Moons orbiting exoplanets not bound to stars may host habitable conditions, study suggests.

NOORDWIJK, THE NETHERLANDS — An astrophysicist at the European Southern Observatory in Garching, Germany, has reported that life could arise on the moon of a planet wandering the galaxy without a star. The gravitational pull between a planet and its moon can create the conditions for liquid water to exist, which is a crucial condition for life. Computer simulations suggest that moons orbiting rogue planets can stay warm for over a billion years, given the right orbit and atmosphere. The findings were published in the International Journal of Astrobiology in March.

“There might be many places in the universe where habitable conditions can be present,” said Giulia Roccetti. “But what we are looking for is places where these habitable conditions can be sustained for hundreds of millions, or billions, of years.”

Around 100 starless planets have been spotted so far, some possibly formed from gas and dust clouds the way stars form, and others probably ejected from their home solar systems. Researchers calculated in 2021 that these orphaned planets might have moons that need not be cold and barren places. If a moon's orbit is not a perfect circle, the gravitational pull of its planet continually deforms it, generating heat. Moons that have a thick, heat-trapping atmosphere might keep the surface warm enough for water to remain liquid. That water could come from chemical reactions with the carbon dioxide and hydrogen in the atmosphere, initiated by the impact of high-speed charged particles from space.

Roccetti and her colleagues ran 8,000 computer simulations of a sunlike star with three Jupiter-sized planets, which showed that many planets that are ejected from their solar system will sail off into space with their moons in tow. The team ran simulations of those moons, assumed to be the size of Earth, whizzing around their planets on the orbit they ended up with during the ejection.

For moons with an atmosphere the same pressure as Earth's, the period of potential habitability lasted at most about 50 million years. For moons with an atmosphere having a pressure 10 times greater than that of Earth, the period of potential habitability lasted for about 1.6 billion years, which is close to the conditions on Venus.

Moons of free-floating planets “will not be the most favorable places for life to arise,” according to astrophysicist Alex Teachey. Teachey believes stars will be far better sources of energy for life because of their incredible power output and longevity.

But Roccetti — although not an astrobiologist herself — thinks moons of orphan planets have a few  important advantages. They will have some, but not too much, water, which many astrobiologists think is a better starting point for life than, say, an ocean world. And not having a star nearby means there are no solar flares, which in many cases will destroy the atmosphere of an otherwise promising planet.

“There are many environments in our universe which are very different from what we have here on Earth,” she says, “and it is important to investigate all of them.”

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