Rogue Planets’ moons might have liquid floor water and thick atmospheres. They might be liveable

The search for life on exoplanets takes a fairly conservative approach. It focuses on life similar to that of the earth. Sure, it is entirely possible that life comes in many exotic forms, and scientists have speculated on all the strange forms life could take, but the simple fact is that earth life is the only form we currently understand . So most of the research is focused on life forms that are carbon-based, like us, with a biology based on liquid water. But even with that narrow gaze, life could be hiding in places we don’t expect.

Since terrestrial life depends on liquid water, the search for life on exoplanets focuses on those within the circumstellar habitable zone (CHZ) that surrounds stars. That is, neither too close nor too far away for liquid water to exist on a rocky planet. For our solar system, this is roughly between the orbits of Venus and Mars. Most of the exoplanets that meet these criteria are super-earths, which closely orbit small red dwarf stars, as red dwarfs make up about 75% of the stars in our galaxy and super-earths are the most common terrestrial exoplanet.

One of the surprising discoveries about exoplanets is that planets the size of Jupiter often orbit near their stars. These “hot Jupiters” are unlikely to have life, but they could have moons that are as warm and humid as the earth.

Known potentially habitable worlds from 2020. Credit: PHL @ UPR Arecibo

And it turns out that large gas planets don’t even have to orbit near their star to have moons with liquid water. For example, we know that Jupiter’s moon Ganymede has an ocean of water beneath its icy surface. Europe is known to have more water than Earth, and even Saturn’s tiny moon Enceledus has liquid water. The interesting thing about these examples is that the presence of liquid water on these moons is not due to the heat of the sun, but rather to the thermal warming due to the gravitational pull of their planet. This, of course, raises an interesting question. If Jupiter and Saturn’s moons can have liquid water, then what about the moons of Jupiter-like exoplanets that don’t even orbit a star?

Illustration of a planet floating freely through the universe with a potentially habitable moon. Photo credit: Tommaso Grassi / LMU

A recent article in the International Journal of Astrobiology explores this question. The interesting thing about this study is that it’s not just about whether an exomon could remain geologically active enough to have liquid water. The answer to that is clearly yes. Instead, this work examines how potentially habitable exomons could form and whether they could hold enough liquid water long enough for primeval life to develop. For moons within a star system, for example, one of the main drivers of the moon’s chemical evolution would be the star’s light and heat. But for the moons of rogue planets, a primary impact would be cosmic rays. This, combined with tidal warming, would fuel the evolution of the lunar atmosphere over time.

To see the effects of these differences, the team modeled an Earth’s mass moon orbiting a rogue planet with the mass of Jupiter. They found that with some reasonable guesswork about its chemical composition and orbital stability, a renegade exomone could hold liquid water on its surface. Much less than on Earth, but enough to allow life to arise and develop in a reasonable time frame.

It is important not that this model is aimed at exomons with a rich atmosphere. If these habitable exomons exist, we could possibly study their atmospheres using infrared and radio astronomy. So the first planet to show evidence of extraterrestrial life could be a villain.

Reference: Patricio Javier vila et al. “Occurrence of Water on Exomons Orbiting Free-Floating Planets: A Case Study.” International Journal of Astrobiology FirstView (2021): 1-12.

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