In a groundbreaking study, scientists have revealed a captivating possibility: Earth-sized moons orbiting rogue planets could potentially sustain liquid oceans for an astonishingly long period, up to 4.3 billion years, without the need for starlight. This research, led by David Dahlbüdding from Ludwig Maximilian University of Munich, challenges our understanding of habitability in the universe. The findings, published in the Monthly Notices of the Royal Astronomical Society, suggest that these moons might possess the conditions necessary to support life, even in the absence of a star.
The study focuses on a Jupiter-like rogue planet and an Earth-mass moon, exploring the moon's ability to retain an atmosphere and generate tidal heat. The results indicate that a 100-bar atmosphere dominated by hydrogen is crucial for maintaining liquid water on the moon's surface. This pressure is approximately 100 times Earth's sea-level pressure, an intriguing prospect for astrobiologists.
What makes this research particularly fascinating is the concept of tidal heating. Moons in eccentric orbits, like those of Io, Europa, and Enceladus, experience gravitational flexing, converting orbital energy into heat. This process could provide the necessary energy for liquid water to exist on these moons, even without a star. The study's authors utilized orbital histories from a previous International Journal of Astrobiology study, simulating the ejection of planet-moon systems and the subsequent decline in tidal heating.
One of the key findings is the role of hydrogen in retaining heat. Unlike carbon dioxide, which can condense in the cold upper atmosphere, hydrogen molecules absorb infrared radiation poorly but create configurations that absorb outgoing heat through close collisions. This collision-induced absorption becomes more effective as hydrogen density increases, making hydrogen a more suitable atmosphere for sustaining liquid water.
The study's model followed 6,945 surviving moons, with a 100-bar hydrogen atmosphere, and calculated their orbital changes and tidal heat over time. The results showed that 43% of these moons reached temperatures suitable for surface liquid water at some point, with the longest interval lasting an astonishing 4.341 billion years. Thinner atmospheres produced shorter intervals and fewer suitable cases.
However, it's important to note that this study is just one piece of the puzzle. The model does not account for a working ocean, detailed interior, surface geology, or biology. Additionally, the assumption of constant gravity with altitude and the omission of moist convection, clouds, and hazes are limitations that need to be addressed in future research.
Furthermore, the study does not establish the existence of these moons or confirm their habitability. Detecting an Earth-sized moon orbiting a rogue Jupiter under 100 bars of hydrogen would be a significant challenge, requiring a bright host star to backlight its atmosphere. The paper expands the range of potential places for liquid water but does not suggest that starless oceans are common or currently observable.
In conclusion, this research opens up exciting possibilities for the existence of life beyond our solar system. It highlights the importance of atmospheric composition and tidal heating in sustaining liquid water on moons. However, it also underscores the need for further exploration and a more comprehensive understanding of the complex factors involved in habitability. As we continue to explore the cosmos, the search for extraterrestrial life remains a captivating and challenging endeavor.
