While some may want to define such a massive moon as a planet caught in a binary system with Kepler 1625b, the researchers define it as a moon since its mass is only 1.5 percent that of the planet it orbits, roughly the same as Earth and our moon.
The hunt for exoplanets - planets outside our own Solar System - provided its first results only 30 years ago. We won't know for sure until researchers can do more tests, of course, but even so, what we do have is very promising.
Given the conditions that both the planet and its potential moon are gas giants, it can not be said that this planet might support life.
In flickering light of a distant sun, scientists may have discovered the first moon outside our solar system.
"If confirmed by follow-up Hubble observations, the finding could provide vital clues about the development of planetary systems and may cause experts to revisit theories of how moons form around planets", he said. Through the Hubble, the team studied Kepler-1625b as it passes between the star that it is orbiting, which is Kepler-1625, and the Earth. Or perhaps, like Earth's moon, it is actually a product of its planet, formed in some catastrophic collision.
After analysing their data, they found that the planet transit began 77.8 minutes earlier than predicted. Only two planets in our solar system - Mars and Venus - don't have moons, Kipping said.
In the Hubble data, they saw a moon tugging along, "trailing the planet like a dog following its owner on a leash".
"Unfortunately, the scheduled Hubble observations ended before the complete transit of the moon could be measured".
"The first ecosistema is an extraordinary statement that requires extraordinary proof", said Columbia University astronomer David Kipping. They found no evidence for exomoons in the vast majority of the systems, but Kepler-1625 showed tantalising signs of the tiny dip in brightness that constitutes an exomoon signature. "And if validated, the planet-moon system-a Jupiter with a Neptune-sized moon-would be a remarkable system with unanticipated properties, in many ways echoing the unexpected discovery of hot Jupiters in the early days of planet hunting".
Scientists have never had enough evidence to confirm that any exoplanet has a moon orbiting it, but that might be about to change.
Although the planet and its possible moon are within the habitable zone of their star, both are considered to be gas giants and "unsuitable for life as we know it", Kipping said.
Even if it might be odd that a Neptune-sized moon could exist out there, at the same time, nothing in physics says that it can't.
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.
NASA's upcoming James Webb Space Telescope could really "clean-up" in the satellite search, .