On Ocean Planets and their traits [closed]

Question

I am working on an ocean planet, and I am wondering what conditions would be expected of a planet whose surface is entirely water.

Or more accurately, how accurate is my exact idea.

My current idea is it is around 1.7 to 2.4x the size of Earth (haven't decided). It has extreme storms, and large waves across the planet due to three relatively close moons. Though if you go deep enough into the water it become calm and underwater settlements can be made. Storms come from the relatively hot temperature of the planet due to proximity to its parent star.

When you go deep enough, it became just a massive sheet of ice due to the pressure.

Is this realistic for a ocean planet? Is my explanation for the heavy storms and tides realistic?

Answer 1

A point of clarification; when you say 'ocean planet', do you mean that the planet is Earth-like in all other respects (rocky, orbiting its star at a similar distance, etc.) but happens to be entirely covered in water OR do you mean that the planet is primarily composed of H2O?

Both versions of the planet could be very interesting, but would inevitably look very different.

If we imagine the former, then many of the details you have suggested do not make sense. On a rocky planet with a hot, molten core, the lower down water levels will not be calmer, nor will they be capable of becoming ice - magma activity and underwater volcanoes will ensure a great deal of heat and movement in lower levels of the oceans, creating currents that will fuel the storms above. Furthermore, if you want there to be no land on the surface, the planet being bigger than earth doesn't do you many favors - land rising above sea level is a result of plate tectonics, which only becomes more pronounced as the mass of the planet grows, usually by orders of magnitude. A smaller planet with less surface area (but, perhaps, a denser core, if you wanted gravity to stay similar to Earth's) would be less likely to develop the amount of continental stress to produce very high mountains, thus making it easier for the whole thing to be immersed in water.

Now, suppose, on the other hand, we are talking about a planet or planetoid that is chiefly composed of water; a sort of Supercomet,a collection of ice and debris that has come together into a singular mass. For this planet, it does make sense for it to be very large, since water isn't very dense. It also makes sense for it to have ice built up around its deeper levels, and for deeper currents being calm and still. Its core would not be ice, but instead all of the metals and other materials sifted from the ice pulled to the center by gravity. As the object grew in mass and developed its own gravity, it could, conceivably, develop an atmosphere - with a stable gravitational field holding itself together and a constant source of light and heat from a sun, the outer layer of the ice could melt and create both ocean and water vapor on the outermost layer of the planet.

One point of contention, however, is the moons. The moons can't be too big or too heavy, or else it wouldn't make sense for them to be orbiting this planet and not the other way around. Something like earth's moon might be too much for a planet composed primarily of water and ice, even if we imagine that the metals at the core are quite heavy. Also, as far as the history of the planet's formation, I'm imagining that this would be formed by a mass of comets coming together over time, and the leftovers of that would more likely create a ring than any moons.

As far as the storms on the surface are concerned, you get those for free either way. If the whole surface of the planet is water and it orbits around a sun, you're going to get awesome storms, probably permanent or semi-permanent ones. The larger ones will likely be predictable, but there's plenty of room for smaller off-shoots to run wild and chaotic.

All of this is pretty soft as Sci-fi goes, and I'm not sure that it would survive stronger scrutiny, but I hope it has been helpful. In short, with a couple of addendums, I think your concept for an ocean planet does, if you'll pardon the pun, hold water. It's a good idea, and with a little fleshing out I think it holds up to a basic understanding of science.

Answer 2

Hurricanes sustaining for a long period of time.

Let us see how storms are formed. As told here:

1. Near the equator, water on ocean's surface evaporates due to Sun's heat.
2. The vapors rise and condense forming clouds.
3. When air is warm enough, clouds form a large thunderstorm.
4. Several large thunderstorms cluster together. Earth’s rotation causes the winds to swirl around its center and wind speed increase.
5. If wind speeds increase to 63 kilometer (39 miles per hour), it becomes a tropical storm. If winds reach 120 kilometer (74 miles per hour), it becomes a hurricane.
6. As a hurricane travels over warm ocean waters, it is fueled by heat and water and its intensity increases. (On land, its heat supply is cut and it weakens.)

On your ocean planet, there is no land. Therefore once a hurricane is formed, it will intensify and remain for a long period of time as we see storms on Jupiter.

Jupiter rotates once on its axis every 10 hours. What is the rotation speed of you planet?

If your planet rotates fast, the storms will be more intense.

Tides don't matter when there are hurricanes.

Answer 3

What do you mean by "1.7 to 2.4 times the size of Earth."? What do you define "size" by in that statement?

You might want to look at the Mohs Scale of Science Fiction Hardness at TV Tropes:

https://tvtropes.org/pmwiki/pmwiki.php/SlidingScale/MohsScaleOfScienceFictionHardness

If you are content with a scale level of 1 (or even less if possible) you can make your planet as large or small, as near to or as far from its star, and give it any climate and any surface conditions, you may desire for your story.

But if you hope your story will be more realistic and plausible and have a higher score, you should try to make your planet scientifically possible. You might want to consider whether you mean radius, diameter, surface area, volume, or mass by the "size" of your planet.

For example, a world with 1.7 times the radius or 1.7 times the diameter of Earth would have a surface area 2.89 times that of Earth and a volume 4.912867 that of Earth. If it had the same overall density it would have 4.912867 times the mass of Earth, but of course the overall density of planets can vary.

Suppose, for example, you want the planet to be habitable for humans, who can sail on its sea while exploring, or maybe even settle there to live in the undersea habitats you mention.

Then you might want to consult Habitable Planets for Man, Stephen H. Dole, 1964.

https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf

On page 12 Dole said that humans would probably not want to live on a world where the surface gravity was more than 1.25 or 1.50 g.

Dole had a formula for how he believed the mass of world would change its density and thus its volume. So on page 53 Dole wrote that a planet with a surface gravity of 1.5 g would have a mass of 2.35 Earth mass, a radius of 1.25 Earth radius, and an escape velocity of 15.3 kilometers per second. A mass of 2.35 Earth mass would be just a bit less than 2.4 Earth mass, while a radius of 1.25 Earth radius gives a surface area 1.56 that of Earth and a 1.952972 volume that of Earth.

On the minimum size, Dole decided on page 54 that a planet with an escape velocity of 6.25 kilometers per second might be able to retain an oxygen atmosphere for geological eras of time. That corresponds to mass 0.195 of Earth, a radius of 0.63 Earth radius, and a surface gravity of 0.49 g. But Dole thought that such a small planet would not be able to form an oxygen rich atmosphere, and on page 57 decided that the smallest planet that could form an oxygen rich atmosphere might have about 0.4 the mass of Earth, a radius 0.78 that of Earth, and a surface gravity of 0.68 g.

In recent decades, many scientists have considered the requirements for a world to be habitable. Unlike Dole, they seem to have concentrated on the more general case of worlds habitable for liquid water using lifeforms in general, instead of the more limited case of worlds habitable for humans (or for beings with similar environmental requirements) in particular. Of course most science fiction writers are interested in the more restricted definition of habitability.

One modern discussion of the mass range of a habitable world is in Heller and Barnes "Exomoon habitability constrained by illumination and tidal heating", 2013, pages 3 to 4:

A minimum mass of an exomoon is required to drive a magnetic shield on a billion-year timescale (Ms ≳ 0.1M⊕, Tachinami et al. 2011); to sustain a substantial, long-lived atmosphere (Ms ≳ 0.12M⊕, Williams et al. 1997; Kaltenegger 2000); and to drive tectonic activity (Ms ≳ 0.23M⊕, Williams et al. 1997), which is necessary to maintain plate tectonics and to support the carbon-silicate cycle. Weak internal dynamos have been detected in Mercury and Ganymede (Kivelson et al. 1996; Gurnett et al. 1996), suggesting that satellite masses > 0.25M⊕ will be adequate for considerations of exomoon habitability. This lower limit, however, is not a fixed number. Further sources of energy – such as radiogenic and tidal Heller & Barnes (2013) – Exomoon habitability constrained by illumination and tidal heating, and the effect of a moon’s composition and structure – can alter our limit in either direction. An upper mass limit is given by the fact that increasing mass leads to high pressures in the moon’s interior, which will increase the mantle viscosity and depress heat transfer throughout the mantle as well as in the core. Above a critical mass, the dynamo is strongly suppressed and becomes too weak to generate a magnetic field or sustain plate tectonics. This maximum mass can be placed around 2M⊕ (Gaidos et al. 2010; Noack & Breuer 2011; Stamenković et al. 2011). Summing up these conditions, we expect approximately Earth-mass moons to be habitable, and these objects could be detectable with the newly started Hunt for Exomoons with Kepler (HEK) project (Kipping et al. 2012).

https://arxiv.org/ftp/arxiv/papers/1209/1209.5323.pdf

So their sources indicate minimum masses for habitable worlds of 0.1 Earth, or 0.12 Earth, or 0.23 Earth, and a maximum mass for habitable worlds of about 2.0 Earth mass.

I note that with similar average density, a world with about 2.0 Earth mass would have less mass than Dole's upper limit. Dole's upper limit had 2.35 Earth mass, 1.25 the radius, of Earth, an escape velocity of 15.3 kilometers per second, and a surface gravity of 1.5 g. A world that has 2.0 Earth mass or less would have less of all those factors.

This Wikipedia article says:

The mass of a potentially habitable exoplanet is between 0.1 and 5.0 Earth masses.[21] However it is possible for a habitable world to have a mass as low as 0.0268 Earth Masses.[58]

https://en.wikipedia.org/wiki/Planetary_habitability#Mass

https://phl.upr.edu/projects/habitable-exoplanets-catalog

The possibility that planets with as low as 0.0268 Earth mass could retain an atmosphere, and thus liquid water, and be habitable for some forms of life, comes from this article:

https://arxiv.org/abs/1906.10561

Low mass worlds will have low escape velocities and rapidly lose their atmospheres as gases escape into outer space. The paper studies low mass water worlds entirely covered by very deep oceans of water, with with atmospheres purely of water vapor. I note that worlds with such vast oceans would have a lot of liquid and so could replace water vapor escaping into space with evaporated surface water for very long periods of time.

And there is one class of water worlds which could have even less mass than the ones discussed in that paper. Water worlds which retain their liquid water with solid lids on top of it. Those worlds would have vast world wide oceans many kilometers deep underneath planet wide ice sheets many kilometers deep.

There are a number of worlds in the outer solar system covered with thick ice sheets. Vast world wide oceans of liquid water have been detected beneath the ice of some of those worlds, and subsurface oceans are suspected on other worlds.

https://en.wikipedia.org/wiki/List_of_largest_lakes_and_seas_in_the_Solar_System

Of those worlds with internal oceans, the smallest is Saturn's moon Enceladus, 513.2 by 502.8 by 496.6 kilometers. Encledaus has a mass of about 1.080 times 10 to the 20^th power kilograms, or 0.00018 the mass of Earth, which is much smaller than the minimum mass of 0.268 Earth mass for water worlds without natural roofs all over them.

https://en.wikipedia.org/wiki/Enceladus

And if life can form & survive in such internal oceans that type of world could be interesting to explore in submarines.

Planets more massive than Earth, but less massive than ice giants, are called super Earths. Super Earths may water planets, planets totally covered with water.

This is considerable scientific discussion about the possibilities that super Earths, including those which are water worlds, are habitable. As mentioned above, it has been claimed that worlds with more than 2 times the mass of Earth would not have liquid metal cores and magnetic fields and plate tectonics.

This article mentions a study which indicates that super Earths with four to six times the mass of Earth would have the longest lasting metallic cores. And thus they should probably have magnetic fields and plate tectonics.

https://www.revyuh.com/news/science-and-research/space/super-earths-are-more-likely-to-have-magnetically-protected-habitability-than-earth-new-study-suggests/

There is a theory that planets need large moons for habitability.

Here is a link to an article which claims that A) large moons are necessary for habitability, and 2) that super Earths are unlikely to form large moons.

https://screenrant.com/super-earth-planets-habitable-can-study-detail/ So the habitability of super Earths, including the ones which are water worlds, is still rather controversial. In any case, humans wouldn't want to visit or settle on any planet which has a surface gravity greater than about 1.25 or 1.5 g. So it might be prudent to restrict the size of your water world.