Life in the dark: Potential urability of moons of rogue planets

TL;DR

The paper investigates whether moons orbiting rogue planets can host urable environments via tidal heating after their host star undergoes a Type II supernova. Using 4,412 two‑dimensional N‑body simulations with homologous mass loss and a CPL tidal‑heating prescription, the authors quantify how explosion‑induced perturbations modify moon eccentricities and semi‑major axes, and assess the viability of long‑term subsurface oceans. The results show moons remain bound in all cases, with eccentricities typically modest but enhanced in resonant configurations or from pre‑existing orbital eccentricity; 12–15% of scenarios produce tidal heating sufficient to sustain liquid oceans, and damping timescales can exceed the Solar System age, implying billions of years of heating. The study highlights rogue‑planet moons as plausible lurkers for abiogenesis and motivates future observational searches with next‑generation telescopes and microlensing surveys.

Abstract

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.

Figures (9)

• Figure 1: The visual representation of the parameters used for modelling the SN II explosion. The progenitor system (top) can evolve into two distinct end states (bottom): the planet-moon system can stay bound to the neutron star or can leave the remnant and travel as a rogue system. The moons stay bound to the planet in both scenarios.
• Figure 2: The eccentricity of the moons as a function of their planet's pre-explosion semi-major axis, assuming two planetary masses. Colours represent pre-explosion planet-moon distances. Only models with $f_{\mathrm{m}}^{(0)}=0^\circ$ are shown as the effect of the pre-explosion true anomaly of the moons is negligible. Grey areas mark eccentricity values that arise from perturbations inherent to the hierarchical three-body problem.
• Figure 3: Top: eccentricity of moons post-explosion as a function of the pre-explosion eccentricity of the planet. Colours represent the pre-explosion eccentricity of the planet, while symbols refer to its pre-explosion true anomaly. Systems where the planets stay bound to the remnant neutron star (and hence do not become rogue planets) are indicated in gray. The mean values of the moon eccentricities for a given planetary eccentricity-true anomaly combination are indicated by vivid-coloured symbols. Dashed lines connect the mean moon eccentricities as a function of planetary eccentricity for the different pre-explosion true anomalies. Bottom: change in the eccentricity of moons post-explosion as a function of their pre-explosion eccentricity. Models with increasing eccentricity are shown in bright colours, while those with decreasing eccentricity are shown in pastel colours.
• Figure 4: Post-explosion peculiar velocities of planets and remnant neutron stars in the fiducial set of models. Colours represent the pre-explosion semi-major axis of the planets, while symbols correspond to different planetary masses.
• Figure 5: Specific power of tidal heating on the moons of rogue planets as a function of the eccentricity of the moons for different values of $\rho_{\mathrm{m}}$ and $Q_{\mathrm{m}}$. The colours in panels A1 and A2 represent the mass of the planets, and in panels B1 and B2, the distance of the moons from the planet. In the light blue bands, $l_{\mathrm{tidal}}/l_{\mathrm{tidal,Eur}}$ and $l_{\mathrm{tidal}}/l_{\mathrm{tidal,Enc}}$ fall between 0.1 and 10, so these moons can be deemed urable. The minimum eccentricities required for urability are indicated by dashed lines.