Oct. 3 (UPI) -- The existence of a large exomoon orbiting a gas giant located 8,000 light-years form Earth best explains a unique pattern of stellar dimming observed by both Kepler and the Hubble Space Telescope.
"We are unable to find any other single hypothesis that explains all the data that we have," astronomer David Kipping told reporters during a phone call earlier this week.
Kipping and his colleague Alex Teachey, both researchers at Columbia University, detailed their investigation of the Hubble and Kepler data in a new paper published Wednesday in the journal Science Advances.
The duo began their research by surveying 284 Kepler-discovered planets with long orbits. Planets too close to their host stars are less likely to feature moons. Of the 284 candidates, one stood out. The transiting data associated with Kepler 1625b revealed some anomalies.
As a planet passes in front of its host star, it blocks some of the light being observed by planet-hunting probes like Kepler.
"We saw little deviations and wobbles in the light curve that caught our attention," Kipping said in a news release.
The anomalies were intriguing enough to warrant 40 hours of observation time using the Hubble Space Telescope.
The Hubble data allowed the two astronomers to more precisely measure the dimming patterns caused by Kepler 1625b. The data confirmed two anomalies that could be explained by the presence of a moon.
Hubble data revealed slight dimming caused by the transit of a potential exomoon. The observations also confirmed Kepler 1625b passes in front of its host star earlier than expected. The orbital anomaly can be explained by the gravitational influence of a moon.
Kipping and Teachey combined the Kepler and Hubble data to build a model of the sun-planet-moon system. The simulations closely matched the observed data.
"When we run our model, the moon hypothesis was the best and only single explanation for the timing effects and the dimming of the star that we see in the data," Teachey said.
The exomoon candidate is roughly the size of Neptune, and it is orbiting a planet the size of Jupiter, making it a seemingly unusual pair. But everything astronomers know about planets and moons is informed by the makeup of the solar system. What's unusual here may not be unusual out there.
Scientists typically use three different scenarios to account for moon formation. Most scientists think an impact scenario explains Earth's moon. Saturn's moons were formed from its rings. Astronomers think many of the gas giants' moons are captured asteroids.
Because the newly identified exomoon candidate is so large, it's unlikely that it was captured or formed from a collision. Previous studies suggest it's possible such large moons can coalesce from a planet's rings.
"You do sometimes form moons that big through the coalescing of disk material," Kipping said. "It's the upper end of the scale but its not incompatible with a disk formation scenario."
In talking with reporters, both Kipping and Teachey insisted their hypothesis remains just that. They want some more observational data to be certain what they're looking at is indeed an exomoon.
"It's our job as scientists to be as skeptical as possible about new ideas and hypotheses, even when those ideas are our own," Teachey said. "We're confident in the work that we've done and that the scientific community will help us get to the bottom of this one way or another."