At a young age, we’re told how the Sun warms the Earth and makes life possible. That idea sticks with most of us for life. But when we want to understand things more thoroughly and we dig more deeply, we learn that Earth its own heat sources that help it maintian habitability: remnant heat and radioactive decay. Other rocky worlds can have these sources, too.

A small percentage of us keep going down this rabbit hole in pursuit of more detailed knowledge about planetary habitability. We discover that there are other heat sources that could potentially power life, like tidal heating. Tidal heating happens on moons, where the powerful gravity from a much more massive planet stretches and compresses the moon as it orbits. That movement creates heat that could allow liquid water to persist and aid habitability That’s what scientists think is happening to some of our Solar System moons like Europa, where the heat likely maintains a warm liquid ocean under a thick ice cap.

New research digs into the questions of moons, tidal heating, and habitability, but with a twist. It examines rogue planets and their moons. It’s titled “Life in the dark: Potential urability of moons of rogue planets.” The authors are Viktória Fröhlich and Zsolt Regály, both associated with the Konkoly Thege Miklós Astronomical Institute of the HUN-REN Research Centre for Astronomy and Earth Sciences in Hungary. Their paper will be published in the journal Astronomy and Astrophysics.

‘Urability’ is a fairly new word that refers to the conditions that allow life to get started on a world. It’s different than the word habitability, which basically describes a world with liquid water that can sustain life. Urability takes other things into account, like the geophysical and geochemical factors necessary for life to arise. “While habitability concerns conditions that allow surface liquid water and the persistence of life, urability addresses the minimal physical, chemical, and energetic requirements for life to originate,” the authors explain. In this research, the authors explore the urability of rogue moons orbiting rogue planets.

“The number of discovered rogue planets (i.e., planets without a host star, also called nomadic, unbound, orphan, wandering, starless, sunless or free-floating planets) has now reached several hundred,” the authors write. “What is more, theories suggest that there could be as many as two Jupiter-sized rogue planets for every star in the Galaxy.”

This artist’s illustration shows a star exploding as a supernovae and a planet caught up in it. While planets too close to an exploding star would be destroyed, some at greater distances could be expelled intact, and their moons with them. Image Credit: David A. Aguilar (CfA)

This means that there is a vast population of not only rogue planets, but also, at least potentially, their moons. So our search for a deeper understanding of habitability and urability is incomplete without considering these worlds.

There are different mechanisms that can eject a planet from its solar system and leave it wandering through the darkness of space. One of them is supernova explosions, and that’s what this work focuses on. The researchers examined the potential for planets ejected by supernova explosions to hold onto their moons. “In this study, we investigated the orbital dynamics of an exomoon orbiting a planet whose host star explodes as an SN II,” they explain.

The pair of researchers employed simulations to test their ideas. Their set of fiducial models began with planets and moons on circular orbits, providing a fixed base for comparison. In subsequent simulations they increased the eccentricity of the planets and the moons. Finally, they simulated two moons orbiting a planet in mean-motion resonance. In all of these scenarios they also varied some of the supernova explosion’s parameters.

“In general, the SN II explosion causes the planets’ orbits to become unstable, resulting in them continuing their journey through the galaxy as rogue planets,” the authors write. Surprisingly, the powerful explosions didn’t tear the moon away, and they remained gravitationally bound to the planets. “Based on our results, planets ejected during SN II explosions retain their moons in all cases,” the authors explain, noting that this result aligns with previous research.

However, the moons’ orbits around the planets was altered and the explosions changed their eccentricities. “The magnitude of the moon’s eccentricity excitation depends solely on the velocity kick received by the planet,” the researchers write. This is the critical part of the study, since an elliptical orbit creates the tidal heating that can underlie urability.

For a planet with an orbit that was initially circular, moons reached an eccentricity of about 0.33. For comparison, Earth’s Moon’s eccentricity is approximately 0.0549, which is nearly circular.

For planets with pre-existing eccentric orbits, their moons reached an eccentriticy of about 0.88. The results also show that for pairs of moons in resonance, their eccentricity can reach about 0.27.

But the important part about eccentricity is how it powers tidal heating on these rogue moons. In situations where the moons orbit their planets at greater than 0.01 AU, and with eccentrities greater than 0.1, there can be effective tidal heating. Between 12% and 15% of rogue moons in these situation experience tidal heating that’s comparable to the heating on the ocean moons Enceladus and Europa. In this case, comparable means between 0.1-10× their approximate values.

*Data shows that Jupiter’s moon Europa has a sizable ocean under its frozen surface. Tidal heating prevents the moon from freezing. The same thing could be happenong on rogue moons. NASA’s Europa Clipper is on its way to find out for sure. Image Credit: Artist’s Illustration: NASA/JPL-Caltech*

The simulations also show that moons in these situations maintain their eccentricity on geological timescales. “Eccentricity damping timescales exceed the age of the Solar System, implying billions of years of continuous heating on the moons. Such worlds represent promising targets for future searches for extraterrestrial life,” the researchers explain.

There could be trillions of rogue moons in the Milky Way according to some theoretical work. While this work focused on ejection by supernova explosion, it applies to other scenarios as well. “Importantly, this reasoning extends beyond supernova scenarios: the potential urability of exomoons applies equally to eccentric rogue planet–moon systems produced by early dynamical instabilities or stellar flybys,” the authors write.

An artist’s illustration of a rogue or free-floating planet drifting through the Milky Way. Image Credit: NASA, JPL-Caltech and R. Hurt (Caltech-IPAC)

Detecting rogue moons is out of reach, but probably not for long. We could be on the precipice of discovering and confirming rogue moons as our technology advances. The authors point out that the moons Titan and Ganymede block out about 2% of the light from their planets when they transit in front of them. That means their transits could be detectable. Gravitational microlensing could also discover them, but that takes a lucky alignment. The Nancy Grace Roman Telescope will help with that, as will the Vera Rubin Observatory.

If this research proves to be accurate, then we’re on the verge of understanding a hidden population of planets and moons whose discovery could force a rethink of solar systems, planets, moons, life, and the entire galaxy. If habitability, or urability, don’t depend on a star, we need to shift our thinking to match Nature.

“Our findings suggest that such eccentric moons, once expelled into interstellar space with their host planet, may remain urable for billions of years, sustained by tidal heating alone,” the authors conclude.