Kepler Space Telescope (NASA)Astronomers think they have discovered the first known moon beyond our Solar System. It is believed to be in orbit around a gigantic planet—an exoplanet—lying 8000 light years away. The "exomoon", estimated to be the size of Neptune, would also be the largest known moon, much bigger than any of the Solar System's moons. The astronomers say that further observations will be needed to confirm the early results.
Detecting an exoplanet transiting its star (NASA)The research team, led by David Kipping and Alex Teachey of Columbia University, New York, made the discovery using the Kepler Space Telescope. Exoplanets are revealed by a slight dimming of starlight as they pass in front of, or transit, their host star. The team noticed an unusual pattern coming from the exoplanet known as Kepler 1625b, which is several times the size of Jupiter and takes 19 hours to transit its star.
Investigating further, using the powerful Hubble Space Telescope, Kipping and Teachey observed a second, much smaller dip in the star’s brightness a few hours after the planet’s transit. This hinted at a moon lying close to the planet, or, in the words of Dr Kipping, "trailing the planet like a dog following its owner on a leash". Both Kepler 1625b and its possible moon are gaseous, like, for example, Jupiter or Neptune, and not rocky like Earth. The moon, known as Kepler 1625b-i, orbits some three million kilometres from the planet.
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