# Is this gas giant possible?

I want to create a gas giant planet. The parameters I would like for it to have are:

It should have a large layer of gas within its atmosphere that living creatures including humans can survive and thrive in, and I would like for that area to have zero-g. It should not be ridiculously windy. And it should include plant life and water.

I’m assuming that such a gas giant would be within its solar systems Goldilocks Zone, but it doesn’t have to be.

Is this gas giant possible?

• "Zero-g": Gravitational fields extend infinitely far. For example, our solar system is gravitationally bound to the Milky Way. The Milky Way itself is gravitationally bound to the Local Group, a structure consisting of 54 large and small galaxies, with a diameter of 10 million lightyears. – AlexP Jan 17 '18 at 18:02
• Is it possible that by zero-g it was meant that humans should sense buoyancy due to handwave to the extent that they float in mid-air? – NofP Jan 17 '18 at 18:11
• Do you want a setup such as the one in Larry Niven's The Integral Trees? – Spencer Jan 17 '18 at 18:49
• @AlexP, I meant microgravity. Apologies. – Len Jan 23 '18 at 21:39
• @NofP, yes, floating was what I meant. – Len Jan 23 '18 at 21:40

In Physics, Gravity obeys the Inverse Square Law. This means that the intensity is inversely proportional to the square of the distance from the source of gravity.

Because of this equation for Gravity, which is continuous in $r$,

$F_g=GMm/r^{2}$

a band of zero gravity for a gas giant is impossible, as it would represent a discontinuity. The only way to have a zero gravity environment is to have no gas giant in the first place.

But, taking a step back to make this work:

You can use other forces to counteract Gravity to have a net zero-g environment. Take a look a this question regarding the feasibility of buoyancy as that counteracting force:

Floating a boat on a gas giant

Regarding using another planet as that counteracting force, the result would be a single point in which there is an unstable equilibrium, meaning that an "area" of zero-g would not exist.

• As mentioned in my answer, you would not have "a 2D plane of zero-g," there is only a single point between the two where gravity cancels perfectly. But there would be a large volume where gravity is negligible compared to what is otherwise achievable in proximity to a gas giant. – Lex Jan 17 '18 at 19:05
• Right, correcting that in my answer. I was thinking of a scalar, which gravity is not. Thanks for the catch! – Austin A Jan 17 '18 at 19:12

Rocheworlds

A Rocheworld is a pair of equally sized planets that orbit so closely around there common center of mass that their atmospheres touch. At that point of contact the gravitational pull from the two planets cancel. This would give you a microgravity environment. This is the L1 point of the two worlds and thus would have the gravitational gradients you would expect at L1. This means nothing would be stable there, denser then air organisms would fall toward the worlds, and lighter than air organisms would be pushed away from the center of the envelope. But these are minuscule forces that an organism with any level of activity should be able to overcome. It does mean floating islands and masses of inert vegetation are probably out.

On whether this configuration of worlds is possible, this question addresses sharing an atmosphere. To summarize, if the pull of the worlds on each other is to great that they tear themselves apart. There is a very slim margin in which two terrestrial worlds are close enough to share atmosphere but not destroy each other. But we don't care about the surface of the worlds so I think the better option would be two gas giants. Their thicker atmospheres would mean more overlap and there much larger volume would mean the area of microgravity would be much larger. Of note is that at the far end in this direction, stars that share an atmosphere, there are observed cases of their existence.

Speculative Environment

As for the biology, this is how I envision the ecosystem working. The amount of sunlight reaching the area of micro gravity is small due to the worlds eclipsing it on either end. You could have the orbital plane of the planets perpendicular to the plane of the ecliptic which would allow more light but I feel would reduce stability in an already tenuous system. Putting the worlds closer to their parent star would also hurt stability. My solution is to cover the gas giants with photosynthetic algae. The high gravity on the worlds themselves will preclude the development of complex life which leaves the algae without any predators so they can flourish. Air currents pass these algae across the bridge between worlds, possibly even concentrating the nutrients. This gives a food and oxygen source for the complex life in the microgravity environment in between the worlds.

As mentioned earlier, no organism here could be inert, every living thing that can't survive the high gravity of the worlds below, or the vacuum to the sides, would need to constantly be making micro adjustments to keep itself near L1. This could lead to a king of the hill type power struggle, where organisms are vying to be closest to L1.

Another possible plot point is the storms in such a system. I don't know exactly what they would be like, but I am sure they would be interesting.

Note: Rocheworlds have appeared in fiction, most notably Robert L. Forward's series which coined the term. Though I am unaware of any cases where the inhabitable micro gravity environment has been a key point. In part because in fiction the pair of worlds are usually terrestrial making the bridge between them much smaller in volume.

Note: mostly copied from my answer to Can an inhabitable zero-G environment occur in nature?. Since my answer used gas giants it fits here.

• But wouldn't a Rocheworld scenario be temporary at best? As the two worlds pass by one another they share an atmosphere. But as they pass away from one another their atmospheres would snap back to their own planets, or dissipate in space, or whatever happens. I would need something that would last much longer. I mean, what are the odds of finding two planets, gas giants, that would stay together in some kind of locked orbit, that life could exist in that shared space? P.S. I apologize for making you give the same answer twice. Thank you. – Len Jan 17 '18 at 19:44
• Rocheworlds specifically refer to the case where they are a single planetary system, like the Earth and the moon. Given their proximity they would almost definitely be tidally locked. As to their existence, it is worth looking at contact binaries, the stellar equivalent. They have been observed multiple times and are believed to be stable over billion year time spans. – Lex Jan 17 '18 at 20:02
• Have a look at this, Could gas giants exist as a contact binary – Austin A Jan 17 '18 at 20:07
• Thanks @AustinA I hadn't realized that existed. The first half of the accepted answer is a good explanation of how gas giants would differ from stars, but its math for Roche Limits is broken. – Lex Jan 17 '18 at 20:38
• Yeah i agree, he uses the approximation formula for a rigid body; however, even with the fluid satellite calculation, with constant 2.44 instead of 1.26, the gas giants are still just in the Roche Limit. (I'm pretty sure) – Austin A Jan 17 '18 at 21:23