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For only the second time, astronomers have found what they think is a moon orbiting a planet, orbiting a star that is not the Sun. Like the first potential exomoon, it is gigantic – larger than half the planets in the Solar System. Confirmation could take a while, but the first possible discovery is now looking like part of a pattern, rather than a mistake or a bizarre one-off.
More than 4,000 planets have now been confirmed around other stars, and thousands more candidates are yet to be verified. Since moons greatly outnumber planets in our own Solar System, an abundance of satellites in the galaxy makes intuitive sense. Indeed, the prospects for finding life close enough to study with future generations of telescopes may be higher on moons than planets by sheer weight of numbers.
This is all pretty speculative, however, because existing techniques are unlikely to find most moons. However, in 2017 Columbia University's Cool Worlds team reported signs of a planet-sized object that appeared to be associated with an even larger planet. Now in Nature Astronomy, the same team announce they've done it again.
The exomoon candidate is suspected to orbit Kepler 1708b, a not quite Jupiter-sized planet 5,500 light-years away. We are sure Kepler 1708b exists because the Kepler Space Telescope witnessed strong dips in brightness as it passed in front of the star Kepler 1708.
In examining these dips Dr David Kipping and co-authors found them accompanied by smaller dips consistent with a moon about 12 planetary radii away (between Europa and Ganymede in the Jovian system).
“It’s a stubborn signal,” said Dr Kipping in a statement. “We threw the kitchen sink at this thing but it just won’t go away,”
If real, Kepler 1708b-i, as the moon would be called, has a radius 2.6 times Earth's – making it a probable gas world rather than a rocky object.
Compared to a system whose largest moon is Ganymede – 60 percent smaller than the Earth and not much bigger than Neptune's own moon Triton – the idea of a moon this large is hard to wrap our heads around. The paper considers the possibility Kepler 1708b-i was once a planet of its own, captured after a close encounter.
Such an event is unlikely, but with enough systems out there even odd things are bound to happen now and then, and currently, the supermoons might be the only ones we can find. “The first detections in any survey will generally be the weirdos,” Kipping said.
The first detections are also likely to be questionable ones – after all, we don't yet know exactly what sort of signal to expect. “It might just be a fluctuation in the data, either due to the star or instrumental noise,” said Professor Eric Agol of the University of Washington, who was not involved in the study. However, the team is more confident.
Confirmation will require the use of the limping Hubble Telescope or perhaps the newly launched JWST, but getting precious time on either won't be easy. Even if that can be secured, opportunities will only occur every two years, when Kepler-1708b transits, and we must hope the alignment of star, planet, and moon allow detection.
Kipping's team previously claimed Kepler 1625b has a Neptune-sized moon, but their case remains disputed. One earlier possible exomoon has also been reported. However, in that case, all we know is the mass ratio of the object and a larger body it orbits – the pair could be moon and planet or planet and star. Since that question will probably never be resolved, Kipping's candidates are the leaders to be the first confirmed moons beyond our Sun's influence.
