In September, NASA officially confirmed the existence of a whopping 6,000 exoplanets—a feat so impressive that it’s bizarre to think that, by contrast, the number of confirmed exomoons tallies up to, well, zero. But that imbalance may shift soon if a new proposal by astronomers ends up being as effective as they claim.

An upcoming Astronomy & Astrophysics paper describes how astronomers devised and utilized a novel, alternative approach for identifying exomoons, which successfully turned up a promising exomoon candidate orbiting HD 206893 B, a Jupiter-like exoplanet located about 133 light-years from Earth. Specifically, the team repurposed high-precision astrometry—a mathematical approach to mapping out stellar distances—to carefully evaluate any and all signals near the exoplanet.

The object appears to be around 0.4 Jupiter masses, which is more than seven Neptune masses, and is still much smaller than HD 206893 B at 28 Jupiter masses. So it’s an absolutely gigantic exomoon orbiting an absolutely gigantic exoplanet. Well, if true. As the researchers themselves admit, the alleged exomoon will now have to face scrutiny from the wider astronomical community. Still, they argue, the observation cements astrometry as a promising tool for future exomoon searches. The paper by the collaboration, including lead author Quentin Kral of the Paris Observatory in France, is currently available as a preprint.

We haven’t found any exomoons?

Given the tremendous success of our ongoing exoplanetary hunts, it’s strange that we’ve yet to find any exomoons. So why aren’t the tried-and-true strategies that work for exoplanets not working for their orbiting companions? Indeed, there have been quite a few discoveries of exomoon candidates over the years.

That said, reality isn’t so simple. To take exoplanets as an example, all “discoveries” go through a rigorous process of checking and double-checking for mistakes such as, but not limited to, asteroid sightings, tricks of the light, duplicate findings, etc. In fact, NASA admitted that around 8,000 exoplanet candidates are currently awaiting confirmation from the astronomy community.

The exomoon search faces similar challenges. There’s also the fact that, on the cosmological scale, exoplanets are already considered extremely small. If our solar system is any guide, moons are smaller than their host planets, meaning that exomoons should be even smaller—and therefore trickier to detect.

“Moreover, there is no definition of what an exomoon is, and some ambiguity remains as to whether it may include, for instance, binary planets,” the study explained, adding that the “scarcity of detections contrasts sharply with the ubiquity of moons in our solar system.”

Smaller object, bigger problem

The new model attempts to address some of these challenges by piecing together different, existing approaches for detecting exomoons. By directly measuring the spatial wobble of the host planet, astronomers should be able to calculate the effect of an orbiting moon’s gravitational pull. This would offer researchers greater flexibility when evaluating the possibility of an exomoon’s presence around exoplanets, according to the paper.

To test their model, the team monitored the astrometric positions of the exoplanet HD 206893 B using the GRAVITY instrument on the Very Large Telescope in Chile. They carefully observed the wobbly differences between the motion of the exoplanet and a nearby secondary signal, likely an exomoon candidate. What’s more, astrometric techniques allowed the researchers to calculate the size and orbit of this candidate.

Although the team believes the new exomoon signal is promising, they “emphasize the tentative nature of this candidate, which needs further confirmation using GRAVITY data,” the paper noted. But more importantly, the experiment demonstrates the effectiveness of high-precision astrometry, and current and next-generation instruments will “usher in a new era of comparative exolunar science,” they said.

Indeed, as the researchers note, the new method is intended to complement existing ones. If so, the new study would be yet another instance of multi-messenger astronomy—an observational approach that uses multiple methods to study one signal. Even if this particular signal ends up being a dud, the proposal should certainly be the start of something good. Astronomers are inching closer to confirming the first known exomoon. And in fact, they may already be there.