why aliens might just breathe hydrogen
Hydrogen and helium are the most common elements in the universe. And they might just make large planets warm enough for life.
Our universe is a vast place, and while it’s made of all sorts of elements that compose a nearly incomprehensible amount of stuff created by stars, supernovae, or neutron stars smashing into each other, over 99% of said stuff is hydrogen and helium. When we think of hydrogen and helium, we think of vast clouds of gas and dust that fill out the space between stars in galaxies, or get smeared deep into intergalactic space by gravity during collisions and flybys. Now, researchers say we should also try to think about large, rocky worlds that harbor life specifically thanks to thick atmospheres full of these elements because they’re excellent at trapping heat.
Here’s why. Life is, basically, an exercise in probability. What is the probability that you have a solid surface to create a stable environment? What are the odds that you have chemistry with viable solvents, amino acids, and a sugar to create enough hereditary molecules and mechanisms for safeguarding and propagating them? And do you have an atmosphere warm enough for this biochemistry to be sustainable over long term to enable mutations, variations, and ultimately, evolution? Earth meets all the criteria we just outlined mostly because we’re close enough to our home star to have liquid water and stable temperatures to properly mix very abundant organic ingredients.
Now, there’s a bit of a catch there. Without something around 200 parts per million of carbon dioxide in our atmosphere, our world would be a giant slush ball barely above freezing as its global temperature average. This is why at just over 410 ppm of carbon dioxide and rising, our planet is getting much warmer than we’re used to, much faster than it should to avoid very serious climactic consequences. This is also why Venus — which, on paper, should be warmer, wetter, and more habitable than Earth — became a hellish furnace that can melt lead on its surface with 30,000 ppm of carbon dioxide in its air. In short, we need a good balance of distance and chemistry for life.
life’s cosmic game of goldilocks
And this prompts the question. How far from your parent star could you go and with what kind of atmosphere and still support conditions that can foster any kind of life? It turns out that if you have a large, rocky world that can hold on to an atmosphere with a mass between 6 and 600 sextillion tons — roughly between a a third of Asia and the entire crust of the Earth, you could be looking at up to 8 billion years of cozy temps at distances as far away as Saturn is from the Sun thanks to all that trapped heat. But, of course, this is provided that this hypothetical world has oceans that are deep, but not too deep, and isn’t so massive that it collapses into a core of a gas giant.
That’s the real gotcha with this study. These co-called Hycean worlds, warm, watery, with a thick, warm blanket of hydrogen and helium, are only speculative. There’s very real concern that once you get past three or four Earth masses, a small, rocky planet will collapse under its own weight and start drawing in clouds of cold gas, creating a typical gas giant without a solid surface. Right now, we think that the maximum mass for sustainable, Earth like planets is about twice that of ours, but we’ve yet to confirm this. We do have candidates for Hycean worlds — one of which was estimated at nine Earth masses — but there’s still a lot of data vetting to be done.
Another thing to consider is that while that hydrogen-helium thermal blanket can keep oceans a billion kilometers away from a star warm and toasty for living things because it traps infrared energy, this effect also means that these planets can’t be too close. If a Hycean world with a thick enough atmosphere was as close to its star as Earth is to the Sun, its oceans would boil, even as the additional pressure would raise the boiling point of liquid water on its surface. If this is correct, it means that most of our current planetary candidates are cousins of Venus: lush, habitable havens for life in theory but infernos in practice. We need to learn a lot more before jumping to conclusions.
what happens if you turn off the oxygen?
Hold on though, you might ask. Life on Earth uses oxygen to create ATP, or adenosine triphosphate, as mission critical fuel. Could alien life metabolize hydrogen or helium in the same way? Or are we just saying that if microbes on other worlds could exist on a Hycean world, this is the mass and atmospheric density they’ll need to stay above the freezing point of water? Thankfully, we have example of life thriving on hydrogen right here on our own planet. Bacteria in the deep soil have no access to oxygen, so they’ve been using hydrogen for respiration for billions of years without an issue. Further still, we can take common bacteria like E. coli and have it do the same thing in the lab.
If bacteria and some even more complex cells can use hydrogen for respiration, there is potential for evolution into multicellular life because while we think of hydrogen as a flammable menace, it’s benign to life and would be tempered with always inert helium on another world. In other words, hydrogen is an excellent substitute for oxygen from a biochemical standpoint. Even better, an atmosphere with a lot of hydrogen would be way less dense than an oxygen-nitrogen one, so much so, natives of Hycean planets trying to scale their tallest mountains wouldn’t need to worry about altitude sickness. The air wouldn’t be thin enough for that to matter until they’re 35,000 meters up.
As an added bonus, a fluffier hydrogen-helium atmosphere would extend further into space and give us a better chance to spot it with our telescopes, which would make a habitable world much easier to find. This is exactly why JWST was able to analyze the the atmosphere of one Hycean candidate, revealing a chemical associated with a vast blanket of marine life in its spectral signature. But, yet again, we can’t get too excited. Much more data is needed to confirm this detection, not to mention the presence of a solid surface, and proper readings of its average global temperature. Hyceans sound like paradise worlds for alien life. Whether they really are remains to be seen.
See: Mol Lous, et al. (2022) Potential long-term habitable conditions on planets with primordial H–He atmospheres. Nat Astro, DOI: 10.1038/s41550-022-01699-8
Seager, S., et al. (2020) Laboratory studies on the viability of life in H2-dominated exoplanet atmospheres. Nat Astro, DOI: 10.1038/s41550-020-1069-4