A planet (as well as a dwarf planet) must, according to the IAU definition, have sufficient mass to assume hydrostatic equilibrium (a nearly round shape). Does it mean they would break apart or explode if gravity vanishes, unlike small bodies like comets or minor planets, whose integrity doesn't depend on gravity?
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1$\begingroup$ Small bodies stay in shape by the same force that a rock on Earth stays in shape: mechanichal strength, i.e. chemical bonds that originiate from compression at formation time (or chemistry in the case of the rock). For bodies like 67P, for which the mechanical strength is quoted as that of 'cigar ash', so not much compression can happen without strong self-gravity. $\endgroup$– AtmosphericPrisonEscapeDec 22, 2019 at 9:22
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4$\begingroup$ Questions that ask what the laws of physics say happens in the face of a violation of the laws of physics don't quite make sense. $\endgroup$– David HammenDec 23, 2019 at 3:32
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3$\begingroup$ We normally close questions that ask about impossible hypothetical scenarios. But I suppose this one has some instructive value, because it gets us thinking about the other forces that are normally balanced by gravity, and it has resulted in some interesting answers... $\endgroup$– PM 2RingDec 24, 2019 at 3:20
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1$\begingroup$ I'm voting to close this question as off-topic because "without gravity" is purely fantasy $\endgroup$– Carl WitthoftDec 24, 2019 at 17:20
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3$\begingroup$ @LeosOndra I think you have been dealt with unfairly and unkindly in comments. It's very clear from your other questions here and in other SE sites (e.g. Physics SE) that you take science seriously and you used "gravity vanishes" as a Gedankenexperiment (also this) to explore the concepts of hydrostatic equilibrium, and not to explore "pure fantasy". That you have generated six answers in which some important points can be expressed indicates that this is by definition a good question! $\endgroup$– uhohDec 30, 2019 at 5:47
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6 Answers
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Even rocky planets would explode. I think there are two ways to see this.
From the perspective of forces, the earth is in equilibrium between the compressive force of gravity and the elastic resistance to compression of the materials that make it up. By Newton's third law, the mantle is pressing upwards on the crust with a force equal to the weight of the crust. If gravity disappears, you will still have that upward force with no downward force to balance it, so you will get upward acceleration.
You may think that the tensile strength of the crust will hold the planet together. But the upward force is the same as the weight of the crust. We know that very large stone structures cannot support their weight under tension.
You may also think that iron and rock aren't very compressible, so the mantle wouldn't expand very far. But the pressures are very great. The core has a density at least 25% greater than iron does under normal pressures. Plus, the temperature is around 6000K, equal to the surface of the sun and much higher than the boiling point of iron at atmospheric pressure. So the expansion won't just be a matter of some cracks forming.
From an energetic point of view, the potential energy of all the parts of the earth spread out across the protoplanetary disk was converted to heat and elastic potential energy when the earth formed. At least one study suggests that about half that heat remains, and in any case heat from radioactive decay has been added over time. If all the energy remained, it would be enough to disperse the earth with the same velocity that parts came together on average when it formed. Given that the energy is at least on the same scale, I think it's reasonable to expect an explosive breakup.
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1$\begingroup$ This might be true for Earth but a generic planet can have a lower inner T. Even with a warmer inner core we are dealing with molten stuff non gases. A table able to sustain one ton doesn't launch a book away. $\endgroup$ Dec 24, 2019 at 5:34
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The hydrostatic equilibrium only dictates that the object holds a spherical shape, it does not determine whether the matter will in a solid form.
Gravity acts on planets, dwarf planets, comets, minor planets, and asteroids to keep them as a coherent form. Gravity acts on all matter in the universe. On smaller scales, human scale and smaller, other forces dominate over gravity, but larger than that, gravity begins to take over.
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2$\begingroup$ Hydrostatic equilibrium means that pressure gradient must counterbalance gravity force - that is pressure must increase to the center of the body. In the Sun, which is made of gas (basically), central pressure is some 2x10^11 bars, so without gravity the star would exlode. A small peeble floating in space holds together with other electromagnetic force, not gravity. So without gravity nothing changes. I seek boundary between the two cases. $\endgroup$ Dec 21, 2019 at 21:28
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1$\begingroup$ "Without gravity the Sun would have never formed". Naturally, but I am interested in what would happen if you (hypothetically) turn gravity off. $\endgroup$ Dec 21, 2019 at 22:38
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$\begingroup$ As I said on human scale and smaller gravity is not the dominant force, so your pebble comment is not relevant. I go back to my earlier comment that comets and minor planets do depend on gravity for their integrity. I think you should edit out the comet and minor planet part of the question so as not to have such glaring inaccuracies within the question. $\endgroup$– Bob516Dec 21, 2019 at 22:51
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1$\begingroup$ I think the answer of what would happen to a planet verses a star is vastly different as they are made up of different states of matter. Check with Physics SE on that, I might be wrong. $\endgroup$– Bob516Dec 21, 2019 at 23:45
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$\begingroup$ Up vote because I also said so. Surprisingly my answer moved from +2 to zero so I hope this one stay on the positive side $\endgroup$ Dec 27, 2019 at 7:55
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i guess if there were no gravity then the planets would not have been formed in the very first step because they are formed, as are stars and all other celestial objects, after the collision of particles which formed clusters, which then acquired sufficient gravity to pull more mass.
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1$\begingroup$ You are right, without gravity there would be no galaxies, no stars, no planets and no human beings. But my original question is not about world without gravity from the beginning. I ask what would happen if you turn gravity off. $\endgroup$ Dec 22, 2019 at 18:28
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$\begingroup$ large bodies like stars will probably explode due to the immense energy radiated by their core... i guess it's depending on the size of the body whether it will explode or not... according to me the force by which they will explode will be directly proportional to the mass $\endgroup$ Dec 23, 2019 at 9:56
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No, rocky planets would not explode. However, without gravity there would be no force to make the object round though. Atmospheres would escape to space though. Since air pressure is quite nice gradient from vacuum to normal air pressure, I would assume no major explosion would occur if gravity suddenly vanished.
Gas planets however would indeed explode rather violently. Their cores are degenerate, and thus particles do have a lot of kinetic energy. Without gravity this energy would need to go somewhere.
EDIT: gas giants would explode, rocky planets mostly not
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$\begingroup$ I disagree, gasses would no longer be held in by gravity, and likely depressurize rather explosively (like how an over pressurized bottle can burst once the bottle looses its integrity). $\endgroup$– JonathanDec 22, 2019 at 17:38
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1$\begingroup$ Thanks for the comment. IMHO, overpressurized bottle would be in a sense any ordinary glass full of water on a table on Earth, if you switch gravity off (gravity of the planet). The reasons is hydrostatic equilibrium of water (the pressure increases toward the bottom). $\endgroup$ Dec 22, 2019 at 18:25
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1$\begingroup$ I stand corrected @Jonathan. I was just thinking about rocky planets. Gas giants would boom pretty intensively I think $\endgroup$– tuomasDec 22, 2019 at 18:32
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While, as far as we know, this is impossible, it is interesting to consider what would happen. Most likely the planet would break apart and possibly, depending on several factors, explode. Rotation speed of the planet, strength of the planet, and presence of liquids or gasses on the planet would all come into play.
Here is how I would imagine it playing out on Earth. Without gravity binding the atmosphere to the Earth, it would fairly quickly expand out into the vacuum of space. The oceans would then boil away without atmospheric pressure holding them in place. Without the pressure compressing the magma inside the earth, it too would likely out gas and push the crust of the Earth apart. The centrifugal force from the rotation of the Earth (usually more than countered by gravity) would also aid in flinging the Earth apart. In the case of Earth, I would imagine this would be fairly explosive, and would be rather spectacular to watch from space. I do not know how fast it would explode, but it would almost certainly at least break apart, and probably explode. I would imagine that only the iron core would remain intact (if even that).
A planet that is not rotating quickly, and that is solid all the way through might still hold together (e.g. possibly Mercury), but it may still break apart depending on how much gas (if any) is trapped inside.
For Saturn, its rings would stop orbiting, and expand out into space. The gas would expand out into space as well, not being held by gravity. I believe this would happen very explosively, since the internal high pressure of the gas is now no longer resisted by gravity. It would likely completely explode into space, except for possibly a rocky core (which would also likely be blown apart by gasses trapped inside, and by the force of the explosion). Centrifugal force would also likely come into play, but I would imagine the explosiveness of the de-pressurized gas would be a much greater factor. This would definitely be interesting to watch from a distance!
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1$\begingroup$ Just a note to the Saturn rings. Without gravity every single piece of ice would tend to flight in a straight line along tangent to its original circular orbit. But the inner pieces have greater velocity so they would collide with outer pieces.But in long term a piece would either fall to the planet (but it would break apart as well) or move to outer space. $\endgroup$ Dec 22, 2019 at 18:33
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2$\begingroup$ @Leos Ondra again there won't be gravity making the pieces fall back. $\endgroup$ Dec 23, 2019 at 10:06
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$\begingroup$ @Alchimista You are right, I used wrong words. So: "a piece would either move to the planet and collide with it ...". $\endgroup$ Dec 24, 2019 at 11:22
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$\begingroup$ I think the rings would all go out into space (none of it colliding with the planet), except for the fact that Saturn would probably violently explode and overtake the rings! $\endgroup$– JonathanDec 25, 2019 at 3:46
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A solid planet is kept on place and shape by electromagnetic forces, ie chemistry. Imagine a giant piece of rock. If you turn off gravity nothing change, the big rock would expand for a certain amount, and that is. Obviously this is oversimplified. If gravity was off it could have not formed depending on its history, or formed without acquiring a nearly spherical shape.
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2$\begingroup$ IMHO, definition of planet means that planet is not kept in shape by electromagnetic forces but by gravity. That's the reason they are round. $\endgroup$ Dec 22, 2019 at 18:17
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$\begingroup$ @Leos Ondra. Right but gravity is switched off by the question. According to your humble opinion then the planet explodes or simply gets to be call asteroid or so :) $\endgroup$ Dec 23, 2019 at 8:17
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1$\begingroup$ @CarlWitthoft which one. The answer? There is no claim in it. And rotation or liquid / gaseous part are not considered. This is about a solid ball in a vacuum. On Earth is hard to imagine a chemical bonding spread from pole to pole, of course. By the way, dust particles in free fall stick to each other by means of electromagnetic interactions, too. $\endgroup$ Dec 25, 2019 at 7:46
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$\begingroup$ By how much would it expand? In what time frame? $\endgroup$ Feb 19, 2020 at 8:46