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On Earth, photosynthetic organisms require additional nutrients to survive; in addition to "feeding" off of sunlight, plants respire and absorb water from the ground in order to grow.

However, the Earth is a closed system chemically speaking, so fundamentally all chemical resources are recycled through the biosphere. Would it be possible for this process to take place within a single organism, such that the only input the creature needed was sunlight to provide energy for its chemical processes?

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Using some limits from other answers - I will restrict myself by saying the creature must grow, be conscious, reproduce, and eventually die.

It is my own opinion that this would be possible - though I do not believe such a creature would ever actually evolve in the real universe. It requires many unique complex processes and requirements because of its niche environment(space), and to reach this complexity, there should be less complex organisms which it could evolve from. This is a problem because there isn't really another similar environment that would easily provide some sort of evolution "cross-over", though I could be wrong.

The only input to the system is sunlight, so the creature needs all other materials to be carried with it. This will have an impact on all the biological processes of the creature. The creature I imagine is actually quite close to @Envite's answer when I think about it.

The creature starts with being born. It has a bunch of "working materials" around it at the start - this is because of its parent. Other than that, there is only the emptiness of space and sunlight.

The creature consumes the material and starts to grow - using the sunlight as its source of fuel. It could be a complex creature, capable of great thought (though it has nobody to teach it anything) or it could be a very simple creature.

Eventually, it has consumed all the material. It self-replicates its child, probably into an egg form, and as it dies it releases (bacteria/chemicals/whatever) that it created during its lifetime which turn it into usable material once again.

The shell of the egg protects it from the bacteria, and the bacteria dies. Then the child is born and the cycle begins again.

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If the creature needs to grow and reproduce, a matter is required to build the new parts. Just energy itself is not enough. Because of that, plants need minerals and nitrogen from the soil and also take oxygen, carbon and hydrogen from the surrounding air.

If no growth or reproduction is required, a living (= running typical metabolism of the living organism, capable of regeneration and possibly limited growth with expense of some other part dying and decomposing) system can be self-contained. A single usual plant would probably survive in a closed system with enough sunlight, sufficient initial amount of water and minerals and some bacteria and fungi to convert the dead parts into usable minerals.

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  • $\begingroup$ e=mc^2, given enough energy, mass can be created $\endgroup$
    – user2813274
    Nov 13, 2014 at 15:59
  • $\begingroup$ Yes but really immense amounts of energy would be required in comparison to the requirements for the normal metabolism. $\endgroup$
    – eigenvalue
    Sep 23, 2015 at 20:11
  • $\begingroup$ "normal metabolism" would likely be even slower than that of plants, but seeing as how it's likely going to be a while before it actually gets anywhere anyways, it would have plenty of time as well $\endgroup$
    – user2813274
    Sep 27, 2015 at 0:31
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Planet Earth as a whole (as stated in the Gaia Theory in its stronger form) is a living being that lives by feeding only solar energy, and keeping all other resources in a closed circuit, gravity bound. So answer is yes.

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    $\begingroup$ This is more philosophy than science based. $\endgroup$
    – jawo
    Nov 12, 2014 at 13:06
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    $\begingroup$ @Sempie: No, not really. $\endgroup$
    – Lightness Races in Orbit
    Nov 12, 2014 at 13:08
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    $\begingroup$ It doesn't feed on only solar energy, what about all the meteorite and comet impacts? What about the gravitational influences of the other objects in the solar system? $\endgroup$
    – Michael McGriff
    Nov 12, 2014 at 13:58
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    $\begingroup$ Strong Gaia Theory (unless common Gaia Theory, which is accepted) says that Earth is a living being, for some definition of life, of course. That definition does not necessarily include reproduction. But anyway, It has not been proven that Earth can not reproduce. Stem cells from Earth (also known as humans) may be capable of terraforming another planet, or even to create a planet. $\endgroup$
    – Envite
    Nov 12, 2014 at 21:42
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    $\begingroup$ This is a good answer. A human being actually contains more bacteria cells than human cells. So if we get to be considered an individual being, why not the whole Earth? Even if it needs an individual consciousness as a whole, we can't preclude the possibility that that won't one day happen. i.e. Asimov's Foundation series' Gaia. $\endgroup$
    – Blake Walsh
    Nov 12, 2014 at 22:27
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This strongly depends on your definition of "Life".

The most common definition is the biological-life in which living systems always depend on nucleic acides.

Since there is no unequivocal definition of life, the current understanding is descriptive. Life is considered a characteristic of something that exhibits all or most of the following traits .. (click link for further information) - Wikipedia - Life/Definition

There are theories of lifeforms which do not depend on carbon (As we do).

But while reading these theories, you'll notice that those who set up these theories, tried to recreate life as we know on different chemistry but did't really looked "how life else could work".

For example, I remember this one episode (Dont know the #, sry) of Star Trek - The next generation in which they met a giant crystal which was floating in space. This crystal had some kind of consciousness and had to consume energy to stay alive. In the episode the creature consumed the warp-field or something, not sure. But you see the point I guess.

I do not think life has to be 'biological' as we know. My personal definition of life, which doesn't really go against the common one (due there is none), is that living things have these:

  • consciousness (doesn't mean awareness of themself, but can)
  • ability to consume and create things ( as like metabolism )
  • ability to replicate under specific circumstances

According to this and Abiogenesis in theory life could develop almost everywhere. The point why it does not is the razerblade problem. Try to stack several razerblades on each other,... they'll collapse. Just under rare circumstances it's possible that they do not, and that's probably the point of life.

We already found amino acids on asteroids, so this concept is not that hypotetical, even if it's not proven at the point.

Another point:

If you look at the lowest developed animals or bacteria we know, like Trichoplax (which btw is different than any other animal we know), you will find structures and metabolism which are really basic and oftn don't need atmosphere to work. So the atmosphere itself is not the problem, the pressure isn't either due a evolution in space-low pressure literally evovles with this pressure.

A lack of nutrient isn't probable too. Look at corals and other sessility animals you'll see that they do not, or almost not, need any nutrients to "stay alive" but to grow and replicate. This principe supports life in areas where almost no nutrients exist like in space. (Even in space, there are particles which could be consumed).

So, if you accept this definitions, nothing stops you from creating any fictional form of life which fit's this requierements.

And as always, stay plausible and keep causality up.

EDIT:

Life only depending on light is implausible. Just as said before, your lifeform must create matter out of light wich seems to be impossible. But as I said, even in space there are atoms and molecules which could be used as nutrients, even if they're very rare.

This means your direct question must be answered with NO, but with a slight change it is possible.

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    $\begingroup$ This started off pretty coherent, but degenerates into who knows what pretty fast....! As always, stay plausible and keep causality up. $\endgroup$
    – Lightness Races in Orbit
    Nov 12, 2014 at 12:20
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    $\begingroup$ This is the crystal life form I believe you're looking for: en.memory-alpha.org/wiki/Crystalline_Entity $\endgroup$
    – Michael McGriff
    Nov 12, 2014 at 13:35
  • $\begingroup$ Yes exactly what I was talking about. $\endgroup$
    – jawo
    Nov 12, 2014 at 13:37
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    $\begingroup$ I would argue that your comment on Corals is simply incorrect, they actively pull nutrients from the water. Additionally your comment on Trichoplax, while relevant to the question should mention that while atmosphere and temp are not an issue, temperature certainly would be. Otherwise this is a well thought out answer. $\endgroup$
    – James
    Nov 12, 2014 at 14:30
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You need to define what you mean by creature.

If you want your creature to be considered living, then you still need to define what this means.

There is currently no universally accepted definition of life. However most definitions require the ability to grow and reproduce. Unless your creature can transform light into mass it would be unable to fulfill this criteria.

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  • $\begingroup$ Which an organism could turn light into matter. It's possible, and generally biological organisms are much more efficient than machines. $\endgroup$
    – DonyorM
    Nov 12, 2014 at 11:54
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    $\begingroup$ @DonyorM: Biological organisms are machines. $\endgroup$
    – Lightness Races in Orbit
    Nov 12, 2014 at 12:20
  • $\begingroup$ @LightnessRacesinOrbit Maybe so, I mean human made machines. My point is that just because humans can't efficiently create matter from energy, doesn't mean a creature can't. We see this all over the place. We don't have any photo device as good as our eyes. $\endgroup$
    – DonyorM
    Nov 12, 2014 at 12:32
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    $\begingroup$ @DonyorM: I'd argue that our cameras are generally as good as or massively superior to human eyes; it's our ability to process multiple images into a consciously-accessible image of high dynamic range, variable focus and not to mention perspective that goes beyond what a single, static JPG tells you. But that's comparing apples and oranges. For starters, we have two eyes, which helps massively; comparing that to a single camera with no processing of the resulting images is grossly unfair. $\endgroup$
    – Lightness Races in Orbit
    Nov 12, 2014 at 13:07
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    $\begingroup$ @DonyorM On the other hand, just because some organisms are more efficient at some tasks than some machines, that doesn't mean that an organism could create matter from light. Regardless of how efficiently you do it, E = mc^2 still holds, so you still need a LOT of energy per unit mass. Even at perfect light absorption and perfect conversion efficiency, there's still a limit to how much mass you can create for a given size (cross-sectional area) of creature at a given distance from the sun. $\endgroup$
    – anaximander
    Nov 12, 2014 at 15:56
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I think it would be possible for such an organism to survive based solely on sunlight, especially for short periods, but not to reproduce or grow. I could imagine, for example, a highly efficient engineered organism which used sunlight to break apart its own waste products to continue metabolizing slowly over long periods of time. Such an organism would probably take the form of a hemispherical pressure vessel to withstand the vacuum of space, and even then, would need to have a very low density.

I say 'engineered' because such an organism wouldn't really be able to evolve naturally. Even the densest clouds of gas in space only have densities of around 10^6 molecules per cubic centimeter, compared to 10^19 molecules per cubic centimeter for air. Space is too sparse and too cold to create the kinds of conditions under which enough chemical reactions are happening to build even the most basic of life forms.

I could imagine, however, circumstances in which such a life-form comes to exist. Perhaps some advanced civilization needs a solution to clean up huge spills of organic chemicals in space. A simple life form could be engineered to slowly drift through the spills, consuming the chemicals to reproduce. The life forms could then be collected at a later point in time to reclaim the lost resources, at the point where the cloud is becoming to sparse and diffuse to sustain them. Even in this scenario, their metabolisms would be exceedingly slow, since a gas cloud in space would diffuse out to a low density fairly quickly, based on the internal pressure of the cloud.

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  • $\begingroup$ I disagree on evolution--I could imagine such a creature evolving on a planet that slowly lost it's atmosphere. It evolves a completely closed-cycle metabolism to tide it over between finding sources of the elements it needs. $\endgroup$
    – Loren Pechtel
    Nov 12, 2014 at 20:37
  • $\begingroup$ How would such an organism get into space? $\endgroup$
    – ckersch
    Nov 12, 2014 at 20:53
  • $\begingroup$ Impact events can throw rocks off a planet that doesn't have too much atmosphere. Of course only microscopic life could survive such an event but once you have it in space it could evolve. $\endgroup$
    – Loren Pechtel
    Nov 12, 2014 at 21:48
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As this is a question for world-building, it sounds like what you are seeking is if such a creature were plausible, given the science we know. Your story or world can work by different rules, but assuming biology and physics as we understand them, like many of the other answers here, I'd have to say it's implausible to think a creature could live on sunlight alone.

That said, a creature could power its life, growth, reproduction, etc primarily from sunlight. To get the extra mass it would have to feed, in some sense, on materials floating in space along with it. Perhaps consuming others of it's kind, or raw materials like gas for volatiles.

Ultimately, it's a question of chemistry. Breaking and forming chemical bonds produce energy for organisms to live. Even earth plants don't "eat" sunlight directly, they use the photons to drive chemical reactions via photo-synthesis.

So, assuming you have some material to work with, the question seems more to be "how big / what kind of life" could fuel its biological processes purely by sunlight.

For that, you can turn to physics. Decide how close to what kind of star your beastie lives, figure out its cross-sectional area facing the star, and you can determine how much energy it could absorb. That's your budget. Compare to some terrestrial plants to get a sense of what's a "reasonable" size, and go from there.

But ultimately, it's your world. If you need or want a biological system capable of self contained nuclear fusion, go for it! The law of dramatic necessity should trump everything else.

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It depends a lot on what you call a creature.

With a bit of stretching you could consider a star a creature (any of the bigger stars in any case):

  • It's born
  • It gathers more materials to grow (using gravity)
  • It changes during lifetime
  • At its death, it creates the seeds for new stars: disruptions moving through space that can spawn new stars if they meet the right conditions (sufficient concentrations of matter)

Now stars are (at the very least on an emotional level) very different from what we would consider life. So you could imagine a creature like a star, but with more consciousness. Like some kind of feedback system where it can detect matter (detectable by gravitation) and potentially jettison some mass to move in the direction of the matter (jettison at high speeds to love a minimal amount of mass for maximum propulsion). It would make it kinda like a giant space amoeba. However, defining the sensory system, feedback loops (decision making basically) and propulsion is essential. That definition would define when your creature is alive and when it's dead and how it reacts.

Alternatively, sidestep evolution and steal from Asimov :D. Intelligent lifeforms engineer lifeform for space. You could envision a solar powered lifeform that would only replicate when it finds enough materials. It would probably have to hunt for new materials to occasionally repair itself though. Since it's engineered, it could be made in conditions that are extremely unlikely to happen randomly. Think of an extremely complicated robot. Be scared of however created it though...

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Yes it is possible for living creature to depend on only sunlight. Physicists were able to create molecules that are made up of light. So if the creature is made up of this photonic molecules then it may use the light from the sun as material for its biological process. If the creature was to be made up of regular molecules it might use light energy to drive its living process but for the materials it may depend on other particles that are emitted by the sun from solar winds or it might have a way to convert energy into matter.

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Earth is not a closed system, even ignoring solar (and other) energy impacting it and heat leeching out.

There's a constant stream of gas drifting out into space. It's a trickly, but it's there.
There's also a constant stream of matter entering the system. Asteroid impacts, solar wind (which are charged particles), etc. etc.

And of course every time we shoot a rocket into space, that's tons of material leaving the system as well.

In space, your creature would be pelted with such stuff all the time. From dust motes and single gas molecules that fill the vacuum of space to asteroid and comet sized rocks, it will get hit by matter. If it can absorb and use that matter, that can allow it to grow.
Combined with efficient use of impacting electro magnetic energy, who knows what may evolve.
But given the environment, it probably won't look anything like life as we know it :)

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Life in its simplest form might be just information. It was once thought of as an inseparable part of either matter or energy and that whatever ever else happens to the information after it travels beyond the event horizon of a black hole would also be lost to this universe along with the matter or energy. Stephen Hawking once was of this opinion. However, he is now of the opinion that the information is sort of stripped off and smeared all over an infinitely large hyper space that is the boundary of the Schwarzschild radius. This information would be expressed in quantum states that may indeed be still coupled to other particles on our side of the boundary. So the real question is, can life be totally destroyed, and it appears that the answer is no.

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A creature could survive solely off sunlight, but it would not be "alive" according to the standard definition of life.

The ability to grow, reproduce, and evolve are considered requirements for an object to be considered "alive". Growth requires matter in addition to energy, and reproduction requires growth, otherwise each new generation will be smaller than the last. Since the question specifies that the creature survives "solely off sunlight", there is no net mass increase. (In theory an organism could collect molecules and dust from the surrounding space, but this would be a very slow process.)

Since this creature cannot reproduce, it cannot evolve, at least not in the standard biological sense of the word. Any space-dwelling organism must have either been created by another being or evolved in a circumstance that did allow it to grow and reproduce.

That being said, as long as it is able to retain 100% of its matter (or alternatively gather as much matter from the surrounding medium as it loses), it can potentially behave like a living organism. Observation, reaction, and even thought do not technically require new matter to occur, only energy (animals on Earth gain their energy by eating food but that energy originally came from the sun).

A moving, self-regulating, thinking, immortal being could survive indefinately in orbit around a star. (It couldn't use rockets, but movement could be accomplished by tilting 'solar sails'). However, it couldn't grow or reproduce, so it wouldn't be considered "alive" according to the standard scientific definition.

It could be argued that the planet Earth is itself such an 'organism', as it is a functionally closed system as far as matter is concerned (the amount it has acquired from meteors, comets, and atmospheric gas exchange is negligable and irrelevant to life's survival) but supports life (and intelligence!) from sunlight alone.

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  • $\begingroup$ "the amount [of matter] [Earth] has acquired from meteors, comets, and atmospheric gas exchange is negligable and irrelevant to life's survival" While absolutely not the only form of life even at the time, certain species of dinosaur might disagree with you. :-) $\endgroup$
    – user
    Mar 7, 2016 at 10:40

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