The loneliest places in the universe might actually be some of the best places for life

A world, cold and alone, drifting through the inky blackness between star systems. Sounds pretty desolate, right? We're talking about free-floating planets, those cosmic wanderers that don't bother with orbiting a sun, just cruising solo through the void.

Astronomers reckon there could be a whole bunch of these vagabond rogue planets out there, maybe as many as 21 for every star in our Milky Way galaxy. That's a truly staggering number, a cosmic fleet sailing in eternal night. For a long time, we figured these lonely giants were just that: lonely. Definitely not the kind of place you'd pack a swimsuit. But what if they're not so lonely after all?

Now, picture a moon, an exomoon if you will, clinging to one of these rogue planets. No star for warmth, just the cold embrace of interstellar space. How could anything possibly stay warm enough for, say, liquid water, which we think is pretty important for life? Well, here's where things get interesting.

When a planet gets booted from its star system, its exomoons can get a bit … strange. Their orbits get stretched and squeezed, and all that gravitational tug-of-war generates something we call tidal heating. It's like kneading dough, but with entire celestial bodies, warming them from the inside out. So, while there's no sun, there's a built-in furnace.

But figuring out how to keep those exomoons cozy and warm was a real head-scratcher. Early models, bless their hearts, tried to cook up scenarios where thick, carbon dioxide-rich atmospheres could trap enough heat from that tidal flexing to keep water sloshing around, according to a new paper appearing in the preprint journal arXiv.

The idea was that CO2 would act like a big, insulating blanket. The problem? Carbon dioxide is a bit finicky. Under the immense pressures needed to trap enough heat, it tends to condense, turning from a gas into a liquid or even a solid, leading to what we call atmospheric collapse. Not exactly conducive to a long-term liquid water party. It was a clever idea, but it just didn't hold water. Literally.

Here's the delightful twist: It turns out hydrogen, that most abundant and unassuming element, might be the unsung hero. Instead of relying on temperamental CO2, a new breed of models shows that exomoons with thick, hydrogen-dominated atmospheres can be surprisingly good at holding onto heat.

It's all thanks to a process called collision-induced absorption, or CIA. Essentially, when hydrogen molecules get squished together in a dense atmosphere, they briefly team up to absorb infrared radiation, effectively trapping heat. This ingenious mechanism can keep surface temperatures just right for liquid water, potentially for truly mind-boggling stretches of time — we're talking up to 4.3 billion years.

So, how did astronomers cook up this new recipe for habitability? They didn't just pull it out of a hat. They used some seriously sophisticated tools, combining a radiative transfer code called HELIOS to model how heat moves through the atmosphere with an equilibrium condensation chemistry code named GGchem to figure out the precise chemical makeup of these bizarre worlds. It's a grand challenge tackled with clever computational solutions, painting a picture of these extreme exomoons where tidal heating and those thick, hydrogen-rich atmospheres conspire to create billions of years of potentially habitable surface conditions.

Now, before you go packing your bags for a hydrogen moon vacation, it’s important to remember that science is a journey, not a destination. This self-consistent atmospheric model, while brilliant, is still built on a few approximations and assumptions. For instance, the HELIOS code, while powerful, assumes a constant gravitational pull, which might get a little wonky for super-thick atmospheres on moons with low gravity.

And the models are currently only looking at "dry" atmospheres, not considering how water vapor itself might influence the temperature profile, or how condensation might affect things. Also, GGchem calculates chemistry for each atmospheric layer in isolation, without thinking about how atoms and molecules might move between those layers.

And hey, just because a world can have liquid water doesn't automatically mean it's teeming with life. We're still learning the intricate dance of habitability.

But here's the exciting bit: this is just the beginning of understanding these rogue worlds. Future research will undoubtedly dive deeper, exploring other atmospheric compositions beyond just hydrogen, and pushing the models further by adding in more complex atmospheric physics, like clouds and more nuanced ways to handle water vapor.

This new understanding of exomoons around free-floating planets throws open a massive, unexpected cosmic real estate market for life. Who knew the loneliest places in the universe might actually be some of the coziest, just waiting for us to figure out their secrets?