Moons Orbiting Planets Without Stars as Potentially Habitable Locations for Sustaining Life

An artist’s impression of a Jupiter-like free-floating planet in dark space. Credit: NASA/JPL.

Free-floating planets are worlds ejected from developing solar systems during the first few million years of planetary system evolution. They were first discovered during a joint Japan-New Zealand survey of the Milky Way in 2006 and 2007. Since then, hundreds of free-floating planets have been discovered, with computer simulations predicting them to be more abundant than stars in the galaxy.

Dynamical simulations suggest that free-floating planets could retain their moons but with disrupted orbits. When a moon’s orbit is not perfectly circular, tidal heating occurs (gravitational interaction with the planet causes friction, resulting in internal heating). Where a thick atmosphere is present, the generated heat would maintain any surface water in its liquid state, or, in the case where no ocean was initially present, could lead to the formation of an ocean from chemical reactions of carbon dioxide and hydrogen in the atmosphere.

Giulia Roccetti from the European Southern Observatory and colleagues ran 8,000 dynamical simulations of a Sun-like star with three Jupiter-sized planets. They aimed to determine if and for how long moons around free-floating planets could retain liquid oceans and, consequently, whether these planets had the conditions and the timescales necessary for the emergence and sustainability of life. Results showed that habitable conditions could last up to 50 million years under atmospheric conditions similar to Earth, 300 million years in moons with atmospheric densities ten times Earth’s, and 1.6 billion years where densities were ten times the latter. Moons closest to their planets were more likely to be retained during ejection and produce habitable conditions. Results suggest that while a solar system is a far more likely setting for life, life could be sustained around free-floating planets. However, the question remains: even if conditions can sustain life, are all the ingredients in place to initiate it? READ MORE