Over the last several decades, astronomers have gotten really good at discovering exoplanets. These objects, ranging from the size of Earth's moon to 1.7 times the size of Jupiter, are planets that reside outside our sun's solar system. Thanks to advanced instruments like the Kepler Space Telescope, we now know about 3,851 exoplanets orbiting around stars in 2,871 systems.

As the illustration below of discoveries within our own Milky Way shows, there are a lot of alien solar systems in just our own celestial neighborhood.

This artists’s cartoon view gives an impression of how common planets are around the stars in the Milky Way. This artist's illustrated view gives an impression of how common planets are around the stars in the Milky Way. (Photo: ESO/M. Kornmesser)

One thing, however, that's been really hard for astronomers to document are exomoons orbiting these exoplanets. We know they're out there — Jupiter alone has 79 moons, including a dozen discovered just this year — but observing them has proven to be extremely difficult. Thanks to astronomers David Kipping and Alex Teachey, however, we may have found the most compelling evidence for an exomoon to date. And like everything else in space, it's stranger than anyone could have predicted.

That's a big moon

In a new paper published in the journal Science Advances, Kipping and Teachey describe their potential exomoon, named Kepler-1625b-i, as a gaseous body many times larger than any moon in our own solar system. Located some 8,000 light-years from Earth, it orbits another gas giant called Kepler 1625b. In their paper, they described the scale of the moon as "jarring."

"The closest analog would be picking up Neptune and putting it around Jupiter," Kipping told The Atlantic.

That two gas giants could be in orbit around each other is puzzling; in particular because it doesn't fit the standard formula seen in our own solar system for moon formation. "If confirmed, Kepler-1625b-i will certainly provide an interesting puzzle for theorists to solve," they write.

The pair discovered the exomoon candidate in 2017 after analyzing data from 284 Kepler-discovered planets with wide, stable orbits. Only one planet in particular, Kepler 1625b orbiting a star in the constellation Cygnus, gave any hints of hosting a moon.

"We saw little deviations and wobbles in the light curve that caught our attention," Kipping said in a release.

An illustration of the transit of Kepler 1625b and its potential exomoon. An illustration of the transit of Kepler 1625b and its potential exomoon. (Photo: NASA, ESA, and L. Hustak (STScI))

Like other exoplanet discoveries, the researchers used the transit method –– which causes a dip in a star's light when an object moves across it –– to record the orbit of Kepler 1625b. Leveraging the sensitive optics of the Hubble Space Telescope, they noticed something odd occurring in the wake of the planet's transit across its host star.

"After the transit ended, Hubble detected a second and much smaller decrease in the star's brightness approximately 3.5 hours later," they wrote. "This small decrease is consistent with a gravitationally-bound moon trailing the planet, much like a dog following after its owner."

Further supporting their moon theory, they also found that the planet transit occurred more than an hour earlier than predicted. Something appears to be gravitationally "tugging" on the planet, similar to the way that our own moon helps to induce a wobble in the rotation of Earth.

"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 and I just 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."

Cautious optimism

While the initial data is promising, the scientists caution that it will take several more observations with Hubble, and possibly even with the more advanced James Webb Space Telescope, due for launch in 2021, before we can officially record our first exomoon.

As for a promising name, Teachey already one idea that's sure to play well with the Internet scene.

"I'm partial to Endor myself," he joked to Geekwire. "I'm more of a 'Star Wars' fan."

Michael d'Estries ( @michaeldestries ) covers science, technology, art, and the beautiful, unusual corners of our incredible world.

We may have just spotted the first 'exomoon'
Astronomers David Kipping and Alex Teachey describe the exomoon Kepler-1625b-i as a gaseous body many times larger than any moon in our solar system.