Evidence of possible exomoon detected for the first time

Artist’s impression of the exoplanet Kepler 1625b transiting the star with the potential exomoon in tow

Artist’s impression of the exoplanet Kepler 1625b transiting the star with the potential exomoon in tow

Since planets and satellites orbiting stars do not emit light on their own, it is impossible to directly observe those outside the solar system.

"This would be the first case of detecting a moon outside our solar system", said David Kipping, Assistant Professor at the Columbia University in NY.

Kipping has spent a decade working on the "exomoon hunt".

Even if it is confirmed as the first ever exomoon discovery, there are still questions to be answered, such as how a gas giant came to have a gas giant moon?

The study, titled "Evidence for a Large Exomoon Orbiting Kepler-1625b", is freely available at the Science Advances website. "Science can't operate by teams such as ourselves refusing to publish our results and hiding behind closed doors", Kipping says in an online outreach video posted concurrently with the paper. "It's the unknown of unknowns which are ultimately uncharacterizeable". There's no analog for such a large moon in our own system. The planet Endor itself is a gas giant, but the Forest Moon is a habitable world, peopled by small furry sentient creatures.

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. Astronomers think numerous gas giants' moons are captured asteroids. However, the pair of astronomers behind the find say it's much too soon to completely prove the exomoon's presence.

Kipping and his colleague Alex Teachey made the discovery after analyzing data from almost 300 distant planets discovered using the Kepler space telescope. Kepler-1625b is the only planet found so far around this star.

While these observations mount further evidence for the existence of Kepler-1625b-i, they are not, as yet, conclusive. About 3.5 hours after Kepler-1625b's transit completed, HST recorded a second smaller decrease in the star's brightness, a dimming indicative of a moon "trailing the planet like a dog following its owner on a leash," Kipping said.

The astronomers were allocated 40 hours of observation time on the Hubble telescope.

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Kepler, now virtually out of gas after an extraordinary mission, detected more than 2,600 exoplanets by looking for the tell-tale dip in brightness that occurs when a planet transits, or moves in front of its parent star as seen from Earth, blocking some of the star's light.

"You do sometimes form moons that big through the coalescing of disk material", Kipping said. Unfortunately, the scheduled Hubble observations ended before the complete transit of the moon could be captured.

Although the object itself can not be seen, there are hints it exists, according to the researchers: The planet moves around its star in a way that indicates something else is pulling on it gravitationally, probably a moon. "It was a shocking moment to see that light curve". In the meantime, they're encouraging other scientists to join in.

"If we want to do moon hunting in the future, we will have to look at planets further [than one astronomical unit, or the distance between the sun and Earth]", Teachey said.

The researchers believe the star system to be 10 billion years old, which means it's had time to evolve. The exomoon orbits roughly 3 million km from its planet.

Based on the moon's distance from its star, its surface temperature was estimated to be 80C, which is considered just on the upper end of conditions that could support life. He points out that our moon - the one with the astronaut footprints - does more than just inspire poets. Had they used the newer data, "we might not have asked for that [Hubble] time". There are three primary theories about how moons form. Finding moons around those planets is even harder. However, searches are biased toward the largest moons that might be out there, because bigger things are easier to detect. There are no indications of tidal capture among our Solar System's moons. And this moon's size defies explanation.

The Hubble Space Telescope is a project of global cooperation between ESA and NASA.

"With exomoons we are stretching the limits of our instrument detection thresholds and precision", astronomer Laura Mayorga, who wasn't involved in the new study, said via email. But for now, the researchers welcome comment and criticism of their hypothesis from other astronomers as part of the scientific process. "Including rocky exomoons in our search for life in space will greatly expand the places we can look".

"If this does pan out and turn into a true discovery, it would be really revolutionary, but I don't think we're quite there yet", says Megan Bedell, an astronomer at the Flatiron Institute in NY.

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