No sun, no problem? How life could thrive on moons of starless 'rogue' planets

Moons orbiting starless "rogue" planets could stay warm enough to host liquid water for billions of years, a new study suggests, potentially creating long-lived habitats for life in the depths of space.

Using computer models, researchers found that temperatures on an Earth-size moon orbiting a Jupiter-like rogue planet could remain warm enough to support liquid water on its surface for up to 4.3 billion years — nearly as long as Earth has existed.

"The cradle of life does not necessarily require a sun," study lead author David Dahlbüdding, a researcher at the Ludwig Maximilian University of Munich in Germany, said in a statement.

The new study focuses on natural satellites of exoplanets, known as exomoons, specifically those orbiting free-floating rogue planets. Astronomers have yet to confirm the existence of an exomoon beyond doubt, but mounting circumstantial evidence suggests the first discovery may not be far off.

Rogue planets are byproducts of chaotic young planetary systems, where close gravitational encounters can fling worlds out of orbit around the host star and into interstellar space. Recent research suggests that these nomadic planets have a significant probability of retaining their moons even after being ejected. The violent process can, however, dramatically reshape those moons' orbits, stretching them into elongated paths around their planets.

As a moon moves closer to and farther from its planet along such elliptical paths, the planet's gravity repeatedly squeezes and flexes its interior. In our own solar system, this process powers the intense volcanic activity of Jupiter's moon Io and helps keep subsurface oceans from freezing on icy moons such as Europa and Saturn's Enceladus.

Known as tidal heating, the process generates internal heat through friction and, according to the new study, could be strong enough to keep liquid water oceans from freezing even in the cold of interstellar space.

Whether that heat can remain at the surface depends largely on the moon's atmosphere, the researchers say. Earlier studies suggested that carbon dioxide could provide enough greenhouse warming to keep such moons habitable for up to 1.6 billion years. In the extreme cold of interstellar space, however, carbon dioxide can condense, causing the atmosphere to collapse and allowing heat to escape, the new study notes.

Hydrogen, however, behaves differently under thick, high-pressure conditions, the study argues. The team's simulations show that, when hydrogen molecules collide, they can briefly absorb heat that would otherwise radiate away into space. This allows a dense hydrogen atmosphere to act like an insulating blanket, trapping warmth far more effectively.

The results, published in February in the journal Monthly Notices of the Royal Astronomical Society, show that under these conditions some exomoons could remain warm enough for liquid water — and therefore be potentially habitable for life as we know it — for up to 4.3 billion years.

The findings could "significantly broaden the spectrum of possible environments that could harbor life," the statement read, suggesting that "life could arise and endure even in the darkest regions of the galaxy."