A planet in one of my solar systems has 100% of its surface covered in deep water, many hundreds of kilometers deep. Some criteria relating to the planet.

  • It's fairly close to its host, a sunlike star.
  • It has no major moons, and has a somewhat breathable atmosphere around 5 times as dense as the earth's
  • It has a mass of approximately 7 earths, and a surface gravity of 14.3m/s squared.
  • The water itself would be at or near boiling almost all of the time at the equator and likely falls back to the surface in the form of rain around the poles.

My issue comes with depicting it in artwork or renders, with no knowledge of what would happen to air currents on such a world.

  • $\begingroup$ What's the rest of atmosphere composition? $\endgroup$
    – Mołot
    Nov 13, 2017 at 7:31
  • 4
    $\begingroup$ Your planet works out at approximately 14,000 kilometer radius with 500 kilometer water depth, with a light core with 4.0 g/cm^3 density. Air pressure would be around 7 atm, with boiling point of water near 170 °C. At the equator, the cloud base is zero as you have supersaturation all the way - rainy fog (on the other hand, this ought to increase the local albedo and bounce sunlight back into space and decrease temperature). Oxygen content around 5%, but the rest needs be noble gases or at 7 atm you'll die of inert gas narcosis. $\endgroup$
    – LSerni
    Nov 13, 2017 at 19:06
  • $\begingroup$ On the other hand that goes well with your nickname - Neon is a noble gas and would work :-) $\endgroup$
    – LSerni
    Nov 13, 2017 at 19:07

3 Answers 3


Some thoughts about your world.

  1. It has a mass of approximately 7 earths, and a surface gravity of 14.3m/s squared.

    It will be tricky to have a world with a mass 7 times that of earth but a gravity not even double earth's. I am not sure how that would work.

  2. On earth cloud level can be right at the water (I once saw a cloud emerge from Lake Michigan after the air temperature dropped suddenly). Or it can be high in the air. This depends on the lifted condensation level. Warm air holds more "dissolved" water than cool air. As air cools it cannot hold the water it has and the water turns back into liquid. That is why you see your breath on a cold day. That is what determines where clouds form.


The lifted condensation level or lifting condensation level (LCL) is formally defined as the height at which the relative humidity (RH) of an air parcel will reach 100% with respect to liquid water when it is cooled by dry adiabatic lifting. The RH of air increases when it is cooled, since the amount of water vapor in the air (i.e., its specific humidity) remains constant, while the saturation vapor pressure decreases almost exponentially with decreasing temperature. If the air parcel is lifting further beyond the LCL, water vapor in the air parcel will begin condensing, forming cloud droplets. (In the real atmosphere, it is usually necessary for air to be slightly supersaturated, normally by around 0.5%, before condensation occurs; this translates into about 10 meters or so of additional lifting above the LCL.) The LCL is a good approximation of the height of the cloud base which will be observed on days when air is lifted mechanically from the surface to the cloud base (e.g., due to convergence of airmasses).

If water in the boiling zone is boiled from undersea volcanoes then there will be loads of steam and clouds right on the surface, like a pot of boiling water. If boiled by the sun, I have to think the air above the water will also be very hot and so the clouds will be very high, where the air is cooler. These clouds will also be very dense because of the vast amount of vapor coming from the boiling sea, and probably with a flat bottom from an atmospheric thermocline. In fact I think such dense clouds would probably reflect a lot of sunlight back out and make it difficult for the sun to boil the sea. Hmmm...

In the more normal parts of your world I can think of no reason why the clouds would not be like earth clouds over your water world. More dense air means a volume of air can carry more water but there will be a temperature where the water condenses out, just like on earth. At the height where that temperature is you will get your clouds. You can pick which type you like for artistic effect.

  • $\begingroup$ Well, the planet has such low gravity because it's more than 60% water with very little metal in the remainder. it's far less dense than earth as a result. $\endgroup$ Nov 13, 2017 at 2:39
  • $\begingroup$ You have some reading to do about gravity, Neon. Gravity depends on mass, not density. You could start here physics.stackexchange.com/questions/193017/… $\endgroup$
    – Willk
    Nov 13, 2017 at 3:24
  • $\begingroup$ Ad 1) See my question about biggest planet with 1g surface gravity. It might be easier than you think. $\endgroup$
    – Mołot
    Nov 13, 2017 at 7:32
  • 1
    $\begingroup$ @Will but volume, and therefore distance from the centre to the surface depends on density, and gravity depends on distance between the centres of the objects. $\endgroup$
    – komodosp
    Nov 13, 2017 at 13:29
  • $\begingroup$ This sounds like a Venus in the making. Very high clouds may be exposed to UV radiation splitting water molecules, letting hydrogen escape to space. That much water vapour will also cause massive greenhouse effect, and atmosphere and ocean are probably quite efficient at distributing temperature between equator and poles. Recent research seems to indicate that global oceans don't last in the habitable zone, in fact. But this planet can be young (at geological scales), so that's not a show-stopper. $\endgroup$
    – Eth
    Nov 14, 2017 at 18:49

Apart from what you have listed, there are two more factors that will have a big impact.

The first of these is speed of rotation. The faster this is, the greater the Coriolis force will be. A slow rotation (and therefore a weak Coriolis force) will make the air rise at the equator and descend at the poles. There will therefore be a single thick cloudy band around the equator, and one thick clearer band around each pole. Faster speeds of rotation will deflect this air more, leadings to a greater number of such alternating bands.

One other possibility is that the planet is tide-locked to the star (a realistic possibility since you said that the planet is "fairly close") to its parent star. There has already been another thread on this forum on the climate of such planets.

The second of these is axial tilt. The greater the axial tilt, the more pronounced the seasons. In terms of how the planet looks from space, these will mean that the bands gradually move towards the pole of each hemisphere during that hemisphere's summer.


I think it would look like Venus. At near boiling, most of the atmosphere is going to be water vapour. With a higher gravity you will get a much thicker atmosphere, especially without a moon to create tides in the atmosphere.

The amount of water vapour air can hold increases about 7% per degree C at Earthly temp/and pressure. (20 C) This rapid increase is one reason why the equator didn't cook in the last warm period.

You need to come up with a plausible explanation for much less atmosphere, which I suspect means a lot of research into planetary development -- a nascent science, as we don't have many examples with historical data.

I expect two circulations happening: The top of the atmosphere will be powered by the star, and the lower limit of it will be the lower limit of solar penetration. The air will be fairly dry -- the lower atmosphere will act as a water sink. Circulation here will be fast. You have only dry air, so air is the only transport mechanism between the equator and the pole. There will be a shortage of nucleation centers (no dust...) so cloud formation will be delayed, then abrupt.

Below that the heat is from residual radioactivity and leftover scraps of geology. This is a small source of heat so you will get slow circulation toward the poles. While there is lots of water vapour in this air, there is no real reason for it to mix with the upper atmosphere. When precip in the upper atmosphere happens it will drop into the lower atmosphere and be trapped there.

Because of the much higher density of the atmosphere, 100 C will be far below boiling point.

The surface will be dark, stirred by an occasional puff of wind as a parcel of water vapour lifts free of the surface and makes a lazy climb upward.

Volcanoes would be a source of updrafts, but most of the surface will be very uniform in temperature (crustal heat leaks equally everywhere)

You may have a candidate for the world's most boring planet contest. (grin)


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