Moons Orbiting Rogue Planets Could be Habitable

When looking for signs of life beyond the Solar System, astrobiologists are confined to looking for life as we understand it. For the most part, that means looking for rocky planets that orbit within their star’s circumsolar habitable zone (HZ), the distance at which liquid water can exist on its surface. In the coming years, next-generation telescopes and instruments will allow astronomers to characterize exoplanet atmospheres like never before. When that happens, they will look for the chemical signatures we associate with life, like nitrogen, oxygen, carbon dioxide, methane, and ammonia.

However, astrobiologists have theorized that life could exist in the outer Solar System beneath the surfaces of icy moons like Europa, Callisto, Titan, and other “Ocean Worlds.” Because of this, there is no shortage of astrobiologists who think that the search for extraterrestrial life should include exomoons, including those that orbit free-floating planets (FFPs). In a recent study, researchers led by the Max Planck Institute for Extraterrestrial Physics (MPE) determined the necessary properties that allow moons orbiting FFPs to retain enough liquid water to support life.

The study was led by Giulia Roccetti, an astrophysicist with the European Southern Observatory (ESO) and the Ludwig-Maximilians University Munich (LMU), who specializes in planetary atmospheres and habitability. She was joined by researchers from the MPIEP, the Max Planck Institute for Astronomy (MPIA), the Excellence Cluster ORIGINS research network, the Université Côte d’Azur, and the Center for Nanoscience at LMUM. The paper that describes their findings recently appeared in the International Journal of Astrobiology, a publication maintained by Cambridge University Press.

The study of exomoons is a major challenge using current techniques, and no confirmed detections have been made to date. That is expected to change in the coming years as the Nancy Grace Roman Space Telescope (RST), and 30-meter ground-based observatories like the ESO’s Extremely Large Telescope (ELT) commence operations. Alongside the James Webb Space Telescope (JWST), these observatories will enable Direct Imaging studies, where light reflected from an exoplanet’s atmosphere or surfaces is used to confirm the presence of a planetary system.

These studies could also reveal exomoons, which would appear as tiny specks of light orbiting their parent body. Concurrently, the discovery of countless free-floating planets (FFPs) in our galaxy has challenged our understanding of planet formation and the early evolution of planetary systems. These “rogue planets” are thought to have formed in a system and were eventually ejected due to dynamic instabilities. Assuming these planets have moons in tight orbits, they would likely bring them along for the ride.

The Excellence Cluster ORIGINS is an interdisciplinary research network that includes LMU, the ESO, five Max Planck Institutes, and the Leibniz Computing Center (LRZ). In a previous study, the ORIGINS team demonstrated that Earth-sized moons orbiting Jupiter-sized gas giants might have liquid water. As MPE astrophysicist Tommaso Grassi (a co-author on the paper) said in an MPE press release:

“We modeled this environment and found that, under specific conditions and assuming stable orbital parameters over time, liquid water can be formed on the surface of the exomoon. While the final amount of water for an Earth-mass exomoon is smaller than the amount of water in Earth oceans, it would be enough to host the potential development of primordial life.”

According to the ORIGINS study, the processes of evaporation and condensation (aka. wet-dry cycles) are key to the early evolution of exomoons, providing the necessary chemical complexity for accumulating molecules and the polymerization of RNA. Moreover, their results indicated that the orbit of exomoons around FFPs becomes less eccentric and more circular over time. This reduces the tidal forces acting on the exomoons’ interior, thus lessening the internal heating that leads to interior oceans.

In this latest study, the ORIGINS team collaborated with the MPE-led team to develop a new, realistic model that can calculate the evolution of lunar orbits over billions of years – the sort of timescales necessary for the evolution of life. Their results indicate that exomoons tightly bound to FFP have a reasonable shot of supporting life. As Roccetti explained:

“In this way, we found out that exomoons with small orbital radii not only have the best chance of surviving their planet’s ejection from its planetary system, but also remain eccentric for the longest period of time. They can thus optimally produce tidal heat. In addition, dense atmospheres favour the preservation of liquid water. Thus, Earth-sized moons with Venus-like atmospheres with close-in orbits around their orphan planets are good candidates for habitable worlds.”

There are some exciting discoveries anticipated for the coming decade. On April 13th, the European Space Agency (ESA) will launch the JUpiter Icy Moon Explorer (JUICE) to explore Ganymede and Europa. By October 2024, NASA will send the Europa Clipper to join in these efforts, focusing on Europa and the water plumes on its surface. And in June 2027, NASA’s Dragonfly mission will launch for Saturn’s moon Titan, where it will study the moon’s surface, atmosphere, and methane lakes for potential signs of life.

These missions will have a profound impact on the search for extraterrestrial life, which is entirely focused on Mars right now (a rocky planet at the edge of our Sun’s HZ). If and when these missions find potential evidence for life in the outer Solar System, it will knock the definition of “Habitable Zone” on its ear. This will have immense implications for astrobiology, demanding that future surveys dedicate time to studying icy moons and rocky planets. As always, the search for extraterrestrial life is confined to looking for signs of life “as we know it.”

As the scope of what we know expands, so will the search, greatly increasing the odds that we will find life out there.

Further Reading: MPIEP, Cambridge University Press