There's no analog for such a large moon in our own system. Things like asteroids and comets are far too small to resolve given our current technologies. There are ways we could now detect them, but orbital dynamics makes doing so fairly unlikely. "But we were unable to find any other single hypothesis which can explain all of the data we have".
Jupiter's moons are believed to have formed from a ring of material; but again, none of Jupiter's moons are the size of Neptune. They found two signs suggesting an exomoon could be in tow. The first involves the amount of light blocked while the exoplanet orbits its host star. Based on computer modeling, the surface temperature on both the alien planet and the moon would be on the order of 175 degrees Fahrenheit (80 degrees Celsius), Kipping said.
The alternative is what are called transit-timing variations. The orbital anomaly can be explained by the gravitational influence of a moon. This is consistent with the planet and moon orbiting a common centre of gravity, causing the planet to wobble from its predicted location.
For Teachey and Kipping, the best and simplest explanation is that Kepler-1625b has a moon.
 Despite its size, the mass of the candidate moon is estimated to be only 1.5 percent of the mass of its companion planet. Knowing when you're looking at a moon and not a planet is tricky.
Finding exoplanets is a hard task, let alone exomoons. For practical reasons, however, we probably shouldn't expect to. This particular planet - or exoplanet - is about the same distance from its star as Earth is to the sun. This comes about because of the methods we use to detect them. A primary goal of the Kepler mission is to identify planets that are in the habitable zones of their stars, meaning it's neither too hot nor too cold for liquid water-and potentially life-to exist. Star Warshad the moons of Yavin IV and Endor. In most cases, the moon ends up getting ejected from the system.
Both the study team and critics alike say that further confirmation is still needed, however, adding an element of caution to what would be a landmark discovery.
The light of a star known as Kepler-1625 dims slightly as a large planet - Kepler-1625b - moves across the star's disk as viewed from Earth. And they noticed weird deviations in the "light curve" generated by the 19-hour-long transit of Kepler-1625b, a planet about three times heftier than Jupiter that orbits a star about as massive as our own sun. In the intervening time, the Kepler data has been cleaned up to control for some minor biases in its instruments.
This artist's impression depicts the exomoon candidate Kepler-1625b-i.
But the Hubble data doesn't. The observations also confirmed Kepler 1625b passes in front of its host star earlier than expected.
In the meantime, a new release of Kepler data smoothed away many of those bumps, weakening the original case for a moon search. And Hubble shows an additional dimming later in the planet's transit.
"It's raising new questions about the sort of the dynamical processes that go on to create the planets and moons", Teachey said. Such a scenario is unlike anything that's seen in our own solar system, of course, and it doesn't fit any of the standard theories for moon formation.
"The first exomoon is obviously an extraordinary claim and it requires extraordinary evidence", Teachey told reporters in a teleconference organized by the journal that published the findings. "We're learning new things how to do these observations better".
Faced with that skepticism, Teachey and Kipping are undoubtedly applying for more Hubble time during its next transit.