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Odd moves provide clues to exomoon’s presence

Published: 22nd November, 2018 at 04:00
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Deviations in exoplanet Kepler-1625b’s orbit hint at the presence of what could be the first known exomoon.

It’s a space oddity for real: astronomers have spotted an exoplanet that wobbles. The wobbling could be due to an exomoon – a moon orbiting a planet outside our Solar System. If confirmed, it would be the first exomoon to be discovered.


Alex Teachey and David Kipping of Columbia University used NASA’s Hubble Space Telescope and Kepler space telescopes to gather data on what they think is a huge moon – similar in diameter to Neptune – in orbit around Kepler-1625b, a gas giant exoplanet 8,000 light-years away.

Astronomers search for exoplanets by watching for the temporary dimming of a star’s light caused by a planet passing in front of it, referred to as a transit. But while observing Kepler-1625b (discovered in May 2016) they spotted something unusual.

“We saw little deviations and wobbles in the light curve that caught our attention,” explained Kipping.

What they noticed was a second, much smaller dip in the star’s light after the exoplanet’s 19-hour transit, a dip that would be consistent with a following moon.

They also noticed that Kepler-1625b began its transit 1.25 hours earlier than expected because it had wobbled away from its predicted location. Such a wobble could be explained by the presence of another body orbiting a common centre of gravity.

“A companion moon is the simplest and most natural explanation for the second dip in the light curve and the orbit-timing deviation,” said Teachey. “It was a shocking moment to see that light curve, my heart started beating a little faster and I just kept looking at that signature. But we knew our job was to keep a level head and test every conceivable way in which the data could be tricking us until we were left with no other explanation.”

This is an extract from issue 329 of BBC Focus magazine.

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Inside Mars: What will we uncover beneath the Red Planet's surface? © Andy Potts, NASA


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Jason Goodyer
Jason GoodyerCommissioning editor, BBC Science Focus

Jason is the commissioning editor for BBC Science Focus. He holds an MSc in physics and was named Section Editor of the Year by the British Society of Magazine Editors in 2019. He has been reporting on science and technology for more than a decade. During this time, he's walked the tunnels of the Large Hadron Collider, watched Stephen Hawking deliver his Reith Lecture on Black Holes and reported on everything from simulation universes to dancing cockatoos. He looks after the magazine’s and website’s news sections and makes regular appearances on the Science Focus Podcast.


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