Rogue Exomoons: A New Frontier for Liquid Water and Potential Life Beyond Stars

Global - Ekhbary News Agency

Rogue Exomoons: A New Frontier for Liquid Water and Potential Life Beyond Stars

The vast cosmic ocean is teeming with celestial bodies that defy conventional understanding. Among the most enigmatic are free-floating planets, often dubbed 'rogue planets,' which wander interstellar space untethered to any star. Recent astronomical estimates suggest these solitary wanderers could number in the billions, potentially outnumbering the stars in our own Milky Way galaxy. Intriguingly, some of these rogue planets are undoubtedly accompanied by their own moons – 'rogue exomoons' – with a subset potentially reaching Earth-like dimensions. A new, compelling paper, recently accepted for publication in the prestigious *Monthly Notices of the Royal Astronomical Society* and also available in pre-print on arXiv, unveils a startling possibility: some of these distant, starless moons might possess liquid water on their surfaces, dramatically expanding the traditional parameters for habitable environments.

The notion of liquid water existing on a moon without the direct warmth of a nearby star seems, at first glance, counter-intuitive. Liquid water typically requires an environment sufficiently warm, a condition primarily facilitated by the radiant energy of a host star. So, how could a moon orbiting a rogue planet, by definition devoid of a proximate star, sustain surface liquid water? The answer, as proposed by David Dahlbüdding of the Ludwig Maximilian University of Munich and his co-authors, lies in the powerful, yet often overlooked, phenomenon of tidal heating. While the mechanics are nuanced, tidal forces are unequivocally capable of generating significant internal heat within a celestial body. This is a well-established principle within our own solar system, where tidal heating powers the subsurface oceans of icy moons like Enceladus and Europa, and even fuels the intense volcanic activity on Io, all without direct solar energy input. The gravitational tug-and-pull exerted by their respective gas giant planets is sufficient to heat the interiors of these moons, melting ice into vast oceans.

However, a critical distinction arises: within our solar system, none of these tidally heated moons maintain liquid water *on their surfaces*. Enceladus and Europa, for instance, are encased in massive ice sheets that act as protective blankets, insulating their subsurface oceans and trapping the heat generated within their cores, allowing water to remain liquid at depth. This raises a crucial question for rogue exomoons: how could they retain surface liquid water without the insulating benefit of a thick ice sheet to prevent rapid heat dissipation?

Early investigations into this challenge, notably a prior paper by Giulia Roccetti, also from the Ludwig Maximilian University of Munich, explored whether a dense carbon dioxide (CO2) atmosphere could effectively trap tidal heating energy at the surface. For CO2 to form a sufficiently protective barrier, its atmospheric pressure would need to be remarkably high. Yet, simulations revealed a critical flaw: under conditions of high pressure and relatively low heat from tidal forces, CO2 would condense into liquid or ice, leading to an atmospheric collapse and the moon's eventual freezing. This scenario effectively ruled out CO2 as a viable solution for sustained surface liquid water on these starless worlds.

The current research, however, proposes an ingenious alternative: hydrogen. Unlike CO2, hydrogen does not condense into a liquid except at absurdly low temperatures, ensuring atmospheric stability. The challenge with hydrogen, under normal circumstances, is its transparency to infrared energy, meaning any heat from tidal forces would simply radiate away into space. Yet, at very high pressures, H2 molecules undergo frequent collisions, forming momentary dipoles. This process, known as Collision-Induced Absorption (CIA), enables hydrogen to absorb infrared light. Consequently, a high-pressure hydrogen atmosphere effectively transforms into a colossal greenhouse blanket, allowing the moon's surface to remain warm enough for liquid water to exist, sustained solely by the tidal heating from its host planet. This discovery fundamentally redefines the conditions under which a world can be considered potentially habitable.

To substantiate their hypothesis, the researchers conducted an extensive series of simulations. They modeled the temperatures of moons with varying atmospheric compositions and pressures, and assessed the impact of different levels of tidal heating on the potential for liquid water. Employing both a radiative heat transfer model and an equilibrium chemistry model, they ensured their simulations accurately reflected plausible atmospheres on these rogue moons. The findings were truly remarkable. At Earth-standard atmospheric pressures of 1 bar, liquid water could persist on the surface of these exomoons for an astonishing duration of up to 95 million years. Even more impressively, at higher pressures, reaching up to 10 bars, surface liquid water could endure for billions of years. Such extended periods are more than ample for the emergence and evolution of life, fundamentally shifting our understanding of where life might arise beyond the traditional 'Goldilocks Zone' around stars.

These findings underscore the profound implications of rogue planets and their moons in the broader search for extraterrestrial life. If billions of these systems exist throughout our galaxy, and if a significant fraction possess Earth-sized moons with hydrogen-rich atmospheres, the sheer number of potentially habitable locations in the cosmos could be vastly underestimated. This research not only refines our understanding of habitability but compels us to reconsider the definition of life-sustaining environments, suggesting that the universe may be far richer in hidden oases than previously imagined. The exploration of these starless worlds represents a thrilling new frontier in astrobiology, promising to unveil secrets that could reshape our perception of life's place in the cosmos.

Ekhbary News Agency