A free-floating planet (FFP) is a planetary-mass object that orbits around a non-stellar massive object (e.g. a brown dwarf) or around the Galactic Centre. The presence of exomoons orbiting FFPs has been theoretically predicted by several models. Under specific conditions, these moons are able to retain an atmosphere capable of ensuring the long-term thermal stability of liquid water on their surface. We model this environment with a one-dimensional radiative-convective code coupled to a gas-phase chemical network including cosmic rays and ion-neutral reactions. We find that, under specific conditions and assuming stable orbital parameters over time, liquid water can be formed on the surface of the exomoon. The final amount of water for an Earth-mass exomoon is smaller than the amount of water in Earth oceans, but enough to host the potential development of primordial life. The chemical equilibrium time-scale is controlled by cosmic rays, the main ionization driver in our model of the exomoon atmosphere.

中文翻译：

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围绕自由漂浮行星运行的系外卫星上存在水：一个案例研究

自由漂浮行星 (FFP) 是一种行星质量天体，它围绕非恒星大质量天体（例如褐矮星）或银河系中心运行。几个模型在理论上已经预测了围绕 FFP 运行的系外卫星的存在。在特定条件下，这些卫星能够保留能够确保其表面液态水长期热稳定性的大气层。我们使用一维辐射对流代码对这种环境进行建模，该代码耦合到包括宇宙射线和离子中性反应的气相化学网络。我们发现，在特定条件下，假设轨道参数随时间推移保持稳定，外卫星表面可以形成液态水。地球质量系外卫星的最终水量小于地球海洋中的水量，但足以承载原始生命的潜在发展。化学平衡时间尺度由宇宙射线控制，宇宙射线是我们的系外大气层模型中的主要电离驱动因素。