An artist's impression of a rogue exoplanet that has an exomoon but no star. Credit: Tommaso Grassi / LMU

10 Reasons Why Moons of Rogue Exoplanets are a Good Place for Alien Life

Scientists found a new potential type of host for alien life in the universe.


Scientists modeled the conditions on the surface of a satellite of a massive rogue exoplanet that left its home star system. They took into account the mass of bodies, their orbit, chemical composition, and chemical evolution over ten million years. The calculation showed that if a number of conditions are met, a significant amount of liquid water can be formed on the satellite, although much less than on Earth. In turn, even the smallest amount of water could be enough to support the most basic life forms.

During their existence, planets can change their orbit. Basically, this comes from interactions with other massive bodies, planets, or companion stars of the parent star.

For example, a similar scenario explains well the unusual chemical composition of Mars: some scientists suggest that it formed in the zone of the main asteroid belt, and only then, under the influence of Jupiter’s gravity, shifted to its present orbit.

With a stronger interaction, the orbit may cease to be closed, and the planet will fly away from the star system into outer space, becoming an orphan planet.

At first glance, it may seem that the conditions on such rogue exoplanets or their satellites will be as far as possible from the terrestrial ones since a body devoid of a star is doomed to freeze. In fact, this is not always the case, at least for the satellites of these planets.

Studies of the moons of Jupiter and Saturn have shown that the Sun is far from the only source of heat for them. The satellites of the giant planets are heated from the inside due to friction from the tidal deformation that occurs during orbital rotation.

An additional contribution to this heat is made by the decay of radioactive elements that are located in the interior of the nucleus. As a result, there is an under-ice ocean inside Europa, Enceladus, and a number of other satellites.

Everything you need to know about the possible existence of water in moons of rogue exoplanets

1. A group of scientists led by Patricio Ávila from the University of Concepcion conducted simulations and found out under what conditions the satellite of an orphan planet can have liquid water on the surface.

2. First of all, in order for the system of the planet and the satellite to be stable, the planet must have a large mass. For strong tidal heating, the satellite must have an elongated orbit and, again, revolve around a massive body.

3. Therefore, for modeling, scientists chose rogue exoplanets with a mass of Jupiter, around which a satellite of the Earth’s mass rotates in an elliptical orbit with an eccentricity from 0.001 to 0.5.

4. Further, the greenhouse effect is needed to retain heat. To calculate the required level of opacity, the researchers used the Patmo algorithm, which calculates the interaction of the atmosphere with electromagnetic radiation along a straight segment, taking into account many factors, including photochemical reactions taking place in the gas.

5. Finally, the formation of water requires a certain chemical composition of the celestial body. In all the scenarios considered, scientists assumed that the satellite’s atmosphere is 90 percent carbon dioxide and 10 percent hydrogen.

6. The main driver of chemical reactions for such an atmosphere will be cosmic rays, which will decompose CO 2. to CO + O. Then the released oxygen will bind with hydrogen and form water, but at the same time, the water will decompose into OH + H under the action of cosmic rays.

7. To calculate the chemical evolution of the atmosphere of the rogue exoplanet’s satellite and calculate the equilibrium points of various chemical reactions, the researchers used a simulator.

8. Calculations have shown that water is formed both at a comparatively low atmospheric pressure and at a much higher one, about 10 bar. Differences in the intensity of cosmic radiation are practically irrelevant in the long term.

9. In a scenario with low pressure for 10 million years, little water is produced: one hundred thousand billion tons, which is tens of thousands of times smaller than the Earth’s hydrosphere. In the high-pressure scenario, about 13 times more water is generated.

10. According to scientists, such a celestial body can be considered quite viable – worse than Earth, but better than Mars. However, the rogue planet’s satellite’s orbit will gradually round out, and over ten million years will no longer provide sufficient tidal heating to keep the surface temperature above the melting point of water. Only the presence of other moons will allow this period to be extended.

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Koberlein, B. (2021, June 11). The Moons of Rogue Planets Could Have Liquid Surface Water and Thick Atmospheres. They Could be Habitable. Universe Today.
LMU München. (2021, June 9). Exoplanets: Liquid water on exomoons of free-floating planets.
Starr, M. (n.d.). Rogue Exoplanets Lurking in Space Could Have Habitable Moons, Scientists Say. ScienceAlert.
Ávila, P. J., Grassi, T., Bovino, S., Chiavassa, A., Ercolano, B., Danielache, S. O., & Simoncini, E. (2021, June 8). Presence of water on exomoons orbiting free-floating planets: a case study: International Journal of Astrobiology. Cambridge Core.


Written by Vladislav Tchakarov

Hello, my name is Vladislav and I am glad to have you here on Curiosmos. As a history student, I have a strong passion for history and science, and the opportunity to research and write in this field on a daily basis is a dream come true.

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