Billions of Sunless Rogue Planets Could Actually Be Cradles for Life

Forget the Goldilocks Zone. Life could start in the darkness.

Astronomers have spent decades hunting for life where it seemed to make sense: on Earth-like planets orbiting Sun-like stars at just the right distance. But a new study from a team of European researchers suggests some life could be hiding in the dark on a completely different type of planet. Specifically, on the moons of rogue planets that don’t even orbit a star.

These wandering planets often take their moons with them on an endless voyage into the abyss. While these moons lack a sun to warm them, they have something else: a gravitational “hug” from their parent planet that is so tight it generates heat from the inside out.

Life Doesn’t Need a Star

The traditional Habitable Zone (sometimes called the Goldilocks zone) is kind of like a campfire. If you sit too close, you burn, but sit too far and you freeze. You need the right distance for liquid water, which is necessary for life as we know it.

But rogue planets are as far from that as you can imagine. While we’re thinking about the optimal distance from the campfire, rogue planets are wandering the cold dark forest by themselves.

Rogue planets are planets that were ejected from their solar systems. This typically happens early in solar systems’ evolution, when young planets that get too close to the star can fling each other out of orbits. The result is a free-floating planet that wanders through the galaxy without a parent star. But sometimes, these planets keep their moons along with them. These moons are where life could be.

When a moon orbits a massive planet, especially on an elongated orbit, the planet’s gravity pulls harder on one side of the moon than the other. This constant stretching creates friction in the moon’s core. We see it in our solar system as well; it’s why Jupiter’s moon Io is the most volcanic place in our solar system and why moons like Europa or Ganymede have liquid water despite being so far from the Sun.

However, heat alone isn’t enough.

How To Build Life

Gravity can generate enough heat to keep water liquid, even in this unlikely rogue planet scenario. But it’s not enough to ask whether water might stay liquid. The question is whether these worlds might support chemistry relevant to the origin of life.

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The researchers suggest that these dark moons might actually be better than Earth-like planets at starting the engine of life.

Biologists believe that the emergence of life requires very specific conditions. Specifically, it needs several wet-dry cycles. In other words, you need several stages of dehydration (drying) and rehydration (wetting) to concentrate molecular building blocks. These cycles, which can happen in volcanic pools that evaporate and refill this rain, can concentrate molecules and link them together into long chains like RNA.

On a sunless exomoon, the tides would be massive. As the moon moves through its orbit, the changing gravitational pull could cause the oceans to heave and recede, creating perfect wet-dry zones on any available land.

The researchers, led by David Dahlbüdding and Tommaso Grassi from the Max Plank Institute for Extraterrestrial Physics, also modelled the atmospheres of such planets. They found that these hydrogen worlds would likely be rich in ammonia, which makes water less acidic. This is exactly what you need to help RNA strands grow and stay stable.

A Numbers Game

The problem with these studies is that they show what could be possible. But you still need some conditions to be just right. In that sense, whether life really can exist on such planets becomes a game of odds. If life emerging is like winning the lottery, you need a lot of tickets to improve your odds.

This is where things really get interesting, because the number of these rogue planets is staggering.

Current estimates suggest there could be at least two ejected rogue planets for every single star in our galaxy. Some surveys suggest the number could even be as high as 21 rogue planets per star. In a galaxy of hundreds of billions of stars, we are talking about a trillion wandering worlds. So, there are a lot of lottery tickets.

If even a fraction of those worlds have Earth-sized moons, the total “habitable real estate” in the universe might be mostly in the dark.

Ultimately, this research could even help us understand how life emerged on Earth, the researchers conclude.

“These environments are interesting to model because they push planetary modelling into unusual regimes, but they also serve to understand the environments in which potential life precursors emerged on Earth,” Tommaso Grassi, MPE Scientist.

We tend to think life needs a star because Earth does. This study reminds us that a star may be only one way to pay the energy bill. Gravity can do it too. Pressure can help. Chemistry can conspire. A world can be severed from its sun and still, under the right conditions, hold onto warmth for billions of years.

Some of the most promising habitats in the universe may be places no dawn ever reaches.

The study is published in the journal Monthly Notices of the Royal Astronomical Society.