Life might also be possible on very special moons
Liquid water is one of the most important conditions for the emergence of life as we know it on earth. Now, for the first time, the necessary properties of moons around free-flying planets have been determined in a new, interdisciplinary collaboration in order to store liquid water for a sufficiently long time and thus make life possible.
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Artist’s view of an exomond with liquid water. |
Liquid water is a crucial component for the emergence of life on earth. Although only one planet has been known to have formed on which life originated, the scientific community assumes that the occurrence of liquid water plays a central role in the chemical evolution that can lead to the development of life elsewhere. In and outside of our solar system, the habitable zone defines an annular region around the central star in which it is neither too hot nor too cold for liquid water on planets.
Moons can also be habitable – even if they belong to planets beyond the habitable zone. To do this, however, they must have a heat source other than stellar heat, such as changing tidal forces. For example, thanks to tidal heating, there is an ocean of liquid water beneath the icy crust of Saturn’s moon Enceladus.
The discovery of dozens of free-flying planets (FFPs) in our galaxy has changed understanding of the early evolution of planetary systems and theories of planet formation. These lonely wanderers were probably expelled from their planetary systems by dynamic instabilities and thus no longer have a parent star. However, if they have moons in tight orbits, they can gravitationally lock them to themselves. This works best for Jupiter-like planets with Earth-sized moons. This creates new, unexpected places where life might form.
In a previous study of liquid water on moons of starless planets, researchers from the ORIGINS cluster demonstrated that Earth-sized moons around Jupiter-like planets may actually have liquid water. The results suggested that the amount of water possible on the lunar surface is only a fraction of the total volume of all Earth’s oceans, but that’s still a hundred times the water content of Earth’s atmosphere. This amount is already enough to stimulate chemical processes that can lead to life. Local wet-dry cycles (evaporation and condensation), as recently shown by ORIGINS scientists in a study on the first steps of evolution, provide the necessary chemical complexity that might promote assembly of molecules and polymerization of RNA.
The orbit of exomonds around FFPs becomes less eccentric and more circular over time. This reduces tidal forces and consequently heating efficiency. In a unique collaboration, doctoral student Giulia Roccetti (ESO, previously a master’s student at LMU) under the guidance of ORIGINS scientists, Prof. Barbara Ercolano (astrophysics), Dr. Karan Molaverdikhani, Dr. Tommaso Grassi (astrochemistry) and Prof. Dieter Braun (biochemistry) developed a new, realistic model that can calculate the development of lunar orbits over long periods of time. These are time scales of a few billion years, which are necessary for the development of life.
“In this way, we found that exomonds 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 time,” explains Roccetti. In addition, dense atmospheres favor the preservation of liquid water. Thus, in particular, Earth-sized moons with Venus-like atmospheres that are close to their planet are candidates for habitable worlds.
The team reports on their results in a specialist article published in the journal International Journal of Astrobiology will appear.
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