Astronomers have found the first compelling evidence of a moon outside of our Solar System - orbiting a gigantic planet, over 8,000 light years away from Earth.
Analysing data from the Kepler and Hubble space telescopes, the researchers believe the so-called ‘exomoon’ is the size of Neptune, and orbits a planet the size of Jupiter.
Over the last 30 years, astronomers have discovered over 3,500 ‘exoplanets’ - planets orbiting stars other than the Sun - but this is the first evidence of an exomoon.
The two researchers, from New York’s Columbia University, examined data on 284 planets outside our Solar System and only one of them, Kepler-1625b, showed evidence it may host a moon.
Lead author Alex Teachey and his co-author, Professor David Kipping, observed Kepler-1625b while it was in transit, passing in front of its parent star, Kepler-1625.
They noticed a momentary dimming of starlight, which Mr Teachey, a graduate fellow in astronomy, said is "indicative of something gravitationally tugging on the planet".
"We've tried our best to rule out other possibilities such as spacecraft anomalies, other planets in the system or stellar activity, but we're unable to find any other single hypothesis which can explain all of the data we have", he added.
Mr Teachey said he is "urging caution" in interpreting the findings, and that more observations are needed to confirm the existence of the exomoon - which would be named Kepler-1625b-i.
He added: “The first exomoon is obviously an extraordinary claim and it requires extraordinary evidence."
The research paper has been published in the Science Advances journal.