We May Have Found Our First-Ever Exomoon, Opening A New Chapter in Space Exploration

• Scientists think that they have gathered compelling evidence for the presence of an exomoon—a moon orbiting around a planet outside of our Solar System.

• By analyzing the movements of a mysterious cloud of sodium moving around the planet WASP-49 b, the team was able to construct a compelling argument for the presence of an exomoon.

• While many candidates have been put forth, there has never been a confirmed detection of an exomoon.

Moons probably aren’t exclusive to our Solar System. Most things probably aren’t (including life, but that’s another discussion). But for as many far-off stars and exoplanets we’ve found at this point in our space-investigation history, we haven’t yet definitively found an exomoon—a moon located anywhere other than our little star system.

We’ve definitely found candidates, but we have yet to confirm anything—in part because several candidates have become less likely with more data, and in part because of just how truly difficult exomoons are to see. In general, they’re dim, small, and block less light from stars for less time than planets. That said, a new study published in The Astrophysical Journal Letters might have gotten us a step closer than ever before to spotting a natural satellite around another world in a whole different system.

The secret? Sodium.

The team behind this new study—lead by Caltech researcher Apurva Oza—turned some of our best observational equipment on a mysterious cloud of sodium that has been seen floating around the planet WASP-49 b (which itself orbits the star WASP 49). This cloud was first discovered in 2017, and Oza has been studying it pretty much since then in the hopes that it is indicative of a volcanic exomoon.

See, by all accounts, the sodium shouldn’t be there. WASP-49 b (a hot, Saturn-like planet) and the star it orbits are both made primarily of hydrogen and helium. The trace amounts of sodium they do have could in no way maintain this cloud, which seems to be actively replenished at a rate of 220,000 pounds of sodium per second. But, nonetheless, there it is.

So, the team dug in, and they managed to find several pieces of evidence that seem to point fairly compellingly in the exomoon direction. For one, the cloud twice seemed to jump in size as if being refueled, despite not being right next to the planet at those times. And for another, the cloud is moving in the wrong direction for it to be a planetary atmospheric phenomenon. “We think this is a really critical piece of evidence,” Oza said in a press release. “The cloud is moving in the opposite direction that physics tells us it should be going if it were part of the planet’s atmosphere.”

On top of their observations, the team ran models to try and understand the behavior of this cloud. Eventually, they found that its seemingly irregular movements in front of, behind, and around both WASP-49 and WASP-49 b—as well as its apparent lack of tether to any particular spot on the surface of WASP-49 b—were best matched by the presence of a moon on an eight-hour-long orbit around the planet.

These already exciting data points are even further strengthened by the fact that we have actually seen a moon behaving in pretty much this exact way right in our own cosmic backyard. Jupiter’s moon Io is the most volcanic object in our Solar System, fueled by the constant push-and-pull deformations of tidal forces exacted on it by its planetary host. It spews so much gas and debris that, according to NASA, it can create clouds around Jupiter that are up to 1,000 times the radius of the planet itself. If this potential exomoon is anything like Io, it’s firmly in the realm of possibility that it would create a cloud of sodium that we could see long before we were able to detect the planet directly.

The potential existence of this moon is exciting, but if we want to confirm its presence, we’re going to need to act fast. (Well, fast on a cosmic scale, anyway). The tidal forces needed to generate the amount of volcanic activity needed to make a cloud of sodium like this are extremely strong, and are likely taking a serious toll on the structural integrity of the moon. In addition, if you spew a bunch of the stuff you’re made of out into the universe via big volcanoes, you don’t have that material inside of you anymore, making you even weaker. Over time, if we are right about the nature of this object, it is basically going to disintegrate into nothing more than debris around a distant star. “If there really is a moon there,” Oza said, “it will have a very destructive ending.”

Good thing our telescopes are getting better and better. Eventually, there’s a good chance we’ll be able to know for sure exactly what is making this cloud. Until then, we’ll just have to patiently settle for promising evidence.

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