Warm Moons of Rogue Planets express
In the Galaxy, there are plenty of rogue planets, thrown out by supernova explosions. These free-floating planets lack a sun, but often retain their moons. Simulations show that after the cataclysm, the moons' orbits become barely elongated, and that's enough to kickstart internal heating. In 12–15% of cases, the heating power rivals what keeps the oceans of Europa and Enceladus warm. Amazingly, this process fades incredibly slowly: for moons farther from the planet than ten times its radius, the heating will last longer than the Solar System has existed. So water oceans under the icy crust can remain liquid for billions of years. The next extraterrestrial life might be discovered not around a star, but in the eternal darkness of interstellar space.
Original abstract
as written by the authors arXiv:2511.03392
Free-floating planets are thought to be numerous in the Galaxy and may retain their moons after ejection from their natal systems. If those satellites acquire or preserve orbital eccentricity, tidal dissipation could provide a long-lasting internal heat source, potentially creating urable environments (capable of enabling abiogenesis) in the absence of stellar radiation. We explore (i) whether moons remain bound to planets expelled by a core-collapse supernova, (ii) how the explosion reshapes their orbits, and (iii) under which circumstances tidal heating can sustain urable subsurface oceans. We carried out three-dimensional N-body simulations with an 8th-order Runge-Kutta scheme, modelling homologous stellar mass loss for progenitors of 10 M$$_{\odot}$$. Post-explosion orbital elements of single moons and resonant moon systems were analysed, and tidal heating power was estimated with a constant phase-lag model for several tidal dissipation functions and moon densities. All simulated moons survive the supernova and remain bound to their planets. The explosion excites moon eccentricities up to $$\sim7\times10^{-4}$$ and $$\sim3\times10^{-3}$$ for single moons of planets with circular and eccentric orbits, respectively. For resonant pairs, an eccentricity of $$\leq2\times10^{-2}$$ is preserved. The semi-major axis of the moons changes by $$\leq0.2\%$$. For 12-15\% of cases -- preferentially moons at $$a\leq15\,R_{\mathrm{planet}}$$ and with $$e\geq10^{-3}$$ -- the specific tidal heating power lies between 0.1 and 10 times what is estimated on Europa or Enceladus, sufficient to maintain liquid oceans beneath an ice crust. Eccentricity damping timescales exceed the age of the Solar System for $$a\geq10\,R_{\mathrm{planet}}$$, implying billions of years of continuous heating on the moons. Such worlds represent promising targets for future searches for extraterrestrial life.
There may be more rogue planets in our Galaxy than stars — so warm moons with oceans in the Universe could be unimaginably numerous, like grains of sand on all the beaches of Earth.