For a space-based story I am writing, I was hypothesising about how to have organisms that can thrive the vacuum of space.

Ignoring plausibility of such a creature evolving, I realised a few problems with such an organism:

  • How would metabolic processes (e.g. respiration) work?

I was thinking that if these creatures were situated in a hydrogen-rich nebula, they could perhaps use that as the main fuel for their respiration and metabolic processes, however, is such an alternative even viable? Clearly, an exothermic reaction has to be in place, with the other reaction having to be some sort of fuel you can also find in space (another sort of space organism perhaps?) So, how would it work if it even could at all? Thanks!

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    $\begingroup$ Hi, while this is an interesting idea, it's a very broad question. You are asking us to help you design every aspect of this creature, something you could write a book about. I suggest you start with just one question. Whichever one you think is the underlying one that the others depend on. Or just the most pressing. You can ask followup questions later if you need to. $\endgroup$
    – Cyn
    Commented Jun 1, 2019 at 3:14
  • $\begingroup$ @Cyn Although I believe you have a point, I have a few concerns: 1. Follow up questions can lead to duplicate markings 2. You don't have to help with every aspect, just the ones which you can do. $\endgroup$
    – bio
    Commented Jun 1, 2019 at 3:25
  • $\begingroup$ Yeah, be reasonable: one question per question is the rule, and questions get closed for having just two. Nine is a shade excessive. If you're concerned about being flagged as a dup, then pick the one question that you feel is the core point. Once you have an answer for that, you can see if the other questions even make sense to ask in the light of that answer. For example, if your core question is "can a creature made of organic compounds live in the vacuum of space" and you get an unequivocal "no" answer... the other questions kinda disappear. $\endgroup$ Commented Jun 1, 2019 at 3:43
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    $\begingroup$ Much better now, thanks bio. I've retracted my close vote. Note that if you click on the link to the left of your name, that says when it was last edited, you can see the full revision history. $\endgroup$
    – Cyn
    Commented Jun 1, 2019 at 5:01
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    $\begingroup$ A good followup question either builds on the first question or is about an entirely different aspect in the same universe. But yes, not only do you need to be sure that you aren't asking a dup of someone else's question, you need to be sure you aren't duplicating your own. $\endgroup$
    – Cyn
    Commented Jun 1, 2019 at 5:02

1 Answer 1


Easy answer first:

Clearly, an exothermic reaction has to be in place, with the other reaction having to be some sort of fuel you can also find in space

You wonder, "another sort of space organism perhaps?" Nope, because what would those organisms eat? They have exactly the same issue so you're just rtying to kick the problem of "how would deep space organisms work" down the road a bit without actually answering any of the questions. So lets not do that.

You'll be happy to know that the universe is full of stuff that provides useful exothermic reactions and may be readily found in nebulas! Your beasties are gonna have to be fusion powered. This provides a means to produce other elements and provide a reasonable and pretty efficient means of moving around.

Here are the problems though.

Next, nebulas aren't like thick clouds of stuff. A really, really dense nebula might manage $10^4$ particles of stuff (where stuff mostly means "atoms" or "atomic nuclei") per cubic centimetre. For comparison, the air your currently breathing manages more like $10^{19}$ particles in the same volume. To hoover up much of that stuff, you're gonna have to be moving around, and to get any sensible distance any reasonable time in something the size of a nebula (which can easily be hundreds of lightyears across) you're going to need to use a pretty decent rocket, and fusion reactions can certainly provide that.

Sure, if your local nebula had enough oxygen in it, you could try and use more convention (from our point of view) respiration, but the amount of energy you liberate from buring hydrogen (~120MJ/kg) pales into comparison with the amount you liberate from fusing it (tens of terajoules per kilo, so that's hundreds of thousands of times more) and that's very, very important when your food is so spread out over such a vast volume. If you used hydrogen-burning chemical rockets to get around, you'd take millions of years to get anywhere.

So your things don't respire as such, but more move around at moderate speed, sucking up everything in their path and fusing it. That's a little bit like some kind of interstellar basking shark, but a little more like a Bussard Ramjet ( I can't find a good online reference for them, bafflingly; the Starflight Handbook is pretty good, but comes in dead tree form only, I believe). Going into detail about these rockets is really beyond the scope of this answer, but the key thing is that they use a giant magnetic field to sweep up fuel from space and (where necessary) a giant laser to help ionise distant fuel to make it feel the effects of the magnet scoop. There's a lot to be said about the problems of ramjets and their (potential lack of) usefulness, but again: outside of the scope of this answer, but as a means of collecting interstellar material they're not totally awful.

So, to summarise:

  • Fly around a nebula scooping up the local material.
  • Use a giant magnetic field to help scoop stuff up, maybe a big laser to help.
  • Run the gathered fuel through a fusion reaction to make heavier elements, provide electricity and thrust.
  • Use a fusion rocket to be able to travel through a vast nebula at a reasonable speed, scooping up a reasonable amount of stuff as you go.

Doesn't sound very biological, does it? That's because of how astonishingly hostile space is, with so little stuff to do anything with and such a large amount of high-energy radiation about. If you've got billions of years and a stellar mass to play with things aren't so bad, but for everything else being biological just seems pretty pointless and futile, and the best you can manage is to be as machine-like as possible. Sorry about that!

Really, the problem is this. If you were an organism adapted to life in space, why would you live out in the void between stars? There's nothing there! There's precious little to eat, no useful light, nothing to do, the scenery won't change much over the next few millenia, etc etc. Surely, the only reason you'd be out there was if you were travelling somewhere more more interesting, and in that situation you'd just hibernate until you got there.

Planetary systems have lots of matter concentrated into useful, dense forms and they'll probably have far more metal (in the chemical sense... don't listen to the astronomers when they try to tell you what "metals" are!) than you'll ever find in deep space, they have sunlight you can use for solar power, there's a bunch of different means of propulsion and manoevering readily available to you... what's not to like?

Look for your space monsters around stars, protoplanetary disks, asteroid belts, ring systems, cometary clouds, actual planets. Ones in deep space are lost, dead, or hibernating and travelling at high speed.

  • $\begingroup$ Hmm, I could see some organisms moving further and further outside an atmosphere attempting to escape some danger on a planet, and soon they have escaped from an atmosphere altogether. Also, perhaps at the bottom of this food chain there could be some sort of hydrogen-based photosynthesis from a space plant. Can you please delve into these possibilites too? $\endgroup$
    – bio
    Commented Jun 2, 2019 at 7:31
  • $\begingroup$ @bio the problem with that is, much like the idea of escaping earth and going to live on mars, is that the place you are fleeing to is vastly more hostile and dangerous than the place you're escaping from. Could you suggest "some danger" that is so dangerous that you must flee your planet forever, yet so benign that it gives you all the time you need to reconstruct yourself into a spacefaring species? $\endgroup$ Commented Jun 2, 2019 at 9:50
  • $\begingroup$ @bio also, photosynthesis generally requires ready access to a large amount of reactive volatiles, of the sort you might find in a planetary atmosphere. You might be able to make some sort of ultra-slow-growing vacuum lichen to grow on comets, but it isn't going to be able to repair the damage it suffers from cosmic radiation in time to save its own life. Extremophiles on earth have it so, so easy by comparison with space. $\endgroup$ Commented Jun 2, 2019 at 9:53
  • $\begingroup$ @bio but feel free to post a separate question about the plausibility of vacuum plants. $\endgroup$ Commented Jun 2, 2019 at 9:54
  • $\begingroup$ Here is such a danger - humans who are trying to hunt these organisms for whatever reasons, and who, although have developed air flight, don't have the time, money or resources to develop space travel which is efficient enough to create a profit from selling off the space organism's product. $\endgroup$
    – bio
    Commented Jun 3, 2019 at 2:06

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