According to the authors of the study, it's possible that the second dip in light could be caused by another planet orbiting the star, but the Kepler Space Telescope didn't find any evidence of a second planet in the system during its observations of the star.
In principle this anomaly could also be caused by the gravitational pull of a hypothetical second planet in the system, but the Kepler Space Telescope found no evidence for additional planets around the star during its four year mission.
As the planet moved in front of the star, the star's light dimmed too much.
For the first time, astronomers have discovered what could be an exomoon, a moon outside our solar system.
The Hubble Space Telescope is a project of global cooperation between ESA and NASA.
Kipping and Teachey discovered it among 300 exoplanets in Kepler's catalogue, all of which produce predictable dips in starlight that occur as an orbiting body passes in front of its sun - a phenomenon called a "transit". That's why the astronomers need another look with Hubble, hopefully next spring. Teachey said that it's likely this possible exomoon is "in some ways the lowest hanging fruit". The moon's Neptune size was on a scale that had "hardly been anticipated" and "defies easy explanation" based on current theories of moon formation.
He added: "We're not cracking open Champagne bottles just yet on this one".
A hunt for exomoons - bodies that orbit these distant planets - has proceeded in parallel. Kipping and Teachey weren't even sure how such a moon might have formed.
Teachey and Kipping compared the observations to a range of various models - including some that did not include a moon - that could potentially explain the data.
This NASA photo taken on July 20, 1969 shows astronaut Edwin E. "Buzz" Aldrin, Jr. saluting the USA flag on the surface of the Moon during the Apollo 11 lunar mission.
In the meantime, a new release of Kepler data smoothed away many of those bumps, weakening the original case for a moon search. A moon could cause that kind of an uncertain, wobbly path, they noted.
This decrease in dimness is consistent with "a moon trailing the planet like a dog following its owner on a leash", Kipping said.
Thousands of planets have been detected around faraway stars in recent years.
An illustration of the exoplanet Kepler 1625b with its large hypothesized moon. He has led the field over this time, so I am delighted that his persistence has paid off. "But we were unable to find any other single hypothesis which can explain all of the data we have".
The study was published Wednesday in the peer-reviewed journal Science Advances. That makes it plausible that the new exomoon is just as off-the-menu as the "hot Jupiters" that surprised early exoplanet hunters, says Stephen Kane, an astrophysicist at the University of California, Riverside, who was not part of the research team.
However, astronomers said they had not dwelled on the question of habitability, since the moon is thought to be composed of gas.
He and his colleague David Kipping are careful to call their discovery a "candidate moon", noting that more studies are required to confirm its existence. In the meantime, they're encouraging other scientists to join in.
Whether confirmed or not, the subject offers insight into how rare - or how common - our own solar system might be. They were amazed instead by how different this moon was from the roughly 180 known in our solar system. With a radius that's around a third that of its parent planet, this object is unlike any moon of a giant planet in the solar system.
Given that both the planet and its potential moon are gas giants, no one is suggesting conditions that might support life.
It's not like the exomoon in "Avatar" or Endor from "Star Wars", Teachey said, "but going forward, I think we're opening doors to finding worlds like that".