I am trying to create a planet with low gravity that keeps an atmosphere and has an ocean that covers the whole planet. Basically I am developing beings (separately from this question) who can live in both water and air and I want them to be able to swim in the ocean and also fly/float in the sky, so I thought low gravity might be a solution to achieve this.

However, there’s another post that already asks the question Life in extremely low gravity water and the answers there (plus my own research) led me to believe it’s incredibly unlikely if not outright impossible to achieve my goal in nature.

Then I thought lets do it artificially. Ultimately most of the physics would still be the same, still not possible. So the I came up with...

Could soft matter infused with nano or pico sized magnets be used to clump together so that we could create environments with it?

I envision a planetoid sized sphere of water infused with finely spread nano/pico sized magnets throughout, to serve as the ocean. The magnets would keep the water together, and in turn the size of this mass could be large enough to create its own gravity thus keeping the atmosphere around it. You might not even need a rocky core since the water would kind of be the core.

And everything that lives there, or even visits, could become covered in the magnets, or the magnets could seep into them, so that they float, but not off into space.

In this way we create an artificial world with the needed parameters.

Addendum: I guess I am basically describing a kind of Ferro fluid, but one that doesn't exist in todays technology. Lets suppose a futuristic level of technology where it can have far greater applications than what current Ferro fluids can.


A) Once the planet is built using this method I wonder if the magnets would have to remain, or if having amassed the water would it still need them...

B) I suggested in another iteration of this question (sandbox) that this technology could/might be used to create environments other than this one. I'd like to hear if you think that's possible too...

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    $\begingroup$ @Len - Two things make magnetic particles ineffective (if noncontiguous, obviously you could hold in an atmosphere with a solid dome) to hold an atmosphere. 1) the smallest a magnet can be is 1 atom, and it has to be a ferromagnetic atom, which makes it substantially larger than most atmospheric elements, and 2) magnetic dipoles' effect dies off by the inverse cube, rather than the inverse square. So if gravity can't hold something, magnetism definitely cannot. $\endgroup$
    – jdunlop
    Commented Mar 2, 2018 at 20:07
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    $\begingroup$ @Len - I understand that the basic idea is that a nano magnet would be able to hold a drop of liquid around it. Maybe that will work - but the same schema would definitely not work for gas (at least if are not talking about highly ionized gas). $\endgroup$
    – Alexander
    Commented Mar 2, 2018 at 20:08
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    $\begingroup$ @Len - Water molecules are bigger than single atoms, so you could conceivably have a latticework of magnetic particles holding them in place. It'd be indistinguishable from having a solid barrier, however, by definition. $\endgroup$
    – jdunlop
    Commented Mar 3, 2018 at 1:28
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    $\begingroup$ @Len - That being said, of course sufficient water will have its own gravity. Sufficient anything has its own gravity. But while a planet made entirely of water (with some contaminants) is possible, once you have enough water under pressure, it starts behaving strangely. You'd have what amounted to a rocky core, made mostly of hot ice. $\endgroup$
    – jdunlop
    Commented Mar 3, 2018 at 1:34
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    $\begingroup$ Why use low gravity rather than denser air? $\endgroup$
    – DonielF
    Commented Mar 6, 2018 at 4:22

2 Answers 2


Assuming there is a possibility that the inhabitants had full molecular nanotechnology, it could be an underwater version of a utility fog.

enter image description here

Basically lots of little 12 sided nano machines, with an arm on each face, all hold hands with their neighbors. They can move their arms freely, grabbing and releasing as needed, but with the right command the arms can go rigid, and you suddenly have a solid object.

Fom the wiki:


Hall thought of it as a nanotechnological replacement for car seatbelts. The robots would be microscopic, with extending arms reaching in several different directions, and could perform three-dimensional lattice reconfiguration. Grabbers at the ends of the arms would allow the robots (or foglets) to mechanically link to one another and share both information and energy, enabling them to act as a continuous substance with mechanical and optical properties that could be varied over a wide range. Each foglet would have substantial computing power, and would be able to communicate with its neighbors.

In the original application as a replacement for seatbelts, the swarm of robots would be widely spread out, and the arms loose, allowing air flow between them. In the event of a collision the arms would lock into their current position, as if the air around the passengers had abruptly frozen solid. The result would be to spread any impact over the entire surface of the passenger's body.

While the foglets would be micro-scale, construction of the foglets would require full molecular nanotechnology. Hall suggests that each bot may be in the shape of a dodecahedron with 12 arms extending outwards. Each arm would have four degrees of freedom. The foglets' bodies would be made of aluminum oxide rather than combustible diamond to avoid creating a fuel air explosive.

Hall and his correspondents soon realised that utility fog could be manufactured en masse to occupy the entire atmosphere of a planet and replace any physical instrumentality necessary to human life. By foglets exerting concerted force an object or human could be carried from location to location. Virtual buildings could be constructed and dismantled within moments, enabling the replacement of existing cities and roads with farms and gardens. While molecular nanotech might also replace the need for biological bodies, utility fog would remain a useful peripheral with which to perform physical engineering and maintenance tasks. Thus, utility fog also came to be known as ″the machine of the future".

  • $\begingroup$ To be completely honest, I'd already thought of this, but didn't think it could give me the gravity/magnetism part, or how it would hold the water. Then I thought "what if the two were combined somehow?" I think that may end up being the best answer, but I want to hear if anyone has something I haven't thought of... Thank you for your answer. $\endgroup$
    – Len
    Commented Mar 2, 2018 at 19:59
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    $\begingroup$ I think they could be made to hold water. If a shell of them retracted their arms then they would essentially become a solid chunk. Take a number of overlapping layers and the gaps between them would be too small for water molecules to squeeze through. I do not know if they could be made watertight and clear though... $\endgroup$
    – AndyD273
    Commented Mar 2, 2018 at 20:21

Planets made out of water

It sounds like you're trying to build a planet out of water.

Water produces gravity the same way rocks do. Regolith is about 2 times as dense as water so an Earth-sized planet will have 1/2 the gravitational pull of the Earth. There's no need for magnets at all. If you just put enough water in the same place, at a low enough temperature, you'll create an Earth-sized ball of water that'll stick together due to gravity. You can do that with (a lot of) ordinary spaceships.

Will this water evaporate into space? Yes, and that's okay.

The Earth loses air (including water vapor) to evaporation all the time. Eventually, all of our gasses & liquid water water will evaporate away. We already lost most of our hydrogen and helium this way. Nitrogen, oxygen, carbon dioxide and water are heavier than hydrogen and helium, so it takes longer for them to evaporate. That's why our atmosphere is made of nitrogen, oxygen, carbon dioxide and water vapor.

The stronger a planet's gravitational field is, the slower its atmosphere will evaporate. The weaker a planet's gravitational field, the faster its atmosphere will evaporate. A planet with 1/2 the Earth's gravitational field will keep its water for a very long time, almost as long as Earth's will. Even a planet with 1/10 of the Earth's gravitational field will keep its water for a long time--at least by human timescales.

How much gravity do you want? The more gravity you want, the more water you should put in the same place and the longer (due to gravity and having more water to begin with) it'll take to evaporate. The colder the planet, the longer it'll take to evaporate too.


Magnets tend to be really weak because attractive force of magnets decreases at the cube of the distance between magnets. The most powerful tiny magnets we have are protons, neutrons and electrons. Their force is a rounding error compared to the electrostatic effect. (Magnetism basically is just an artifact of electrostatic forces when you mix them with special relativity.)

Usually you have to look very closely at atomic spectra even to see the magnetic effect of protons and neutrons, compared to electrostatics. The only way to get a powerful magnet is by running current through a wire (usually a superconductor) very fast. This would take a lot of handwavium to accomplish en-masse at a small scale. In addition, that many nanomachines changing the properties of water on a water-based planet could really mess up water-based biology, which depends on water behaving like water.

The attractive force of gravity and electric forces decreases at the square of the distance. Gravitic and electric forces will thus be much more effective at holding your planet together. If you filled water with positive and negative much-more-powerful electric charges, called ions, they would hold your water together better than magnets to reduce evaporation. This way you could have a smaller planet with lower gravity yet the same evaporation rate.

Positive and negative ions dissolved in water are called salt. When you dissolve salt in water, the positive and negative ions separate and stick to positive and negative ends of the water molecules. This holds the water together much more strongly than magnets could.

Magnetic shielding

For the sake of completeness, there's one case where magnetism makes sense to protect a water planet. A planet with a north and south magnetic pole gains some protection from the solar wind, a blast of positive and negative particles emitted from the sun which can knock air molecules away from a planet. However, gravity is a far more important factor in this sort of thing.

A few things to note

  • The water in the middle of a big enough ball will freeze due to pressure.
  • A planet-sized ball of water will heat up during assembly for the same reason meteors heat up when they hit the Earth. (Release of gravitational potential energy.) It'll take some time to cool off.
  • Electromagnets work by reducing the effect of one half of a positive-negative electrostatic pair. In this sense, the most powerful magnets are just a weak, inefficient, roundabout way of producing electrostatic forces. You'll get better results by using electric charges directly.
  • Water remains at a liquid state when it's at an appropriate pressure and temperature. If you want to preserve Earth-like temperatures then you also have to preserve Earth-like pressures. Pressure is the weight of all the air above you. Decreasing the gravity of a planet reduces the weight of the air, so you'll have to add air to compensate. Thus, a low-gravity water planet will require a thicker atmosphere than Earth's. (Though much of this atmosphere may consist of water vapor, depending on the temperature, pressure and gravity, as well as what other elements are present in the atmosphere.)
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    $\begingroup$ Uh... holy $#!@... thank you. Great answer! Very helpful. p.s. My initial idea was that this water planet was artificially created with hand wave super science via using the magnets to grab the water and amass it until it reached the planetoid size. Once the mass of water was big enough to create its own gravity (and was relatively stable) the magnets would leave the water and clump into the core; becoming the core that creates the magnetosphere. Possible? $\endgroup$
    – Len
    Commented Mar 6, 2018 at 19:35
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    $\begingroup$ What you're describing is (almost) literally how the Earth formed.The Earth has a magnetic core. $\endgroup$
    – lsusr
    Commented Mar 7, 2018 at 0:42
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    $\begingroup$ Glad to be assistance in worldbuilding. What you're describing is actually pretty similar to how the Earth works. Heavier substances which produce the magnetosphere sink while lighter ones (such as water) float. The hard part about building a planet isn't getting the water to stick together (it does that on its own once it's close enough) but to get it together in the first place. Magnets can't get water to clump together over long distances because magnets don't work at long distances. You'd get better results building the planet with a coilgun (AKA Gauss rifle) around a magnetic core. $\endgroup$
    – lsusr
    Commented Mar 7, 2018 at 0:52
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    $\begingroup$ So the Earth was formed because the gravitational force of its metallic core attracted water to it. Then the magnetosphere helped protect the resultant atmosphere. $\endgroup$
    – lsusr
    Commented Mar 7, 2018 at 0:59
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    $\begingroup$ Go ahead and spin the core. You'll want to spin the whole planet (water & core) anyway to create day & night cycles. Besides day & night cycles, the only noticable effect you'll get from spinning the core is the Coriolis effect, not convection. This has important large-scale effects on things like the shape of hurricanes, but that's about it. It won't produce heat because heat is created when things rub against each other, and a spinning planet is all rotating together at the same angular speed. If you want to keep the planet warm, make the core radioactive, like Earth's. $\endgroup$
    – lsusr
    Commented Mar 7, 2018 at 21:42

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