Astroboffins may have found the first exomoon lurking beyond the Solar System

It's so big that it's possible that it could have its own little moons

exomoon
Artist's impression of Kepler-1625b and its hypothetical moon. Image credit: Dan Durda

Scientists have spotted what may be the first Moon to be discovered outside the Solar System, according to a paper published in the journal Science Advances on Wednesday.

Finding exoplanets is a difficult task, let alone exomoons. David Kipping, assistant professor at Columbia University, has kept at it for almost a decade. Now, working together with Alex Teachey, a graduate student, they may have finally hit the jackpot.

Kepler-1625b is an exoplanet that orbits Kepler-1625, a Sun-like star, about 8,000 light years away. It was detected by the transit method, where a star’s brightness dips periodically when a body blocks the light as it crosses the star's orbit. Observations of the light curves made by the Hubble Space Telescope reveals two anomalies, clues that there might be something else lurking there too.

“The first is that the planet appears to transit one and a quarter hours too early, that’s indicative of something gravitationally tugging on the planet. The second anomaly is an additional decrease in the star’s brightness after the planetary transit has completed,” Kipping said, during a telephone conference.

In one recording of a 19-hour transit ended, there was a second much smaller dip in the light curve that followed three and half hours later. It can be explained by a gravitationally-bound moon following its parent planet. The researchers can’t be completely sure, however, since the observation of the moon transit could not be completed.

The irregular times of the transit can be explained by a moon wobbling around a planet, but there is also a chance it could also be a hidden planet.

"A companion moon is the simplest and most natural explanation for the second dip in the light curve and the orbit-timing deviation," Kipping explained.

"It was definitely a shocking moment to see that Hubble light curve, my heart started beating a little faster as I kept looking at that signature. But we knew our job was to keep a level head and essentially assume it was bogus, testing every conceivable way in which the data could be tricking us."

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The bigger the drop in brightness, the larger the size of the object. In order to be able to catch an exomoon using the transit method, it must mean the satellite has to be pretty hefty. The really surprising part is that its estimated to be about as big as Neptune, Teachey said. It is so big that theoretically the moon might even have its own moons.

Most satellites around planets are teensy. They might be leftover bits from a planetary disk that have coalesced to form a moon or large chunks of material that have escaped the planet after impacts with asteroids. A giant moon like Neptune points to other mechanisms.

“Could it be a capture scenario? It’s sort of raising new questions and just having a moon of this size, has so far not really been anticipated much in the literature,” Teachey said.

The researchers also estimate that the new find is only 1.5 per cent of the mass of its parent planet, and the planet is several times the mass of Jupiter. That would make the ratio between both bodies similar to Earth and the Moon.

Both bodies also lie in the star’s habitable zone. Don’t get too excited, however, the chances of life are pretty slim since the planet is gaseous and not rocky. At the moment, the moon’s existence and properties remains speculative. The researchers hope that details might be confirmed when NASA finally sends the James Webb Space Telescope to space and its powerful enough to see if its a moon or not. "We can expect to see really tiny moons with Webb," Teachey said. ®




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