First moon beyond Solar System discovered

Astronomers may have discovered an exomoon larger than Neptune around a planet several times larger than Jupiter.

More observations will be needed to confirm whether the signals really do come from an exomoon. Image Credit: Dan Durda
Published: October 4, 2018 at 12:00 pm
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A moon the size of Neptune may have been found around a planet outside our Solar System, according to a recently released study.


If confirmed, this will be the first time we have ever discovered a moon around an exoplanet.

“This may yield new insights into the development of planetary systems and may cause astronomers to revisit theories of how moons form,” says Alex Teachey, a graduate student from Columbia University who led the observations.

The Kepler Space Telescope first detected signs of a moon orbiting Kepler-1625b – a planet several times more massive than Jupiter located 8,000 lightyears away – in 2017.

Teachey has now used the superior observing power of the Hubble Space Telescope to study the host star, watching for the dips in brightness caused by the planet moving in front of it.

Hubble observed the main planet transiting for 19 hours infront of the star, then detected a second dip in brightness 3.5 hours later. It’s thought this is the moon trailing behind the planet.

“It was definitely a shocking moment to see that light curve – my heart started beating a little faster and I just kept looking at that signature,” said David Kipping, also from Columbia University who took part in the study.

Unfortunately, the team’s allotted time on Hubble ran out before the end of the second transit.

The pair also noted that the main planet’s transit occurred an hour earlier than expected, as would happen if there was a moon pulling Kepler-1625b’s centre of gravity away from the centre of the planet, causing it to wobble from its predicted location.

It is possible that a second planet could be responsible for these observations, but during its four-year mission Kepler found no evidence suggesting additional planets around the star.


“It is an exciting reminder of how little we really know about distant planetary systems and the great spirit of discovery exoplanetary science embodies,” says Teachey.


Ezzy Pearson is the News Editor of BBC Sky at Night Magazine. Her first book about the history of robotic planetary landers is out now from The History Press.

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