Exomoons: Why study them? What can they teach us about finding life beyond Earth?

Universe Today has had the recent privilege of investigating a multitude of scientific disciplines, including impact craters, planetary surfaces, exoplanets, astrobiology, solar physics, comets, planetary atmospheres, planetary geophysics, cosmochemistry, meteorites, radio astronomy, extremophiles, organic chemistry, black holes, cryovolcanism, planetary protection, dark matter, supernovae, and neutron stars, and how they both individually and collectively contribute to our greater understanding of our place in the universe.

Here, Universe Today discusses the growing field of exomoons with Dr. David Kipping, who is an assistant professor in the Astronomy Department at Columbia University, along with his PhD students, Benjamin Cassese and Daniel Yahalomi, regarding the importance of studying exomoons, the benefits and challenges, potential exomoon candidates, how exomoons can teach us about finding life beyond Earth, and advice for upcoming students who wish to pursue studying exomoons. Therefore, what is the importance of studying exomoons?

Dr. Kipping tells Universe Today, “There’s four reasons to do this: 1) How common are Earth-like worlds? Exomoons may be a significant contributing factor to the cosmic census of habitable bodies; 2) How unique is the Earth-Moon system? The Moon is thought to have played an influential role in the formation and evolution of the Earth, and thus when we detect an Earth-twin we should naturally wonder if it has a Moon twin too.”

Dr. Kipping continues, “3) What are the moon formation channels? In the Solar System, we see at least three pathways, captures (e.g. Triton), impact (e.g. the Moon) and disk formation (e.g. Galilean moons). We would like to understand if there are other methods, and what the details and limitations are of the three methods we know of; 4) When we point HWO [Habitable Worlds Observatory] at an Earth-twin, a Moon-like moon would be unresolvable and thus its light will mix with that of the planet and potentially create false-positive biosignatures. Knowing about moons is vital to our long-term dream of finding life.”

Along with the Earth’s Moon, our solar system consists of more than 200 moons, but only a handful of them are targeted for astrobiology-related research, most notably two of Jupiter’s Galilean moons, Europa and Ganymede, and two of Saturn’s moons, Enceladus and Titan, but all of which have presented significant evidence for possessing interior oceans of liquid water. Along with finding out if the Earth-Moon system is unique, exomoons can teach us if our own solar system is unique given the wide range of moon types, shapes, and sizes, and especially their formation and evolution.

One possible reason for the Earth-Moon uniqueness is due to the tidal forces caused by the two bodies tugging on each other which maintains Earth’s relatively stable axis. As a result, the Earth very slightly wobbles like a spinning top over the course of 26,000 years, meaning its axial tilt only changes by a few degrees during that time, which has allowed our planet to maintain relatively stable climates, enabling life to both survive and thrive. This contrasts to smaller planets like Mars that wobble wildly over the course of hundreds of thousands to millions of years, resulting in large changes in its axial tilt between 15 degrees and 45 degrees, resulting in shifts of its polar caps and drastic climate variations. For context, both Earth’s and Mars’ axial tilts are currently around 25 degrees. But given all the reasons listed by Dr. Kipping, what are some of the benefits and challenges of studying exomoons?

“Some benefits are that finding a moon would automatically tell us more about its host planet,” Cassese tells Universe Today. “For example, we would be able to tell right away that the planet hasn’t gone through any dramatic orbit changes due to scattering with other planets, since that would likely have stripped the moon away. We can also help use the moon’s orbit to measure the mass of the planet, and even of the star, though there are other ways to measure both of those as well.”

“Moons are very difficult to detect and really push the data we receive to their limits,” Yahalomi tells Universe Today. “Therein lies both a challenge and an opportunity. In pursuit of detecting the smallest signals in these datasets, we need to develop new methods and techniques of extremely precise data analysis. I’m working on creating a new analytic framework for studying the gravitational effect that moons have on their host planets. We are working on methods to differentiate between the wobbles caused by moons and neighboring planets in the same stellar system. Without the goal of detecting moons, we would likely not be motivated to develop these statistical techniques, which can then (hopefully) have larger reaching applications.”

As of this writing, NASA has confirmed the existence of 5,678 exoplanets ranging from terrestrial (rocky) worlds to gas giants much larger than Jupiter. in contrast, there have been zero exomoons confirmed to exist anywhere in the cosmos, quite possibly due to the difficulty to detect them, as noted by Yahalomi. Of the 5,678 confirmed exoplanets, 4,193 have been confirmed using the transit method which detects extremely small dips (approximately 1 percent) in starlight when the exoplanet passes in front of, or transits, its parent star.

These dips in starlight are so small that astronomers require several transits to confirm its existence. Therefore, trying to detect exomoons, which could be much smaller than the exoplanet they orbit, is even more difficult. While there are currently no confirmed exomoons, what are some interesting exomoon candidates, including exomoon candidates that these researchers have studied?

“The two candidates we have announced are Kepler-1625 b-i and Kepler-1708 b-i,” Dr. Kipping tells Universe Today. “They both orbit gas giants at relatively wide separations from their star, and both are surprisingly large, 1625b-i is about a Neptune and 1708b-i is a mini-Neptune. In other ways they are quite different, 1708b-i orbits in a tight Europa-like orbit, seemingly coplanar with the planetary orbit. In contrast, 1625b-i appears inclined and at a much wider orbit, looking more like a captured moon. For 1625b-i, we have a mass thanks to transit timing variations of the primary planet and that lands in agreement with our radius measurement obtained from the dip of the moon in front of the star. For 1708b-i, we only have the dip (just two transits), however the false positive rate is well measured here to be ~1%, giving us dome confidence in the signal.”

As noted, of the more than 200 moons in our solar system, only a handful are currently targets for astrobiology and the search for life beyond Earth. These include two of Jupiter’s moons, Europa and Ganymede, and two of Saturn’s moons, Enceladus and Titan. All four have presented evidence for possessing interior oceans of liquid water, with Titan being the only one with liquid bodies on its surface, although comprised of liquid methane and ethane as opposed to liquid water.

Europa has been previously explored by NASA’s Galileo spacecraft while obtaining incredible images of the moon’s small surface. However, the agency’s Europa Clipper spacecraft, which launches this October, will conduct the most in-depth investigation into Europa’s habitability potential when it arrives in 2030. It will conduct 50 flybys of the small moon, sending back the high-resolutions images of its surface while using its suite of powerful instruments to determine if its interior liquid water ocean can harbor life, as we know it as we don’t know it.

Ganymede has also been studied by NASA’s Galileo spacecraft but the European Space Agency’s JUICE (Jupiter Icy Moons Explorer) spacecraft, which is currently en route to Jupiter with a planned arrival of 2031, hopes to also conduct the most in-depth investigation pertaining to Ganymede’s habitability potential, as well. For Saturn’s moon, Enceladus and Titan, both have been mapped and studied in-depth by NASA’s Cassini spacecraft over the course of its 13-year mission studying Saturn and its many moons.

During this time, Cassini both observed and flew through geysers emanating from Enceladus’ south polar region, indicating a liquid water ocean beneath its icy crust, along with landing a probe on Titan’s surface, revealing rounded boulders possibly formed from flowing liquid methane or ethane. Additionally, evidence has suggested that Titan possesses an interior liquid ocean comprised of water, as opposed to methane and ethane on its surface. Given the habitability potential for these moons, what can exomoons teach us about finding life beyond Earth?

“There are at least two ways moons can affect life elsewhere in the galaxy,” Cassese tells Universe Today. “First, moons can influence and stabilize their host planets [see above]. The other is that moons themselves could be great places for life. Some of the largest liquid water reserves in our solar system exist on moons like Europa, and it’s possible that other moons have similar ingredients that we think are essential for life. If moons are anywhere as near common as planets, the potentially habitable real estate of the galaxy would be much larger than we currently appreciate.”

Yahalomi tells Universe Today, “From what we currently know about planet formation and from our solar system where there are hundreds of moons, there really should be exomoons around many of the exoplanets that we have found. Therefore, if we find that there aren’t exomoons, that would reveal that there is something unique in our solar system and something missing in our understanding of planet formation. As we only know about life on Earth, currently, understanding the larger context of planetary demographics and thus better understanding how common or unique our Solar System truly is, could aid in our understanding of the likeliness of life beyond Earth.”

Like the field of exoplanets, studying exomoons involves a myriad of scientific backgrounds and disciplines to decipher copious amounts of data, including astrophysics, computer science, planetary geology, planetary atmospheres, data science, just to name a few. Additionally, powerful instruments like the aforementioned Habitable Worlds Observatory are required to detect exomoons given their infinitesimally small sizes within the data. It is through this constant collaboration between scientists and use of key instruments that will enable scientists to someday confirm the existence of the first exomoon within the cosmos. Therefore, what advice can the researchers off to upcoming students who wish to pursue studying exomoons?

“It’s a fascinating and rapidly growing area,” Dr. Kipping tells Universe Today. “We are finally in the era where we can detect moons akin to those in the Solar System using JWST. Further, there’s rapidly growing interest in discovering very non-Solar System like moons, such as moons around free-floating planets either using JWST of young systems in Orion (google JUMBOs for example) or using the upcoming Roman telescope with microlensing techniques. We are about to breach the detection threshold in a convincing way.”

How will exomoons help us better understand our place in the universe in the coming years and decades? Only time will tell, and this is why we science!

As always, keep doing science & keep looking up!