A new study by researchers from the Max Planck Institute for Extraterrestrial Physics and Ludwig-Maximilians-Universität München has determined that moons orbiting free-floating planets (FFPs), meaning planets without a host star, can maintain a hydrogen-based atmosphere for billions of years.
The long-term maintenance of a hydrogen-based atmosphere could also support liquid water on the surface of such moons, an ingredient considered essential for life to exist.
Since previous studies have found there may be as many free-floating planets in the Milky Way Galaxy as there are stars, the research team behind the discovery said adding the moons of free-floating planets to the list of study targets could significantly increase the possibility of finding life beyond Earth.
Free-Floating Planets and the Search for Extraterrestrial Life
When searching for potentially habitable locations in the cosmos, scientists primarily focus on planets and moons. To date, such studies have explored rocky, Earth-like exoplanets and moons orbiting gas giants, such as Jupiter and Saturn.
Notably, these studies have mostly ignored moons orbiting gas giants that do not themselves orbit a host star. The primary reason for this exclusion is the lack of a star’s heat energy that is needed to fuel life as we know it.
More recently, studies have identified potentially habitable locations that do not require energy from a star. For example, studies suggesting the possible existence of life beneath the frozen icy surfaces of Saturn’s moon Enceladus and Juter’s moon Europa rely on the energy generated by tidal heating, a result of the gravitational tug of war between the moon and its host planet. In theory, such tidal heating should be enough to support life without the need for a star’s heat.
In the new study, the joint Max Planck Institute-LMU team explored whether a similar process could support life on moons orbiting free-floating planets. The study also examined whether the tidal heating of these moons could support a hydrogen-based atmosphere, potentially allowing liquid water on their surfaces.
Tidal Heating Generates a Hydrogen-Rich Atmosphere Lasting Billions of Years
According to a statement detailing the team’s research, free-floating planets form around a star, just as orbiting planets do. However, given the right, unstable conditions, some planets born around a star can be flung into deep space by the complex interaction of gravity and momentum. The resulting free-floating planets are doomed to wander the cosmos in virtual darkness.
It was also believed that these orbital ejections would separate any orbiting moons from their gas giant hosts. As a result, scientists searching for extraterrestrial life have not made free-floating gas giants with orbiting moons a priority.
More recently, LMU scientists proposed that free-floating planets may not lose all their moons when they are ejected from their host star’s orbit. Instead, the process would likely shove the moons into more eccentric orbits. According to the study authors, the tidal forces resulting from this extreme orbit “rhythmically deform the lunar body, compress its interior, and generate heat through friction.”
Although earlier studies had demonstrated that a carbon dioxide atmosphere could support a life-friendly environment for over a billion years, the extremely low temperature of a free-floating system would cause carbon dioxide to condense, thereby removing its protective effect. Still, the study authors wondered if some of the tidally heated moons in these systems could generate a rich hydrogen atmosphere capable of supporting liquid surface water even without a host star.
That’s because, although molecular hydrogen is largely transparent to infrared radiation, when it is put under extreme pressures, a process known as “collision-induced absorption” occurs. According to the study authors, this process occurs when hydrogen molecules collide, forming transient complexes that can absorb thermal radiation and retain it in the atmosphere.
The Cradle of Life Does Not Necessarily Require a Sun
After simulating several different FFP moon scenarios, the team found that this process was possible. Furthermore, their simulations supported the formation of a hydrogen-rich atmosphere dense enough to support liquid water on the surface of an FFP moon for over 4.3 billion years. Because this length of time is almost identical to Earth’s entire existence, the authors suggest this should be more than long enough for life to form.
“We discovered a clear connection between these distant moons and the early Earth, where high concentrations of hydrogen through asteroid impacts could have created the conditions for life,” explained David Dahlbüdding, doctoral researcher at LMU and lead author of the study. “Our collaboration with the team of Prof. Braun helped us recognize that the cradle of life does not necessarily require a sun.”
When discussing the potential implications of their study’s findings, the research team said it expands the list of possible locations for extraterrestrial life to arise and endure to include “even the darkest regions of the galaxy.”
“These environments are interesting to model because they push planetary modelling into unusual regimes, but they also serve to understand the environments in which potential life precursors emerged on Earth,” explained MPE Scientist Tommaso Grassi.
The study “Habitability of Tidally Heated H2-Dominated Exomoons around Free-Floating Planets” was published in Monthly Notices of the Royal Astronomical Society.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.