Further evidence that a giant moon the size of Neptune is orbiting a Jupiter-sized planet 4000 light-years away has been put forth by astronomers using the Hubble Space Telescope.
Hints of what could be the first known exomoon (moon outside the Solar System) first came to light in 2017, after Alex Teachey and David Kipping of Columbia University in New York found some unusual behaviour in its parent planet, Kepler-1625b. NASA’s Kepler Space Telescope discovers planets by watching for the small dip in starlight as the planet moves across the face of its parent star. These dips are periodic as the planet orbits the star, but Kepler-1625b’s transits seemed out of kilter. Sometimes the transits would occur a little earlier, or a little later, than predicted.
Such events are called transit timing variations (TTVs). They are usually seen in compact planetary systems, such as those around red dwarf stars, where the planets are very close together and are able to gravitationally tug upon one another to affect the timing. However, Kepler-1625 is a Sun-like star and there is no evidence for another nearby planet. Some other object is therefore pulling on Kepler-1625b, with Teachey and Kipping reasoning that it must be a moon. The only snag is that to impart such a large TTV, the moon must have a mass similar to Neptune.
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