In their latest scientific work, researchers Viktória Fröhlich and Zsolt Regály decided to analyze the ability of so-called moons to initiate and sustain life on their surface. free planets, i.e. planets ejected, e.g. in the explosion of a type II supernova, from their parent planetary system and since then drifting in interstellar space. The results of their simulations indicate that such moons are able to remain in elliptical orbits around their planets, thus gaining a new, stable energy source in the form of tidal heating.
In their work, researchers emphasize an important feature of such objects. While usually in the case of exoplanets we talk about whether such planets are conducive to the emergence of life, i.e. they have conditions enabling the existence of liquid water on their surface, in the case of the moons of free planets, the minimum physical, chemical and energy requirements for life to arise were considered here. In this approach, geophysical processes are key, including tidal heat, known from Europa and Enceladus.
As part of their project, the researchers conducted a series of simulations: from systems with circular orbits, through the increasing ellipticity of the moons’ orbits around the planets, to systems of two moons in resonance. At the same time, the parameters of the supernova explosion that would throw the planet and its moon into interstellar space were changed. In each variant, the stability of the orbits and changes in the ellipticity of the orbits after the planet was “ejected” were assessed.
The results are surprising: planets ejected from their own solar system by the explosion of their host star generally retain their moons. However, the moons’ orbits are greatly elongated. With originally circular orbits, the ellipticity of the moon’s orbit increased to about 0.33; in the case of moons originally moving on elliptical orbits – it increased to approximately 0.88; in resonance systems – up to approx. 0.27. The scale of this change depended solely on the intensity of the explosion.
This is crucial for tidal heating. When moons orbit beyond 0.01 AU and have an eccentricity above 0.1, an effective heating source is created inside them. Between 12 and 15 percent of such objects reaches a level comparable to Europa and Enceladus. Moreover, the suppression of this ellipticity lasts longer than the age of the solar system, which means billions of years of continuous energy supply from the interior of the moon.
The authors estimate that there could be trillions of such moons in the Milky Way, and the scenario is not limited to supernovae – similar systems could arise from early dynamical instabilities or star flybys. Although discovering such moons of free planets will be extremely difficult, transits and gravitational microlensing can be detected using, for example, the Nancy Grace Roman Space Telescope or the Vera C. Rubin Observatory.
It turns out, therefore, that such moons, after being thrown into interstellar space together with their parent planet, may remain friendly to life for billions of years, even if their only source of energy is tidal heating.
