Exomoons orbiting rogue planets in deep space may harbor alien life

A new analysis reveals distant exomoons are a surprising candidate in the search for extraterrestrial life.
Chris Young
An artist's impression of an exomoon orbiting a planet.
An artist's impression of an exomoon orbiting a planet.


A team led by the Max Planck Institute for Extraterrestrial Physics (MPE) believes moons orbiting rogue planets – planets that have broken free of their host star and are roaming deep space – could be a surprisingly interesting candidate in the search for alien life.

In a new study, published in the International Journal of Astrobiology, the researchers analyzed the properties required for a moon orbiting a free-floating planet (FFP) to retain large amounts of liquid water.

Investigating exomoons

No exomoons have been directly detected to date, though scientists believe they are abundant throughout the universe due to the fact the Solar System's planets host many moons.

The astronomical community hopes that upcoming observatories, such as the Nancy Grace Roman Space Telescope (RST), and the European Southern Observatory's (ESO's) Extremely Large Telescope (ELT) will be able to detect and shed new light on the very first exomoons.

At the same time, the discovery of countless rogue planets, or FFPs, in our galaxy has challenged our understanding of planetary evolution. These planets are believed to have formed in a star system before eventually being ejected due to dynamic instabilities. If any of these planets had moons in tight orbits, they likely continued to orbit their planets as they traveled away from their star.

New analysis shows exomoons could harbor life

The researchers developed a model that allowed them to calculate the evolution of lunar orbits over the course of billions of years. Ultimately, they found that exomoons with tight orbits around FFPs have a reasonable chance of supporting life.

"We found out that exomoons with small orbital radii not only have the best chance of surviving their planet’s ejection from its planetary system but also remain eccentric for the longest period of time," Giulia Rocetti, an astrophysicist with the European Southern Observatory (ESO) and the study lead, explained in a press statement.

"They can thus optimally produce tidal heat. In addition, dense atmospheres favor the preservation of liquid water," they continued. "Thus, Earth-sized moons with Venus-like atmospheres with close-in orbits around their orphan planets are good candidates for habitable worlds."

The team behind the new analysis isn't the only one who believes FFPs might harbor alien life. In fact, one team suggested in a study last year that advanced extraterrestrial civilizations may have discovered how to steer rogue planets for interstellar travel. Against that hypothesis, the idea that alien life might evolve on exomoons orbiting rogue planets doesn't feel anywhere near as farfetched.

Study abstract:

Free-floating planets (FFPs) can result from dynamical scattering processes happening in the first few million years of a planetary system's life. Several models predict the possibility, for these isolated planetary-mass objects, to retain exomoons after their ejection. The tidal heating mechanism and the presence of an atmosphere with a relatively high optical thickness may support the formation and maintenance of oceans of liquid water on the surface of these satellites. In order to study the timescales over which liquid water can be maintained, we perform dynamical simulations of the ejection process and infer the resulting statistics of the population of surviving exomoons around FFPs. The subsequent tidal evolution of the moons’ orbital parameters is a pivotal step to determine when the orbits will circularize, with a consequential decay of the tidal heating. We find that close-in (𝑎≲25�≲25 RJ) Earth-mass moons with carbon dioxide-dominated atmospheres could retain liquid water on their surfaces for long timescales, depending on the mass of the atmospheric envelope and the surface pressure assumed. Massive atmospheres are needed to trap the heat produced by tidal friction that makes these moons habitable. For Earth-like pressure conditions (p0 = 1 bar), satellites could sustain liquid water on their surfaces up to 52 Myr. For higher surface pressures (10 and 100 bar), moons could be habitable up to 276 Myr and 1.6 Gyr, respectively. Close-in satellites experience habitable conditions for long timescales, and during the ejection of the FFP remain bound with the escaping planet, being less affected by the close encounter.

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