Moons orbiting around free-floating planets can keep their oceans in liquid state for up to 4.3 billion years due to dense hydrogen atmospheres and tidal heating — sufficient time for complex life forms to develop, a study has found.
The study findings, published in the Monthly Notices of the Royal Astronomical Society journal, also offer new clues to the origin of life, researchers said.
“We discovered a clear connection between these distant moons and the early Earth, where high concentrations of hydrogen through asteroid impacts could have created the conditions for life,” said lead author David Dahlbudding, a doctoral researcher at Ludwig Maximilian University in Munich, Germany.
Free-floating planets wander through the galaxy without a parent star. They can be formed during the origin of planetary systems, or when nascent planets come too close, flinging each other out of their orbits.
Gas giants — planets composed mainly of hydrogen and helium, featuring thick atmospheres, deep liquid layers, and without a well-defined solid surface — ejected from their orbits in this manner do not necessarily lose all of their moons in the process, according to Giulia Roccetti, physicist at Ludwig Maximilian University.
However, the ejection alters the orbits of the moons, making them highly elliptical, due to which their distances from the host planet constantly changes, researchers said.
The resulting tidal forces rhythmically deform the lunar body, compress its interior, and generate heat through friction.
This tidal heating may be sufficient to maintain oceans of liquid water on the surface even without the energy of a star in the cold of interstellar space, the team said.
They added that the atmosphere of these moons determines whether heat is retained at the surface.
On Earth, carbon dioxide functions as an effective greenhouse gas, the team said, citing previous studies that have shown that carbon dioxide could stabilise life-friendly conditions on exo-moons for periods of up to 1.6 billion years.
Under the extremely low temperatures of free-floating systems, however, carbon dioxide would condense, causing the atmosphere to lose its protective effect and allowing heat to escape, researchers said.
They investigated hydrogen-rich atmospheres as alternative heat traps.
Under high pressures, colliding hydrogen molecules form short-lived complexes that can absorb thermal radiation and retain it in the atmosphere. At the same time, hydrogen remains stable even at very low temperatures, the researchers explained.
Free-floating planets are thought to be common, with estimates suggesting there could be as many such ‘nomadic’ planets in the Milky Way as there are stars. Moons of free-floating planets could provide stable habitats for long periods of time, the team said.
The study’s findings could thus significantly broaden the spectrum of possible environments that could harbour life, showing that life could arise and endure even in the darkest regions of the galaxy, they said.
“We find that such (hydrogen-dominant) atmospheres can effectively trap heat via collision-induced absorption of H2 (hydrogen molecule), maintaining surface temperatures suitable for liquid water for time-scales of up to 4.3 gigayear (4.3 billion years) depending on the surface pressure, while not prone to condensation-induced collapse,” the authors wrote.
“Wet-dry cycling caused by the strong tides together with the alkalinity of dissolved NH3 (ammonia) could create favourable conditions for RNA polymerisation and thus support the emergence of life,” they said.
