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Imagine an earth mass that consists purely and entirely of water that orbits a star similar to ours at a similar orbital distance to Mars(or whatever distance would be sufficient for the planet to form a 'crust' of ice but leaves everything below that as liquid water, as well as whatever kind of core water would form(still don't know exactly what would happen down there)). I know it won't remain entirely water-based forever, asteroids and such and whatever 'dust' is picked up from space and allowed into the planet's insides by the shifting of... glacial plates(ice tectonic plates)? The multicellular lifeforms that live within and on it are planned to have come from materials introduced via asteroids actually, but I don't know if this kind of planet is stable enough to exist for long enough in the first place.

Would a planet made almost entirely of water be able to stay as a planet for long enough for complex multicellular life to form?

Bonus points for determining if it can continue being a stable planet when the sun inevitably expands enough to bring it into a warmer zone of the solar system and the ice crust begins to melt.

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  • $\begingroup$ Why not? There are known "water" exoplanets. They may have a rocky core of some unknown size, is that Ok? $\endgroup$
    – Alexander
    Nov 15, 2021 at 18:29
  • $\begingroup$ @Alexander Any kind of rocky materials the core of the planet I have in mind would have would be due to the gradual accumulation of non-water materials instead of what was probably already there when the mentioned exoplanets started who probably accumulated their water. I guess part of my question is asking if water could hold itself together if a planet was made of the stuff or if it'll dissipate, evaporate, or otherwise stop existing due to the usual things that affect the usually hardier and rockier more solid planets. I know gas giants exist on their own but they have the mass for it. $\endgroup$
    – Rubrikon
    Nov 15, 2021 at 18:37
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    $\begingroup$ An Earth sized waterworld would absolutely have enough gravity to hold on to its water for billions of years. Such waterworld, however, would likely lack magnetic field and be susceptible to stellar wind effect. This will cause increased mass loss (not as significant as to destroy the planet though) and make surface conditions quite harsh for life. $\endgroup$
    – Alexander
    Nov 15, 2021 at 18:43
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    $\begingroup$ How did the water world form with just water? Normally you'd expect a planet to have a whole melange of different elements, and though it varies somewhat based on the predecessor star I would find it hard to explain why there's a planet with hydrogen and oxygen but not, say, carbon. $\endgroup$
    – Cadence
    Nov 15, 2021 at 19:05
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    $\begingroup$ Short answer? If it's entirely composed of water? No. If it's got a substantial amount of compounds which allow for a high degree of bonding and so the forming of extremely complex structures? maybe. Cellular, or an analogy, some simple lifeform, a bit like bacteria? Probabalbly. $\endgroup$
    – Broken ECLSS unit
    Nov 15, 2021 at 20:30

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Probably such a body would not host life, no matter how long you wait.

One can read everywhere that water is the basis for life, however a water only planet would be a terrible place for life to start, let alone evolve.

Life as we know it rely on several elements in a water based solution. If you have only water and at most traces of the other elements, you are making the evolution of life basically more and more unlikely, because you are relying on these trace element to casually be present at the same moment at the same time. Adding to this that you won't get life "first time right", you are simply steering toward impossibility.

Just to give you an example of an element which is crucial to life, some time ago there was a research published which concluded that an ocean planet would be a terrible place for life, because it would lack Phosphorus in a form usable by photosynthetic organism, without which life as it exists on our planet today won't exist.

It turns out that water worlds may be some of the worst places to look for living things. One study presented at the meeting shows how a planet covered in oceans could be starved of phosphorus, a nutrient without which earthly life cannot thrive. Other work concludes that a planet swamped in even deeper water would be geologically dead, lacking any of the planetary processes that nurture life on Earth.

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    $\begingroup$ +1 agree about scarcity of resources. Same counts for nitrogen.. and carbon. There should be a source to feed a (bacterial) base of a food chain. But there's a difficult startup. $\endgroup$
    – Goodies
    Nov 15, 2021 at 19:16
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    $\begingroup$ This doesn't really answer the question. You are mainly addressing abiogenesis, but the OP's premise is that life will be introduced via materials in an asteroid, not from the planet itself. The question is whether a purely water-based planet would remain stable for long enough for such lifeforms (which are already present as per the premise) to evolve into complex life. $\endgroup$
    – JBentley
    Nov 16, 2021 at 11:57
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    $\begingroup$ @JBentley: Such bacteria would starve and die in a water-only environment. Also, the high-purity water environment would cause them to lyse unless they were specifically adapted for it. $\endgroup$
    – Kevin
    Nov 16, 2021 at 16:30
  • $\begingroup$ @kevin That is a potential answer, which was the point I was making. $\endgroup$
    – JBentley
    Nov 17, 2021 at 1:53
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Your planet can't exist--most of it is going to be ice, not water. Check water's phase diagram--once the pressure gets high enough the liquid state no longer exists.

It also will lack the radioactives to support a geological cycle. Heavy stuff falls down and doesn't ever come back up, your world soon can't support life even if it originally could.

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  • $\begingroup$ Not that a planet composed of (nearly) pure H2O couldn't exist, but only the outer layer would be liquid water (or ice). As pressure increases, ice undergoes phase transitions into different forms. Google says 16, from Ice I (ordinary ice) to Ice XVI. (Though Ice XVI is something of an oddity that would form naturally only at negative pressure: en.wikipedia.org/wiki/Ice_XVI ) So there's no real difference between a pure water world, and one that just has oceans a few hundred km deep over a rocky core - see e.g. Europa. $\endgroup$
    – jamesqf
    Nov 16, 2021 at 17:25
  • $\begingroup$ Although the question has several blind spots, it sounds close to a more plausible (and interesting) question. After all, the liquid oceans on this icy planet would be at least as deep as our own oceans, right? $\endgroup$
    – jpaugh
    Nov 16, 2021 at 21:25
  • $\begingroup$ @jamesqf Not all of those forms of ice will be encountered as you simply raise the pressure. Simply raising the pressure on terrestrial temperatures goes through VI, VII, X and ends up at XI. $\endgroup$
    – Loren Pechtel
    Nov 17, 2021 at 0:42
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Lets look to our own solar system for the answers: https://solarsystem.nasa.gov/moons/jupiter-moons/europa/in-depth/

Europa is an ocean moon of Jupiter covered in Ice that is 15-25 km thick. It is fractured in multiple places, and based on observable craters is not more than 40-90 million years old. It is comparable in size to our moon.

Yet in that time frame it has not completely cooled off and turned to ice. This is because it has a large conductive fluid body within it that affects the magnetosphere of Jupiter. This implies that the oceans are still liquid. Since it is at least 40 million years old, there is a high probability of a solid core, so it is likely to be not completely liquid either, but it is as close as we can get.

For life to form, you need 3 requirements, liquid water, the appropriate chemical elements, and an energy source. It is believed that the first two are present on Europa, but it's hard to confirm that the third is also present. Extremophiles on Earth that live in volcanic vents prove that it is possible for them to live in similar vents on Europa if they exist, but we don't know for sure that there is a solid molten core to provide that. Whether hot water would be enough is dubious as we haven't found any similar lifeform on Earth that could survive in those conditions alone.

You want a planet that is only water, no solids at all. Without solids you might not have the appropriate chemicals to leech into the water, and you would be hard pressed to give your world an energy source, not just for life, but to prevent it from completely freezing over in a few million years. It is also unlikely with asteroid impacts to remain completely liquid as well.

With this in mind, I would suggest going with a completely liquid surface, but solid core concept, and not completely liquid planet for your world building, if you want some realism, perhaps.

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  • $\begingroup$ Nitpick: You seem to imply that Europa itself is only 40-90 million years old. It's the outer crust that gets reformed in that time frame, much as plate tectonics recycles Earth's ocean crust. $\endgroup$
    – jamesqf
    Nov 16, 2021 at 17:38
  • $\begingroup$ A planet, by definition, has an energy source: the star it orbits. The sun is the primary energy source for earth-life, and there's no reason that the OP's planet need suffer the same lack of sunshine that Europa does. $\endgroup$
    – jpaugh
    Nov 16, 2021 at 21:29
  • $\begingroup$ @jpaugh: Europa doesn't lack that much sunshine. In space, it's about 1/25 what Earth gets above the atmosphere. But atmosphere, clouds, shade from tree canopies all reduce the amount that reaches Earth's surface, yet there's still enough for plants adapted to low light. Indeed, many of those plants die if kept in full sun. $\endgroup$
    – jamesqf
    Nov 17, 2021 at 3:32
  • $\begingroup$ You act as if I suggest it is only 40 to 90 million years old. My source suggests that. Do you have a conflicting source? The ice is cracked but there isn't any talk of shifting or tectonics. They don't say anything about the ice getting subducted under other ice. For that to happen ice would have to melt and there is no evidence it does. $\endgroup$
    – Chthonic One
    Nov 17, 2021 at 21:11
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In terms of retaining water in liquid phase what you need is a deep gravity well. You need to retain gaseous atmosphere at sufficient pressure to limit vaporisation of your water. Obviously 1G will do it; I don't know the lower threshold but Martian gravity (0.38G) isn't enough. Higher than 1G will certainly work. I think your world needs a significant rocky core to get the requisite gravity well without being crazy large. The higher its mass the closer you can get those moons for high angular velocity and maximum tidal brutality (work it, baby! pop those planet zits!).

Or, maybe a double planet orbiting a common mass centre? You could have some freakiness with the water spilling between the worlds (Wet and Dry?) or even icebergs orbiting Lagrange points! The icebergs will look like stationary comets due to the heat and pressure of solar wind, and they will loom large due to proximity.

In respect of the phosphorus problem, you will need significant vulcanism to keep phosphates in circulation. If you have that, you will get Hawaii style islands. A lot of them. These will not be home to complex land-based life as they will keep exploding.

To get maintain high levels of vulcanism indefinitely you need strong tidal forces from something that won't boil your water off. You need lots of big moons such as appear in fantasy paintings. Normally these are laughable because in the size and number usually depicted they would cause significant vulcanism, but you need that so wey-hey, big moons for the win!

This will also produce complex and violent tidal action. This in turn may produce notable friction heating of the water over time. On the bright side those moons are perfect for Kevin what's his face to look at from the deck of his trimaran while smooching your heroine in between staving off various apocalypses (is that a word?)

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  • $\begingroup$ "what you need is a deep gravity well..." So, you're saying that water might not be dense enough for its own gravity to maintain cohesion? As in, the density of water might not be sufficient to fit inside the maximum radius per unit of mass that allows the water to remain together and harbor an atmosphere? It sounds analogous to the Schwarzschild radius, where, for any given mass, you need to fit it inside a tiny radius in order to form a black hole. $\endgroup$
    – jpaugh
    Nov 16, 2021 at 21:39
  • $\begingroup$ Considering only the Roche limit, water is at least dense enough to maintain an orbit similar to Saturn's, since Saturn is less dense than water. $\endgroup$
    – jpaugh
    Nov 16, 2021 at 21:53
  • $\begingroup$ @jpaugh yes exactly to your longer comment, but also if you want life to develop anywhere other than volcanic vents you'll need shallow seas. While islands from vigorous vulcanism do have coastal fringes they keep covering them in molten rock, or flat out exploding. $\endgroup$
    – Peter Wone
    Nov 17, 2021 at 1:02

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