Such enormous moons are not present in our own solar system where 200 natural satellites have been indexed.
Now, astronomers think they've discovered the first "exomoon" - a moon outside of our own solar system.
Kipping has spent a decade working on the "exomoon hunt".
"We're not cracking open Champagne bottles just yet on this one". Earth's companion keeps our planet's tilt stable and affects the tides.
Astronomers know of three major moon-formation mechanisms: There's gravitational capture (which appears to be the case with Neptune's biggest moon, Triton); powerful impacts (as happened with Earth's moon, which formed from material blasted into space by a long-ago collision); and the merging of material from a disk of material surrounding a newborn planet.
And when E.T. phones home, there's no reason that home couldn't be a moon.
Although the researchers can not say with certainty that an exomoon caused these effects, Kipping argues that the moon hypothesis offers the best single explanation for both anomalies.More news: LG unveils V40 ThinQ with 5 cameras
"There are now 175 known moons orbiting the eight planets in our solar system".
The researchers then used the Hubble Space Telescope to closely monitor the star in question, monitoring the light closely as the planet made a 19-hour transit. What popped out was Kepler-1625b. It would not resemble the fictional exomoon Pandora in the film Avatar, Teachey said. The main dip in stellar brightness would be the planet itself crossing in front of its star. This is consistent with the planet and moon orbiting a common center of gravity (barycenter) that would cause the planet to wobble from its predicted location.
The detection of the candidate exomoon - moons orbiting planets in other star systems - is unusual because of its large size, comparable to the diameter of Neptune. Moons, due to their smaller size and shifting position, are harder to detect. "If they see what we see, I expect some people will be convinced and other people will be skeptical".
The exoplanet was originally found by the transit technique, in which a planet passing in front of its host star, along our line of sight, causes the star's brightness to dim slightly (by around 1% for a Jupiter-sized planet orbiting a sun-sized star) once every orbit.
They estimate the surface temperature of both to be 176 degrees Fahrenheit.
They say the findings will help unravel the origin of planets and satellites.
Planets seem to be quite common, so it seems likely that moons should be common, too.
Our solar system's moons all are rocky or icy objects.More news: India vs West Indies 1st Test, Day 2
Copyright 2018 NPR. To see more, visit http://www.npr.org/. The planet's name is Kepler-1625b. Knowing when you're looking at a moon and not a planet is tricky. NPR's Nell Greenfieldboyce reports. There's no analog for such a large moon in our own system.
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 affects the tides.
KIPPING: That's been a key driver for us for a while, just trying to understand the cosmic habitats out there that we might look for for life.
To verify this, Kipping and Teachey booked 40 hours of time on the Hubble Space Telescope, to get a finer, more detailed look at light curves from the Kepler-1625 system. They checked around 300 planets for any weirdness that might mean a moon.
The Hubble Space Telescope is a project of global cooperation between ESA and NASA. Their results were published Wednesday in the journal Science Advances.
KIPPING: We're still not 100 percent sure about that, but we think it's the leading hypothesis.
Teachey and Kipping are hoping to try again, and have requested time on Hubble for the next transit in May 2019.
MEGAN BEDELL: If this does pan out and turn into a true discovery, it would be really revolutionary. "That's kind of ironic", Teachey says.More news: Chicago police officer jailed after guilty verdicts