NOORDWEEK, THE NETHERLANDS — Life can arise in the darkest places: a satellite of a planet wanders through a galaxy without a star.
The gravitational tug of war between a moon and its planet can keep certain moons so toasty that liquid water can exist there, a condition considered critical for life. Now, computer simulations show that, given the right orbit and atmosphere, some satellites orbiting rogue planets can remain warm for over a billion years , astrophysicist Julia Roccetti announced on March 23 at the PLANET-ESLAB 2023 symposium. She and her colleagues also report their findings March 20 in International Journal of Astrobiology .
“There could be many places in the universe where conditions could be suitable for life,” says Roccetti of the European Southern Observatory in Garching, Germany. But life, apparently, also needs long-term stability. “We’re looking for places where these habitable conditions can be maintained for hundreds of millions or billions of years.”
Viability and stability do not necessarily have to come from the nearest sun. Astronomers have spotted about 100 starless planets, some likely formed from clouds of gas and dust as stars form, others likely ejected from their solar systems. Computer simulations show that there may be as many of these free-floating planets as there are stars in the galaxy.
Such orphan planets can also have moons — and in 2021, researchers estimate that these moons do not necessarily have to be cold and barren places .
If the Moon’s orbit is not a perfect circle, the gravitational pull of its planet constantly deforms it. As a result of friction, heat is released inside the Moon. In our solar system, this process occurs on moons such as Saturn’s Enceladus and Jupiter’s Europa. A dense enough heat-trapping atmosphere, probably dominated by carbon dioxide, could keep the surface warm enough to keep water liquid. This water can result from chemical reactions with carbon dioxide and hydrogen in the atmosphere, initiated by the impact of high-speed charged particles from space.
But such a moon will not be warm forever. The same gravitational forces that heat it also shape its orbit into a circle. Gradually, the ebb and flow of gravity experienced by the Moon deforms it less and less, and the reserve of heat from friction decreases.
In the new study, Roccetti and her colleagues ran 8,000 computer simulations of a Sun-like star with three Jupiter-sized planets. These simulations showed that planets ejected from their solar system often float off into space with their moons in tow.
The team then ran a simulation of these Earth-sized moons orbiting their planets along the orbit they were in when they were ejected. The goal was to see if gravitational heating was occurring and if it lasted long enough for life to start there. Earth may have become habitable within a few hundred million years, although the earliest evidence of life here dates back to about 1 billion years after the planet formed.
Because the atmosphere is critical to heat retention, the team ran their calculations on three alternatives. The team found that for moons with atmospheres of the same pressure as Earth’s, the period of potential habitability lasted at most about 50 million years. But it can take nearly 300 million years at 10 times Earth’s atmospheric pressure, and about 1.6 billion years at another 10 times that pressure. This pressure may seem extreme, but it is close to the conditions of Venus of the same size.
However, heat and water may not be enough for the emergence of living organisms. Moons of free-floating planets “would not be the most favorable places for life to arise,” says astrophysicist Alex Tichy of the Academia Sinica Institute for Astronomy and Astrophysics in Taipei, Taiwan.
“I think stars, because of their incredible power and longevity, are going to be much better sources of energy for life,” says Tichy, who studies exoplanet moons. “The big open question … is whether life can even start in a place like Europa or Enceladus, even if the conditions are favorable to support life, because you don’t have, for example, solar radiation that can help the process of mutation for evolution.”
But Roccetti—though not an astrobiologist herself—believes that the moons of orphan planets have several important advantages. They will have some, but not too much, water, which many astrobiologists believe is a better starting point for life than, say, an ocean world. And the absence of a star nearby means the absence of solar flares, which in many cases will destroy the atmosphere of a promising planet.
“There are many environments in our universe that are very different from what we have here on Earth,” she says, “and it’s important to explore them all.”