# Gravity and a massive gaseous sphere in space

Some research, and some answers from others, have suggested that a massive (planet sized?) sphere of gas in space, with no hard core, but breathable air inside some part of it, can create enough of its own gravity to keep it stable (hopefully long enough for life to form there).

The massive sphere of gas would exert its own gravity thus keeping the gasses contained, even without a "cap". Inside the sphere one would be pulled toward the center, but once at the center one would experience a sense of no gravity since the gravity would be pulling on you from all directions. That is if the center doesn't have massive pressure. Or would it?

Also, if the sphere were close to a sun the gasses would remain gaseous, be warm enough to sustain life, and have light. There might be water there too.

I'm desperately trying to create a zero-g world/environment, that has native animal life in it, that humans can visit and live in. Is any of this possible?

• There exist massive gaseous spheres in space. Some of them are stars, others are planets like Jupiter. None of them are very suitable to life... Jan 18, 2018 at 16:03
• It all depends on the size of the "gaseous sphere". For example, near the core of Jupiter the pressure is so high that hydrogen exists as a liquid metal. Jan 18, 2018 at 16:15
• Do gas planets have to be giants? Or could there be a smaller planet, the centre of which is not so pressurised? Jan 18, 2018 at 16:34
• @len - I'm sure someone a bit more math inclined would be able to put numbers to it...but I'm not 100% sure. 2 thresholds to be wary of : you need enough gas to give enough gravity to keep the ball cohesive, but you need little enough gas to prevent the gas in the center from becoming a solid due to pressure. I get the feeling these are a long way off...enough gas to maintain it's structure is far too much to not have a solid core of compressed gas (so to the question, the core would have massive pressure) Jan 18, 2018 at 16:53
• @BrettFromLA, done!
– Len
Jan 18, 2018 at 20:00

There are a few problems in what you describe, but there is also a solution

## Pure gazeous planet means a blob at the center

One problem of what you discribe is that there is only ONE center in a planet. If you fall to the center, then the next animal in this world will fall on you.

Then the next...

And so on....

## There would be a dense core anyhow

For the same reason, dust and asteroid would accumulate in the center, creating a dense core (possibly liquid)

## But you still can have super-low gravity

For a sphere, gravity equal C.m/r²; where
- C is a constant equal to G/(2*π) ≈ 10−11 m3⋅kg−1⋅s−2
- m is the masse within the said radius

So, if you have a solid core the size of Ceres surounded by and arbitrary amount of gas, the surface gravity will be similar to Ceres gravity (0.03g)

Ceres has a 500km radius. It would be dificult to have much bigger core without significant gravity.

With such a low gravity, I'd say that earthlike air pressure would allow a human to "swim" in the air. If you increase the pressure, gravity would become secondary ( just like the solid but tiny particules that make smoke get up despite gravity )

But as you get higher, so does the gravity (as the mass of the air beneeth you increase). You can't swim or fly as high as you want.

.

note: I'm afraid English is not my native language. Feel free to edit if you have rights

• Great answer! I'm just going to nitpick a little: Cere's escape velocity is about the average speed of air molecules at room temperature. You need a bit more gravity to hold the atmosphere. But if you are willing to go for a bit more gravity, you could dump all of the Earth's atmosphere there... It is two orders of magnitude less massive than Ceres itself. And yes, you would be able to fly by flapping cardboard wings. Would be very awkward, though. Jan 18, 2018 at 19:28
• Water is the most vulnerable, and long term water vapor having escape velocity pretty certainly kills everything we know, but maybe something exotic uses a big fluoride or chlorine molecule or carbon chain. Re-reading the question looks like that's not really apropos here though.
– user25818
Jan 18, 2018 at 20:56
• You are probably thinking of a bubbleworld. Dani Eder calculated that a sphere of gas 240,000 km in radius, capped with a protective layer that supplied enough gravitational counter pressure to balance the pressure of the gas within would create a layer of living space with Earth like pressure and temperature near the surface with 1400 times the surface area of the Earth, and about a million times the useable living volume. See yarchive.net/space/exotic/bubbleworld.html Jan 18, 2018 at 23:14
• @Renan But that woud be the surface escape velocity. As the core is surounded by much gas the gravity in altitude is higher. The escape velocity follow the rules of a gazeous (giant or not) planet Jan 19, 2018 at 9:39
• @madlozoz Earth's surface escape velocity.is 22.times greater than Ceres and Earth still loses a ton and a half of atmosphere per day. It is not that by having an average molecule speed close to escape velocity will cause an atmosphere to go into space in a flash - most molecules collide with each other in the way and change directions - but some molecules in the upper atmosphere will reach speeds way greater than whatever escape velocity you have. Over geological time, such a "tiny" gas giant would have no atmosphere real quick. Jan 19, 2018 at 10:20

Zero Pellucidar

I have been pondering this question. A sphere of xenon, dense enough for animals to be buoyant and large enough to have gravity? You would need calculus to model gas density at any depth and ultimately you have made a gas giant. As regards cruising around in a gas giant, XKCD has had the last word. A nebula? Too nondense. Some space vortex! I think a vortex would hold together in space, but it would slow rapidly from frictional energy losses. Powered by solar wind?

The OP wants earth like atmosphere in a zero-gravity environment. Like the space station. The space station contains its atmosphere in a hollow shell. That is the solution. Your world is inside a hollow planet, and inside the shell of this planet there is no gravity.

A hollow symmetrical sphere exerts no gravity on objects within, regardless of how much gravity an object outside on the surface might experience. https://en.wikipedia.org/wiki/Shell_theorem

This concept got a little bit of attention on WB stack here. Does the thickness and diameter of a hollow earth give an affect to the overall gravity inside?

but I think lurch's explanation of how this works is excellent. I excerpt it from the Halfbakery.

http://www.halfbakery.com/idea/Moon_20VLO_20Device

But the object inside a hollow sphere experiencing no gravity - that does not seem right. An object in the exact center of Pellucidar would experience no gravity as the pull from all sides would cancel out, just as the object falling thru a tunnel piercing the earth will oscillate back and forth until coming to rest at the center.

Would not an offset object (here the interior orbiting device as > >proposed) experience more pull from the nearer wall than the farther (inverse square and all)? — bungston, Feb 29 2012

The object experiences more pull from a given mass on the near side, but >more of the mass of the sphere is on the far side.

Think of four equal masses, spaced 90 degrees around a circle. Put, on a line between two of them, an object, closer to one than the other. That nearer body excerts more pull than the farther. However, the two to the sides cancel out each others lateral pull, but also add resultant force away from the near object. Once you extend this to a sphere (and do a bit of calculus) all the resultant forces cancel out. — MechE, Feb 29 2012

Your hunch about inverse square distance is correct, but it has another effect you've neglected.

OK, you are about to be placed inside the shell of a hollow planet. You > will not be in the center, but far off to one side. You are given a laser pointer - not some puny dollar-store thing, but a megawatt-class laser with beams coming out both ends. Be careful with it!

Now, taking your Darth Maul laser-pointer, you point it in any direction >you please, and move it around to draw a shape. You will notice that if you point one end in a direction where the shell is close, then the other end is directed at a spot far away. Consequently, when you draw your shape, the close end draws a small shape; the far end draws a big shape - but they subtend identical angles.

If you now calculate the gravitation exerted on you by the two indicated >areas, the close (small) one exerts more force on you per unit mass than the far (big) one - because of the inverse square law as you had presumed. However, the "per unit mass" caveat, when applied to the smallness/bigness of the two opposed figures, cancels precisely.

It works for any opposed pair of shapes you can draw, from any position >inside the shell, as long as the shell is uniform.
— lurch, Feb 29 2012

Consider such a planet with a colony on the exterior surface. There is a door in the floor of one building. On opening it we see people floating around down there; they are on the interior side.I wonder what the experience would be of coming up through that door from the weightless side. — bungston, Mar 16 2012

//coming up through that door from the weightless side.// huh... cool. >I'd never thought of it quite that way - it would feel like an anti-gravity repulsion field was trying to keep you away from the door. (Actually gravity toward everywhere except the non-existent mass in the shaft to the doorway, so the imbalance would seem like a repelling field.)

"Shut the door so I can get out!" — lurch, Mar 16 2012

This can be your world: a hollow planet inside of which there is trapped atmosphere but no gravity. The cool thing - as noted above, if you open a door to the outside, Pellucidar will try to keep its atmosphere inside.

• I love this answer, even though its a separate environment/creation. Thank you!
– Len
Jan 23, 2018 at 21:02

You should read the books "The Integral Trees" and "The Smoke Ring" by Larry Niven.

There is a very brief description on Wikipedia here and a slightly longer one here.

The story occurs around the fictional neutron star Levoy's Star (abbreviated "Voy"). The gas giant Goldblatt's World (abbreviated "Gold") orbits this star just outside its Roche limit and therefore its gravity is insufficient to keep its atmosphere, which is pulled loose into an independent orbit around Voy and forms a ring that is known as a gas torus. The gas torus is huge—one million kilometers thick—but most of it is too thin to be habitable. The central part of the Gas Torus, where the air is thicker, is known as the Smoke Ring. The Smoke Ring supports a wide variety of life.

It's old now but based on hard science at the time and so far as I know the base concept is still viable, if unlikely.

You can have a gaseous cloud in space, but it will be subject to some limitations.

The most stringent of them is that a cloud with breathable density and Earth-compatible temperatures and pressure is unstable whatever its size, and will therefore undergo gravitational collapse until the pressure increases enough to balance the gravitational pull. This could be delayed if the cloud is actually a rotating torus, its "internal gravity" spread out around the whole orbit, and this is the mechanism of Larry Niven's Smoke Ring (also, there he has a gas giant resupplying the Ring with atmosphere leaked into space).

Vast volumes of Earth-compatible conditions could exist inside an interstellar gas cloud for a limited time (a few centuries) during a collapse; then the inhabitants would need to flee towards the borders, and they would start to feel the gravitational pull of the central accretion mass.