Episode for August 10th, 2023 Download

There might be many places in the universe where habitable conditions can exist. But just having those conditions isn’t enough, in order for life to thrive, it also needs stability. It took life on Earth about a billion years to appear, but to last, it also needed a cradle that was safe and stable.

Habitability and stability don’t necessarily need to come from a nearby star. It could be sufficient to have an atmosphere, some water and a source of heat.

In our galaxy, there are many planets that wander alone in the dark, without a star to orbit. These rogue planets could have formed from the same material that makes stars, or they could have been kicked out of their original star systems by gravitational interactions. Some of these rogue planets could have moons which remain in tow, and these moons could be more than just cold and barren rocks.

If a moon has a thick atmosphere, such as carbon dioxide, and some water, it could create a greenhouse effect that traps heat and keeps the surface warm. The heat could also come from the tidal forces of the planet, which stretch and squeeze the moon as it orbits around it.

This same process is going on in Saturn’s moon Enceladus and Jupiter’s moon Europa. A sufficiently 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 changed particles from space.

But how long can such a moon sustain life? The tidal forces that heat up the moon also make its orbit more circular over time, which reduces the friction and the heat. Eventually, the moon will cool down and freeze over. The question is: will life have enough time to emerge and evolve before that happens?

Based on our own planet’s history, life took about a billion years to appear on Earth. Can a moon of a rogue planet last that long? Astronomers are starting to believe the answer depends on how thick the atmosphere is. A thicker atmosphere means more heat retention and more stability. A thin atmosphere means more heat loss and more variability. According to computer simulations, a moon with a thick atmosphere could stay warm for anywhere from 300 million to almost 2 billion years.

That’s enough time for life to have a chance to arise and hopefully thrive on a moon of a rogue planet. However, before we get too far ahead of ourselves, let’s remember that warmth and water might not be enough to let living organisms appear. There are many factors that influence the origin and evolution of life, and we don’t fully understand them yet. But it’s probably safe to say that moons of free-floating planets will not be the most favorable places for life to arise. Stars, because they are so powerful and long lived, are still the best nurseries for any nascent life on an exoplanet.

Nevertheless, there are many environments in our universe which are very different from what we have here on Earth. It is important that we investigate them all.