Last year, researchers revealed some tentative evidence that we had just discovered an exomoon. Now, new research strengthens the case and gives us a lot more information about this utterly peculiar potential exomoon.

As reported in Science, the system of Kepler-1625b is very similar to the Earth-Moon system in terms of mass-to-radius ratio, but scaled up 11 times. The planet could be up to three times more massive than Jupiter, which makes the candidate an exomoon, a truly unexpected object. It is roughly the size of Neptune.

The first hints that a moon could be orbiting this planet came from Kepler observations. There were anomalies that couldn’t be easily explained away. Two astronomers from Columbia University obtained 40 hours of telescope time on Hubble and were able to obtain four times better precision on the existing data. It also showed that the transit started 1.25 hours earlier than expected, and the explanation is that the moon was tugging at the planet. When these observations were tested against different hypotheses, the large moon idea was the winner.

“When we run our models, the moon model emerges as the best explanation for the data and has the added benefit for being a single explanation for the timing effects in the dimming of the star that we see in the data. Still, we are urging caution here,” lead author Alex Teachey said in a press conference.

“The first exomoon is obviously an extraordinary claim, and it requires extraordinary evidence. Furthermore, the size we have calculated for this moon, about the size of Neptune, has hardly been anticipated. That too is a reason to be careful here.”

There is nothing in the Solar System quite like it. The biggest moons in proportion to their planet's size are Earth’s own and Pluto’s moon Charon, and they are tiny by comparison. They were also formed through impacts and so are rocky objects; the researchers are unsure if impacts could give rise to such a large gaseous object. Alternatively, it might have formed together with the planet or could be a captured former planet.

The team stresses how cautious we need to be with this research. The exomoon is a well-supported hypothesis and it is certainly a tantalizing one but more observations are necessary. The next transit of the planet will be in May 2019 and if the team can secure time on Hubble, the observations might be enough to confirm the exomoon.

“We hope to re-observe the star in the future to verify or reject the exomoon hypothesis. If validated, the planet-sized moon will be a remarkable system with unanticipated properties, and in many ways echoing the unexpected discovery of hot Jupiters in the early day of planted hunting,” senior author assistant professor David Kipping, also at Columbia, explained.

The Kepler 1625 system is located about 8,000 light years away, and if does contain a moon around the system is expected to be very stable. It is an old system, twice as old as our own. The planet also inhabits the habitable zone of the star, and while it might not have the right condition for life, it shows how much more detailed planet-hunting research has become over the last decade.