Even without sunshine, it can be comfortable enough for the emergence of life

If planets are thrown out of their solar system, water and life can theoretically arise on the moon near that planet. And there are probably a lot of these out-of-control planets in the universe.

A planet floating in space away from any stars seems the least likely place for life to emerge. But if such a planet has a moon, with the right orbit and the right atmosphere, it could provide conditions for life for over a billion years.

Giulia Roccetti, a doctoral student at the European Southern Observatory, explained how this is possible on Thursday during a symposium on the planets at the European research center Estec in Noordwijk.

Although only a hundred of these lonely planets have been observed, astronomers suspect that there are at least as many as stars. These planets were ejected from their solar system after a gravitational pull from another planet or a passing star. And just like regular planets in a solar system, they can have moons.

On such a moon, Chilean astronomer Patricio Javier Ávila calculated in 2021 that it can get surprisingly hot. Although its orbit is not perfectly circular, but an ellipse, the action of the planet’s tides constantly deforms the rock that makes up the moon, generating friction and therefore heat. Planetary scientists see the same thing happening closer to home, with the moons of Jupiter and Saturn.

If such a moon also has a thick enough CO₂the atmosphere – therefore with a strong greenhouse effect – it can warm up enough for the water to become liquid. And water can get there, thanks to chemical reactions, initiated by cosmic ray particles hitting molecules in the atmosphere.

The heat valve also closes

This already fulfills an important condition for the origin of life. Roccetti went even further. She ran this whole scenario with computer models. She first let a young solar system with three planets – the size of Jupiter – grow until a planet emerged. She then calculated how often a moon from such a planet – the size of Earth – stayed connected to her. She then followed them further to see if the situation calculated by Ávila would occur, and above all: how long it would last. Because frictional heat on the moon is extracted from the motion of the moon around the planet. This movement becomes more and more of a circle, and this slowly closes the heat valve.

“There could be many places in the universe that seem ripe for the emergence of life,” Roccetti says. “What we’re looking for are places where these conditions can persist for hundreds of millions or even billions of years.”

The moon of an orphan planet can meet this requirement, she concluded. in his studywhich was also published this week in the International Journal of Astrobiology.

Long molecules as the beginning of life

In his calculations, it only remained warm enough for at most 52 million years if such a moon had an atmosphere as dense as Earth. With an atmosphere ten times denser, it was already 276 million years old, and with 100 times it was 1.6 billion years old. It’s good, that’s the kind of atmosphere that Venus has.

No one yet knows whether life arises more easily on a dark but warm moon than on a starlit planet. But Roccetti sees advantages in it. “There is water, but not much. All sorts of places will likely see alternating wet and dry periods. This is conducive to the formation of the long molecules with which life had to begin. And a star produces solar flares in addition to heat. They can rob a planet that is otherwise just the right distance to support life of its atmosphere. The moon of a solitary planet is not affected by this.

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