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Move over exoplanets, exomoons are the next big thing

Is that an extremely large moon we see outside the solar system, astro-boffins ask themselves

Scientists have spotted a new candidate for a moon existing outside of our solar system, with only a 1 per cent chance the observation could be an anomaly.

More than 4,000 exoplanets have been mapped since the first was found in 1992. Although the finding of worlds beyond the Earth's immediate star system generated much excitement at the time, exoplanets are not so rare a discovery in recent years: US space agency NASA once found 700 in a single haul.

However, the existence of moons outside the solar system has yet to be confirmed. Going with the thinking that there's nothing particularly special about our own solar system, which is host to more than 200 moons, then we might assume they are also commonplace elsewhere.

Columbia University assistant astronomy professor David Kipping and a team of scientists have put together a study of data from the Kepler space telescope, which NASA launched in 2009 and retired nine years later after it had helped discover more than 2,000 exoplanets.

"Given the abundance of moons in our solar system, it is reasonable to presume that exomoons will reside around some exoplanets – which has motivated efforts to detect them," said Kipping's recent paper in Nature Astronomy.

The team applied various mathematical models to a pool of 73 cool giants picked as promising places to find exomoons because they are transiting planets that periodically eclipse their host star, making potential moons easier to detect.

Subsequent number-crunching earmarked Kepler-1708 b-i, most probably a moon more than twice the size of Earth orbiting a Jupiter-like planet which in turn orbits a Sun-like star.

Exhibiting a cautious air, the scientists also calculated that the chances of the candidate being a false-positive rather than the first confirmed exomoon was around 1 per cent.

"We can find no grounds to reject Kepler-1708 b-i as an exomoon candidate at this time, but urge both caution and further observations," according to the research paper, which is titled "An exomoon survey of 70 cool giant exoplanets and the new candidate Kepler-1708 b-i."

The Hubble Space Telescope, the newly launched James Webb Space Telescope or the European space telescopes project PLATO (PLAnetary Transits and Oscillations of stars) could get more sightings of repeated moon transits to confirm the finding, they suggest.

Kepler-1708 b-i is not the only space object in the running to become the first confirmed exomoon. Proposed in 2018, Kepler-1625 b-i42 is another candidate, and also "unexpectedly large," according to the paper.

"Although the reality of Kepler-1625 b-i remains unclear, the existence of this second candidate challenges us to consider the origins of such large moons," the paper said.

As well as confirming the finding, further observations could help explain why such large moons appear so close to gas giants, which "certainly challenge conventional thinking," Kipping et al note, although "plausible mechanisms have been previously proposed."

"Ultimately, the reality of supermoons such as Kepler-1708 b-i and Kepler-1625 b-i will require follow-up transit photometry, as both their nature and supporting evidence demand appropriate scepticism at this time," the paper said. ®

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