Liquid water is widely considered to be one of the essential components of life. But new research suggests that worlds floating in the darkness of interstellar space could still be habitable, even without the warmth of nearby stars.
A team of scientists from the Excellence Cluster Origins at Munich’s Ludwig Maximilian University (LMU) and the Max Planck Institute for Extraterrestrial Physics (MPE) has discovered that a satellite orbiting a free-floating planet could sustain an ocean of liquid water for up to 4.3 billion years. The combination of dense hydrogen atmospheres and tidal heating could allow these distant moons to remain warm enough for life to arise and evolve over potentially vast amounts of time, researchers say.
Rogue planet and wandering satellite
Planetary systems often form in chaotic environments. In the early stages of development, giant planets can come dangerously close to each other, sometimes ejecting neighboring planets from the solar system entirely. These exiled worlds are known as free-floating planets (FFP) or rogue planets because they move through the galaxy without orbiting a star.
Previous research led by LMU physicist Dr Giulia Rochetti has shown that giant planets ejected from star systems may retain parts of their moons after being hurled into deep space.
The satellite will survive, but its orbit may change dramatically. Instead of traveling in a nearly circular path, they often end up traveling in very elongated orbits around the planet.
Tidal heating could keep oceans warmer
As these moons move toward and away from the planet during each revolution, they are continually stretched and squeezed by the powerful gravitational force. This repeated bending generates internal heat due to friction. This is a process known as tidal heating.
The researchers found that this heat could be strong enough to prevent surface oceans from solidifying, even in frigid interstellar space where sunlight cannot reach.
Whether that heat remains trapped near the surface depends largely on the atmosphere.
On Earth, carbon dioxide acts as an important greenhouse gas that helps retain heat. Early research suggested that a carbon-dioxide-rich atmosphere could maintain habitable conditions for extraterrestrial moons for up to 1.6 billion years. But in the frigid environment surrounding the rogue planet, carbon dioxide would eventually condense and lose much of its warming potential.
Hydrogen atmosphere can trap heat
To solve this problem, researchers investigated hydrogen-rich atmospheres.
Hydrogen molecules normally transmit infrared light easily. However, at very high pressures, collisions between hydrogen molecules can create temporary molecular interactions that absorb and trap thermal radiation. This effect is called collision-induced absorption.
Because hydrogen is stable at very low temperatures, the researchers found that it could act as an effective insulating blanket around these moons, retaining heat for billions of years.
Clues about the origin of life
The discovery could also provide insight into how life originated on Earth.
“Thanks to our collaboration with Professor Dieter Braun’s team, we now know that the cradle of life does not necessarily require the sun,” says David Dahlbudding, postdoctoral researcher at LMU and lead author of the study. “We found a clear connection between these distant moons and the early Earth, where high concentrations of hydrogen from asteroid impacts may have created the conditions for life.”
The researchers also suggest that tidal forces may drive important chemical activity. The moon’s constant expansion and contraction can cause repeated wet-dry cycles in which water evaporates and condenses. Scientists believe that these cycles may help create complex molecules essential to life.
Habitable worlds hidden throughout the galaxy
Astronomers believe that rogue planets may be very common throughout the Milky Way galaxy. Some estimates suggest that there may be as many free-floating planets in our galaxy as there are stars.
If many of these planets have moons, the number of potentially habitable environments could be much larger than previously thought. A new study suggests that a habitable world may not need sunlight at all, and that life could arise and survive even in the darkest regions of the universe.
