There is a kind of planet that does not belong to any star. It was thrown out of the solar system early in the chaos of planet formation, drifting into the cold interior of the galaxy with no sun to orbit and no light to receive. For a long time, people’s assumption has been simple: without stars, there is no warmth; without stars, there is no warmth. Without warmth, there is no water; without water, there is no life.
A new study on the origins of the remarkable cluster at Ludwig Maximilian University in Munich and the Max Planck Institute for Extraterrestrial Physics is quietly overturning this hypothesis. The researchers found that moons orbiting these starless rovers, known as free-floating planets, could maintain liquid water oceans for up to 4.3 billion years. This number is no coincidence. This is about the age of complex life on Earth.
Lead author David Dahlbüdding, a doctoral researcher at the University of Munich, did not set out to rewrite the geography of habitability. The research began as an engineering question: What kind of atmosphere could actually retain heat on the moon, which is completely devoid of sunlight?
Left: Evolution of the pressure-temperature distribution T(P) and the radiation-convection boundary (RCB) in the last step. Right: Final VMR curves for all molecules with maximum VMR>10−12. (Source: Monthly Notices of the Royal Astronomical Society)
Two sources of warmth, no need for a star
When a planet is expelled from its home system, the departure reshapes everything around it, including the orbits of any moons that survive the chaos. These orbits extend into long ovals, pulling the moon closer to its planet and then away from the Earth, and so on. This movement is important because gravity is not passive. Each pass deforms the moon’s interior, compressing its rocks and ice and creating friction. This friction is heat.
This process, tidal heating, is not theoretical. Jupiter’s innermost moon, Io, is therefore the most volcanically active body in the solar system. Europa, which also orbits Jupiter, may have a subsurface ocean that remains liquid through the same mechanism. What’s different about the new study is the context: The satellites being modeled don’t have stars contributing any energy at all. Tidal heating takes the entire burden.
Heat alone is not enough. There has to be an atmosphere to trap it.
Early models of potentially habitable exomoons focused on carbon dioxide as an insulating layer. Carbon dioxide is a good greenhouse gas and has been shown to maintain surface temperatures conducive to life for up to 1.6 billion years. The question is what would happen at the temperatures surrounding a free-floating planet. Carbon dioxide freezes. It condenses from the atmosphere, insulation breaks down, and any heat generated by the tides escapes into space.
Dalboutin’s team turned to hydrogen.
Surface temperature Tsurf (𝑋C+O, C/O) for different Psurf (from left to right: 1, 10 and 100bar). (Source: Monthly Notices of the Royal Astronomical Society)
gas left behind
Hydrogen gas has unusual properties under pressure. By itself, it is essentially transparent to infrared radiation, which carries heat away from the surface. But when hydrogen molecules collide at high densities, they briefly connect into temporary pairs that can absorb radiation and retain it in the atmosphere. This effect is called collision-induced absorption, and it becomes a surprisingly efficient heat trap once the pressure is high enough.
Crucially, hydrogen does not condense at the temperatures found around free-floating planets. The atmosphere remains intact.
The team used coupled radiative transfer and chemistry codes to simulate a hydrogen-dominated atmosphere over a range of surface pressures and internal temperatures. At a surface pressure of 100 bar, the model created habitable conditions for up to 4.3 billion years. At 10 bars, this number reaches 699 million years. Even at a pressure of 1 bar, 20% of the simulated lunar orbit would produce a period of liquid water conditions.
“Achieve 750 kW short-term peak power rating” is a quote from another study entirely. One that belongs here comes from Dalbudin, who noted that “the cradle of life did not necessarily require a sun,” adding that the team found “a clear link between these distant moons and the early Earth, where high concentrations of hydrogen from asteroid impacts may have created the conditions for life.”
Stirring the Tide of Chemistry
The connection to the early Earth goes deeper than atmospheric chemistry. Tidal forces don’t just heat the moon’s interior evenly. They pulse, compress and release to the rhythm of the track. On a moon with shallow seas and exposed land, this rhythm might drive wet-dry cycles, periods when water evaporates and then condenses again in the same location.
For 𝑋C+O=10−2 and C/O=0.59, the surface temperature Tsurf increases with the relationship between the tidally provided Earth’s internal temperature Tint and Io-like gravity. (Source: Monthly Notices of the Royal Astronomical Society)
These cycles were thought to be one of the more plausible mechanisms for building long, complex molecules that preceded biology. As the water recedes, the molecules concentrate. When it returns, the reaction resumes. Through this process, RNA strands can grow longer, and the team’s analysis of atmospheric chemistry suggests that naturally occurring ammonia in nitrogen-containing hydrogen atmospheres can provide alkaline conditions that make polymerization and molecular replication more likely.
The atmosphere in this photo is more than just a thermal blanket. It is a chemical player in the story of life’s first steps.
How many moons, how many darkness
Free-floating planets are not rare objects. Current estimates suggest that there may be roughly as many stars in the Milky Way, perhaps hundreds of billions. An early study by physicist Giulia Roccetti of the University of Munich showed that moons can survive planets being ejected from their systems, and that the elliptical orbits produced by the process actually enhance tidal heating on timescales of millions to billions of years.
Of the 6,945 surviving lunar orbits modeled by the team, 43% reached habitable conditions at some point during their evolution under the 100-bar pressure scenario. Even at the lowest simulated pressures, one in five satellites produced at least some liquid water window.
Researchers are cautious about what their models can and cannot claim. The calculations assume dry atmospheric conditions and do not account for cloud formation, which could trap additional heat and further extend habitability timescales. Thick surface oceans may also accelerate orbital circularization, shortening the window. Whether low-mass satellites can maintain dense hydrogen atmospheres over geological time remains an open question.
Still, the researchers describe their habitable zone timescale as a lower limit, not an upper limit.
The time spent in the habitable zone for 𝑋C+O =10−2 and C/O=0.59 is updated using the semi-major axis and eccentricity distributions of Earth mass satellites and their evolution. (Source: Monthly Notices of the Royal Astronomical Society)
Different types of habitable areas
For decades, the search for life beyond Earth has revolved around the concept of a circumstellar habitable zone, the range of orbital distances around a star where liquid water could exist on a planet’s surface. This concept is useful, but it is also a limitation that excludes the vast majority of the mass and volume of galaxies.
This study joins a growing body of work relaxing this restriction. Subsurface oceans on icy moons within our solar system have shown that liquid water may exist far beyond the classical habitable zone. The new model raises the possibility that a surface ocean, driven by tides and isolated by hydrogen, could persist for geologically significant time spans and require no stellar energy at any time.
Current instruments cannot directly detect such satellites. Future space telescopes designed for transit observations or gravitational microlensing surveys could eventually identify free-floating planets and, in some cases, the moons orbiting them. The theoretical framework now allows us to know what to look for when these tools arrive.
The dark regions of the Milky Way may not be as empty as they seem.
The findings are available online in the journal Monthly Notices of the Royal Astronomical Society.
Original story “Hydrogen atmosphere could make exomoons habitable for billions of years” was published in Bright Side of News.
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