All life we know of to date (presumably) formed on a rocky planet (more precisely: earth). Fiction has been written about life in more exotic locations. Though as far as I know, these lifeforms were always on the surface or in the atmosphere of some large(ish) celestial body.

How can I scientifically justify life1 forming without the presence of such a body?

One thing I have in mind is nebulae, which apparently can contain all necessary ingedients for life as we know it! Answer need not necessarily adhere to this premise, any environment where these life forms can float around freely (and possibly evolve to travel interstellar space over time) will do.

1: possibly turtles

  • $\begingroup$ OP, you realize that you accepted answer of which even own author says it is not evolutionary (so scientifically) possible? Or did you withdraw the 'scientifically' condition? $\endgroup$ Commented Dec 8, 2014 at 17:06
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    $\begingroup$ @PeterMasiar evolution is not scientifically (testable, repeatable, observable, and falsifiable) possible, and OP is allowed to accept and build whatever world he wants. $\endgroup$ Commented Aug 25, 2015 at 21:22
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    $\begingroup$ OK so if you deny evolution, I guess you do not need new antibiotics - those 50 years old would work for you just as well as it worked 50 years ago. Good old Darwinian pressure will solve the problem of the evolution deniers - only the fittest will survive. :-) Also, all breeds of dogs were created at once.-- Yes, OP can accept any answer and build whatever word wants - but science is not magic. Evolution IS testable skeptics.stackexchange.com/questions/8822/… and even can explain why some people do not believe in evolution. :-) $\endgroup$ Commented Aug 25, 2015 at 22:32
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    $\begingroup$ Pedantry, but if we were testing evolution experimentally, e.g. deciding which variables to vary, and which to keep as the control, so that we could repeatedly and falsifiably prove it works the way we have observed it to work... wouldn't we technically be testing artificial selection instead? Besides, in regards to the relevant answer, maybe its just "sufficiently advanced technology"? $\endgroup$
    – Toadfish
    Commented Feb 18, 2016 at 12:37
  • $\begingroup$ Fred Hoyle's science fiction novel "The Black Cloud" (1957) dealt with an intelligent, space travelling nebula. Surprised no-one mentioned it. Physicist and pal of Hoyle Tommy Gold may have developed the idea, but Hoyle used it in fiction. Deals with most of the questions the OP raises in a broad manner scientifically. $\endgroup$
    – a4android
    Commented Jun 29, 2016 at 13:05

7 Answers 7


Okay, so what do you need to have interesting life?

It should be capable of movement. This is handy, because it makes it active. Not entirely necessary, depending on the kind of story you're making (an interstellar fungus that infects a space ship might be enough). There's a few options for that:

Solar sails - this would imply extremely light and sparse being, built around a wide area. By deforming the sail, it could control its movement. The cool thing about this is that it potentially allows the being to travel pretty much anywhere - in a nebula, a planetary ring system, whatever. It doesn't need a solid surface at all.

If there is some solid surface, for example with the planetary ring systems, there's a lot more options. It could simply jump between the rocks (probably with some kind of safety tether). In fact, jumping in free-fall environment with the help of a line would allow both speed and control.

And of course, there's the option of just living on one small body. However, that wouldn't really allow anything big, and it's easy to imagine a deadly catastrophe happening very often.

Source of energy is trickier than it might seem. Of course, there's solar power - a good bet, if you can make a collector of some sort. However, your beings will most likely have extremely low body temperatures (I'll discuss this further later), which limits the usefulness of traditional terrestrial fuels like sugars - the reaction rates might simply be too low to allow much to happen. This can be limited somewhat if it can collect enough solar thermal power to keep itself significantly warm, but that of course comes at a cost as well - more damage to be repaired. The huge problem with solar power is the square-cube law - it's hard to imagine how such an organism could evolve in outer space, if it has no way of regulating its own temperature. Perhaps the nebula it evolved in had enough rocky matter for it to find shelter from the deadly radiation (and heat)?

Now, in the nebula scenario, you probably need to traverse a lot of volume to get a meaningful amount of food, both for energy and as a construction material. This implies low metabolism (and low body temperature), because you need to be able to replace failing parts faster than they break. The ring scenario doesn't have as much problem with that, but there's another problem - it seems that the ring particles tend to be pretty homogenous, so there's little chance there would ever appear one that has enough material of all the different kinds necessary to build any autonomous working machine.

So I see those two pals:

  • An animal-plant, built around a large solar sail, harvesting construction materials like a sperm whale, while getting energy from a sun.
  • A jumper, flying from rock to rock in search of food, with a long "tail" (or maybe multiple tentacles) used to tether itself to the passing rocks (and its prey).

The two could even co-exist, the jumper preying on the sail (or vice versa, if the jumpers are really small). Of course, the sail sounds somewhat more likely to evolve in such an evironment.

The cool thing about the sail is that it could mostly be two-dimensional, so it would have a lot of control over its heating etc. If it's a cold animal, it would be able to keep itself from overheating (the unlit part would serve as a radiator, while the lit part absorbs heat). In fact, it might be a rather good thermal balance mechanism, whether you're relatively cold or warm.

The cold one has another benefit - it could get away with being almost invisible, which would help against predators (if any), and it would allow your heroes to run into them without noticing (the big facepalm moment for the science crew) - the comparatively hot hull of the spaceship might destroy it quite easily.

All around, it's rather unlikely you'd get anything like what we have on Earth. Most of the things that work even for extremophilic bacteria would probably not be present (e.g. the right construction materials, with a ready and stable food source without having to move etc.). Even the simplest modern bacteria are made out of tons of different atoms and compounds, most of which can only form in much higher temperatures than those found in most of outer space. Maybe a nebula that's already forming into a star system, but with a proto-star that didn't ignite yet?

  • $\begingroup$ Very flat organism will have huge surface for given volume - in space will emit comparatively big amount of energy which would be hard to replace. It would have hard time to develop by just random changes in evolution. Robot could be constructed and work on outer space, but cannot self-develop. Evolution is blind and has no goal. $\endgroup$ Commented Dec 8, 2014 at 0:21
  • $\begingroup$ @PeterMasiar I get the feeling that you skipped half of my answer :) And yes, it would have a hard time developing by evolution - but that must have happened for the question to get any interesting, so the answers work from that point on :) $\endgroup$
    – Luaan
    Commented Dec 8, 2014 at 15:17
  • $\begingroup$ Well if something cannot start how it can continue? That is difference between fantasy (using magic) and science fiction, which is concerned about plausibility and physics. $\endgroup$ Commented Dec 8, 2014 at 15:19
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    $\begingroup$ @PeterMasiar Well, your definition of science fiction sounds very narrow. Science is about testing and describing the world, not about knowing every single thing about it (although we'd obviously like to get there in time :) ). I've described some of the conditions in which planet-less life might conceivably exist. Why do you automatically assume that if it's impossible to evolve there (and it's not impossible by far - it would just be completely different from Earth life), it's magic? $\endgroup$
    – Luaan
    Commented Dec 8, 2014 at 15:30
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    $\begingroup$ @PeterMasiar I don't think it is worth continuing this if you're just going to ignore half of all that I write :) I said it would be hard for life to evolve in space - but it was very hard for life to evolve on Earth in the first place. Judging whether it's hard enough to be effectively impossible isn't something I'm going to try. I'm not talking about an intelligent designer, and I'm not saying it's impossible to happen by evolution. And I'm not saying that it must have been by evolution either (although that is what I've assumed, of course). Evolution is just one way that fits Earth :) $\endgroup$
    – Luaan
    Commented Dec 8, 2014 at 16:27

The problem with life is that it needs raw materials to work with. All large animals that we know of need to breathe, you need solid matter to consume, you need energy to power all of this.

In space you can get solar energy but there is a big shortage of matter to use to build your own structure and to metabolize to power your own existence and growth.

Even if life does manage to emerge on a comet or asteroid or similar then how is it going to travel to another one? Once you launch from the original there is no way to steer or guide your path.

Think about earth's atmosphere, nothing bigger than a microbe ever permanently lives in the atmosphere. That's one of the big questions with life evolving in the atmosphere of gas giants. There just isn't enough resources there to support them. The vacuum of space is going to be even worse than either of those atmospheres.

You also need to think about temperature, space is cold. Extremophile life here on earth can survive in cold temperatures, but even that would struggle in space and again it had the advantage of being able to develop in warmer conditions then gradually adapt to more extreme ones.

Your best bet that I can think of will be something like a gas giant with a ring system fairly close to a star. Close enough to have liquid water under ice in the ring. (You can't have liquid water in contact with space, it instantly freezes or evaporates). Life could then develop in that water, and grow space tolerant as it comes out through the ice to access more sunlight. The fact that items in the ring are closer together would make it more feasible for seeds to spread from one asteroid to another and as the plant life develops and grows animals could then develop that feed on it.

An entire ecosystem could develop among this ring system, and then as the animals and plants grow and evolve into more advanced forms they could even start so spread out beyond the original ring. For example plants that fire their seeds out at high velocity could spread throughout the entire ring system.

This life would still be confined to the ring though as once anything launched itself away from a rock it would have very limited manoeuvring ability (maybe a few water jets for course correction but certainly nothing powerful enough to achieve escape velocity from a gas giants gravity well). The only way it would break free is if it found an entirely different form of propulsion, either something we aren't aware of yet or maybe something like a light sail.

  • $\begingroup$ Do you need air to breathe? Microbes that live in the deepest trenches of the ocean don't need any air. $\endgroup$
    – Eric J.
    Commented Oct 10, 2014 at 16:36
  • $\begingroup$ Life can easily escape from a gas giant's gravity well. Not on it's own, but through collision with another object or a close pass by an object with substantial gravity (large moon, astroid). Meteoroid fragments have been released from the surface of Mars and made it to Earth. $\endgroup$
    – Eric J.
    Commented Oct 10, 2014 at 16:40
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    $\begingroup$ Life does not need air to breathe. For first algae, oxygen was a poison (and ozone still is even for us). In atmosphere full of oxygen (created as byproduct by algae), more energy-demanding metabolism can allow some organisms outcompete other slower ones. $\endgroup$ Commented Oct 10, 2014 at 21:15
  • $\begingroup$ @PeterMasiar True, I've edited that bit to be more specific. $\endgroup$
    – Tim B
    Commented Oct 10, 2014 at 21:19
  • $\begingroup$ Can we say that life is what understands best what life needs? Oxygen on Earth is produced by life then consumed by life. Reproducing what happened on Earth, we only need to be extremely creative at the first stages of life evolution, then we can trust in life to come with a solution to go to more complex forms. $\endgroup$ Commented Nov 12, 2014 at 10:55

Nebulae are indeed what I know as an alternative location as well. They not only contain the essential ingredients (including discharges to help along) but there's rocks passing through them all the time, which is what leads to panspermia according to some theorists.

The problem with this, as far as life is concerned, is that when we're talking about "ingredients" and "conditions", we're talking about just enough to form bacteria and very simple organisms (in this case "extremophiles" that can survive space and atmospheric re-entry or first-entry in their case). They can chew on rocks for all time, but there just isn't anything to terraform, in order to allow more complex life to arise.

You don't get giant amoebas in space nor space-goats, launching themselves between rocks. Also, this process, while likely in nebulae, would take orders of magnitude more time than it needs on a planet, because everything's so far apart, so you need to allow for a universe that has hundreds of billions of years available to it.

Since I can't think of other cases that would count as "space" I'll stop here. To sum up, you can get life in outer space, but it's really really hard to get complex life in space.


A few things on nebulae, just from the Wikipedia articles here and here: their density still a very good vacuum, less dense than the gas on the surface of the moon or in low earth orbit. They only last about 10,000 to a few million years, which kind of surprised me, since that means some could have changed over historical times.

Those two things make me think live evolving there is very unlikely - the low density means things will happen slowly, and the short life means there just isn't enough time for much chemistry to happen.

On the good side, it seems like some nebulas have a hot/ionized inner bubble surrounded by a cool outer shell. That boundary region could be interesting, since you've got an temperature gradient, mixing, and fast-moving ions as a possible source of energy. Assuming you're willing to hand-wave away the short lifetime, that might be a good place. Maybe assume the nebula was seeded by surviving microbes or at least complex molecules from somewhere?

  • $\begingroup$ I think your last sentence sums up the gist of theories about life emerging in nebulae. Much like panspermia, the idea is that you get simple life which can survive dormant until it finds a suitable location to grow - because it can prevent its own death outside nebulae, it just has to wait for the next one to keep growing. Nebulae aren't really habitable and there's a limit to how complex life can become in them, but if there's lots of them and small objects like asteroids travel from one to another, it just takes time, assuming you can get basic life in them in the first place of course. $\endgroup$
    – mechalynx
    Commented Oct 10, 2014 at 21:36

Already some great answers given, Tim B's description of a gas giant ring-based ecosystem was evocative and mesmerizing, I could really visualize it in my mind… if you want something really scientifically accurate, I would say stick with what he suggested or at least use it as a jumping-off point.

Ironically enough, I have also been thinking quite a bit about this same question recently. I remember hearing somewhere that some nebulae even have liquid water in them, though I'm not sure if this is something like we might imagine as a giant space ocean (which would be flippin’ awesome) or just something more like a few random droplets of water vapor mixed in with lots of other gasses kept barely liquid by Goldilocks-proximity to nascent protostars or something like that (which sounds more plausible, but much less awesome).

So, if you're trying to imagine something like the ever-popular space whales, (or maybe the Moon Entity from that episode of Doctor Who that just aired, eh? ^_^) nebulae are probably a good place to start… but I think the real critical question here is, where is your world on Mohs Scale of Science Fiction Hardness? If it's in realistic physics and chemistry or close to it, then you probably can't get anything more complex than bacteria-analogues in open void, and even then, probably not unless it was close to an atmosphere or ring system or cryovolcano or something. If it's slid more to the science fantasy side, you know, go crazy with your Space Wales and Space Is an Ocean tropes, have at it!


Recently scientists have discovered that bacteria can survive the long haul to space. If bacteria can survive this long in space it is feasible that less complex forms of life like single celled proteins and amino acids can start in space, but in order for it to develop into a complex organism as we know it, it must have stable and constant conditions. The only known way to have a semi stable environment (a planet) unfortunately is classified as not being "in space" by your definition.

If the planet/asteroid/rock had no atmosphere it would be like being in space but while also being tethered to an object of much greater mass the organism could remain at a stable enough "environment" even without orbiting an object with a mass greater than its own.

Basically it is possible and it is probable that life arrived here and did not actually start here.


What about the nebula ITSELF being a life form?

To bacteria looking at us, we would appear to be mostly empty space maintaining a certain form for millions of their lifetimes.

Just a thought.


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