Moons are abundant in our own solar system, with close to 200.

To find evidence for the existence of the exomoon, the team observed the planet while it was in transit in front of its parent star, causing a dimming of the starlight. An aging sun-like star in the constellation Cygnus is known to host a huge, gas giant planet the size of Jupiter, known as Kepler 1625b. More observations were needed before they could draw any conclusions, but the evidence scored them 40 hours of observation time with the Hubble telescope.

Like its moon, Kepler-1625b is also bigger than its counterparts in the Solar System.

Kipping and Teachey noticed unusual anomalies in the transit data of a gas planet, Kepler 1625b, which is several times the size of Jupiter. Such gargantuan moons do not exist in our own solar system, where almost 200 natural satellites have been catalogued.

An illustration of the exoplanet Kepler 1625b with its large hypothesized moon. The gas giant it presumably orbits has several times the mass of Jupiter. They found two signs suggesting an exomoon could be in tow.

Barely noticeable dips in the Kepler light curves, just to either side of the main lightcurve dip produced by the planet, pointed to the possibility a second body making transit alongside the gas giant, and since the location of the secondary dip changed with each transit of the planet, it indicated that the second body could be orbiting around the planet. "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.

Due to the unfortunate timing of the Hubble observations, their time with the telescope ended after the planet transited the star, but before the candidate exomoon could fully complete its own transit. The Hubble observations showed that the planet's transit occurred 75 minutes earlier than predicted based on earlier transits observed by the Kepler spacecraft.

Roughly 7,000 light years away from us, orbiting around a Sun-like star in the constellation Cygnus, is a single, enormous exoplanet, known as Kepler-1625b.

The researchers monitored a planet known as Kepler 1625b as it passed in front of its parent star.

"It's the unknown unknowns which are ultimately uncharacterizable", Kipping said.

"It is an exciting reminder of how little we really know about distant planetary systems and the great spirit of discovery exoplanetary science embodies", study co-author Alex Teachey said of the discovery in a statement. Or it could have been an object that was captured by the planet's gravity from elsewhere. Tidal forces between the two objects would rob momentum from the less massive companion and eventually pull it into a permanent orbit. But there is a chance we could detect the presence of a major feature of our Solar System elsewhere: exomoons. However, Kepler-1625b and its candidate moon are gaseous, not rocky, so such a collision would not have led to the condensation of a satellite. For example, the Trappist-1 system of seven transiting planets exhibits strong transit timing variations. In fact, in terms of mass ratios, radius ratios and separation, 1625b and its moon would resemble a scaled up-Earth-moon system. Three-and-a-half hours later, Hubble detected a much smaller dip in the star's brightness consistent with a large moon trailing the planet. "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", he notes. While this is an exciting discovery, it's still not confirmed - it will take more time on the Hubble Space Telescope to confirm this remarkable find.

Scientists may have bagged the first-ever exomoon.

"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", said Dr Kipping, from Columbia University in NY.