
Researchers plan to check if the moon is really there by using the Hubble Space Telescope to watch for another transit on October 29.