Lost planets and liquid worlds: exomoon habitability

Liquid water is essential for the emergence of life on Earth. The same is assumed to be true on other planets, which is why the "habitable zone" is often defined as the region around a star where planets (and any moons they may have) are neither too hot nor too cold to have liquid water. Some planets, however, do not orbit a star. Ejected from their system, these "free-floating planets" roam our galaxy in almost total darkness. In the absence of direct stellar radiation, the presence of liquid water on these planets seems impossible. However, on the moons orbiting these wandering Jupiter-like objects, the tidal force exerted by their host planet could, in principle, provide the energy needed to form liquid water. The distance from the host planet and the eccentricity of the moon's orbit determine the amount of energy released into the crust and then into the atmosphere.

According to a previous study (Avila et al. 2021, International Journal of Astrobiology, 20, 300), the amount of liquid water potentially available on the surface of the satelites of free-floating planets is only a small fraction of the liquid water found on Earth today. This makes these environments particularly interesting for studying local wet-dry cycles which, as recently demonstrated in an article in Nature Physics Rev (Ianeselli et al. 2023, 5, 185–195), are sufficiently complex chemically for molecules to accumulate and promote RNA polymerization.

To keep water in a liquid state, the atmosphere must be thick enough to serve as a blanket for escaping heat. The presence of carbon dioxide (CO2) keeps the heat in, just as the greenhouse effect does on Earth. This mechanism can also be observed on Venus, in our solar system, where the CO2-dominated atmosphere exceeds 450 degrees Celsius.

Although this mechanism seems to be conducive to the formation of liquid water, astronomers know that the orbits of these moons become less eccentric as they evolve, reducing the efficiency of the heat produced by the tidal force. Giulia Roccetti's paper "Presence of liquid water during the evolution of exomoons orbiting ejected free-floating planets", currently awaiting publication in the International Journal of Astrobiology, presents a model that can be used to forecast the evolution of these moon orbits, which has never been observed before. The authors were able to digitally simulate the ejection of a planet from its star system and observe the effect on the orbits of its surviving satellites. They were then able to make the orbits evolve around the free-floating planet for billions of years.

Giulia Roccetti and her colleagues found that only a fraction of the moons could retain heat for a few billion years, which corresponds to the time needed for life to appear on Earth. Another important conclusion from this work is that the moons appear to have a relatively dense atmosphere, at least as thick as that of the Earth. Higher surface pressure increases the chance of having liquid water over a long period of time, as is the case on Venus-like moons where surface pressure is a hundred times greater than the Earth's.

This study significantly contributes to increase the likelihood that these exotic objects could exist and, consequently, be observed. However, several questions remain to be addressed: notably what is the role played by other heating processes, such as the moon's geological activity, how do moons interact with each other in the system, and could there be potential mechanisms that lead to the formation of life.



 

Références

" Presence of liquid water during the evolution of exomoons orbiting ejected free-floating planets " accepted for publication on International Journal of Astrobiology, Roccetti et al. 2023, International Journal of Astrobiology.

Also Read

« Life on distant moons », communiqué de presse de référence (Origins Cluster Munich, Allemagne).

Contacts

Aurélien Crida, professeur Université Côte d'Azur, et Andrea Chiavassa, chercheur CNRS, laboratoire LAGRANGE, Université Côte d’Azur / Observatoire de la Côte d’Azur / CNRS.

Giulia Roccetti, PhD à l’ESO-Garching