The moon is estimated to be only 1.5 per cent the mass of its companion planet, which itself estimated to be several times the mass of Jupiter.
The researchers note that in principle this anomaly could be caused by the gravitational pull of a hypothetical second planet in the system, although Kepler found no evidence for additional planets around the star during its four-year mission.
One of the astronomers, David Kipping, said: "We've tried our best to rule out other possibilities such as spacecraft anomalies, other planets in the system or stellar activity, but we're unable to find any other single hypothesis which can explain all of the data we have".
Lead author Alex Teachey said, 'This may yield new insights into the development of planetary systems and may cause astronomers to revisit theories of how moons form'. If confirmed, this would be the first discovery of a moon outside our Solar System. It's comparable to so-called hot Jupiters, gas giant exoplanets that are closer to their stars than Jupiter is to its own, and warmer. The pair would have a similar mass and radius ratio to the Earth and its moon but scaled up by a factor of 11.
Transiting planets are worlds that can be detected by a drop in light as they pass in front of their home stars. They found the best match to be a Neptune-sized exomoon, named Kepler-1625b i, on an inclined orbit at a distance of 35 to 45 planetary radii from Kepler-1625b. The planet goes around the yellow parent star every 287 days at the same distance as the Earth orbits the sun. So, did Kepler-1625b capture its satellite?
Hubble was also able to measure that the planet began its transit earlier than expected, consistent with the "wobble" that occurs when a planet and moon orbit the same center of gravity.
That second dip in light may have resulted from a moon trailing the larger planet, which is orbiting the host star. The researchers were awarded with 40 hours of observation time using Hubble, and the data they gathered were four times more precise than what Kepler had captured. Such gargantuan moons do not exist in our own solar system, where almost 200 natural satellites have been catalogued. The team then used the Hubble telescope to monitor the planet and detected a second dip in brightness. But Kepler didn't find any other planets around this star. "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", David Kipping described his feelings. This catalog included 284 planets found by Kepler with wide orbits around their host stars.
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If indeed a moon, it would be about 2 million miles (3 million kilometers) from its planet and appear twice as big in its sky, as the moon does in ours. "Both bodies, however, are considered to be gaseous and therefore unsuitable for life as we know it", Kipping said.
"We hope to re-observe the star again in the future to verify or reject the exomoon hypothesis".
They estimate the surface temperature of both to be 176 degrees Fahrenheit. Our moon is thought to have formed through an early collision with a larger body that blasted off material that later coalesced into a ball.
KIPPING: So when we look for an Earth twin, I think one of the most obvious things you might ask is, does it have a moon twin, because that seems to have a large influence.
"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. While not a candidate for life themselves, Jupiter-like planets in the habitable zone may harbor rocky moons, called exomoons, that could sustain life.
Those observations could come from another go-round with Hubble, or they could come from NASA's James Webb Space Telescope, which is due for launch in 2021.
Until now, no exomoons have been confirmed. "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.