Two Planets Touching: Is it Possible?

[Gondur]

Hello,
This may not be possible
But, suppose there were two planets about the same size as Earth.
Suppose both planets touched each other.
Think of sticking two balls of ice cream together with an ice cream scoop.

Imagine walking on the surface of one planet up to the point where the two planets meet.

You look up to see an upside down world.
Would it be possible to walk on the upside world..?

 

[jerromyjon]

Imagine walking on the surface of one planet up to the point where the two planets meet.

I think I would feel very light at this point, depending on the size of the contact area you could jump from the surface of one planet to the surface of the other with very little effort.

 

[anorlunda]

Phillip Jose Farmer wrote a series of scifi books on that theme called "World of Tiers" They are fun stories to read.

 

[jim mcnamara]

Moved to GD. We went well off-topic, but the thread is okay. I guess. PS: Both planets would be torn into chunks long before they ever got close enough to "touch". FWIW.

 

[BillTre]

While not touching, Pluto and its major moon Charon orbit each other, are tidally locked (handy for jumping from one to another), and have a center of gravity outside of Pluto. The are about 960 km apart.

There are other things in space that look like you might image what would happen to two bodies barely touching, which that they would get mashed together like two snowballs. The http://www.skyandtelescope.com/astronomy-news/comets-break-up-and-make-up/. This might work with small gravitationally weak bodies. I expect something as gravitationally significant as a planet would tear apart other bodies as @jim mcnamara said.

Barely touching is likely to be a transient condition. I would not want to be too close to where they touched since I would expect the area of contact to spread.

 

[DrStupid]

Both planets would be torn into chunks long before they ever got close enough to "touch".

For what reason? The tidal forces wouldn't be sufficient. This is a rough estimation (with the gravitational field of corresponding point masses) for a contact system of two equal Earth-like planets:

There are strong deformations but the planets are far away from braking into pieces. Using the correct graviational field of the system would give a different result, but I have no doubt that the system would remain stable.

 

[Vanadium 50]

c.f. Rocheworld by Robert L. Forward.

 

[XZ923]

Just thought I'd chime in here; since the planets in question are of unknown composition I would say it's anyone's guess. For example, in Star Wars Legends there's a system called Anoth where two large planetary bodies are almost touching each other; they're basically orbiting each other in such close proximity that the atmospheres are touching. Although they never say what the exact nature of the system is, the atmospheres have such a strong electromagnetic charge that the planets don't collide with each other, but remain in this state. The repelling EM charges of the atmosphere apparently balance out gravity.

Could a planetary body produce an EM field strong enough that it could interact with a similar body to keep gravity from smashing them together? I have no idea. I can't imagine how it would be possible, but I'm not an astrophysicist. I would point out that in the movie Interstellar, there were ice clouds that should have been brought down by gravity, and that movie had Kip Thorne consulting (though IDK if he had anything to do with those clouds; my understanding is that his job was to deal with the relativity parts of the plot). My point is that in sci-fi pretty much anything is technically possible (with the exceptions of things like exceeding c ). Hence the "fi".

 

[Algr]

for a contact system of two equal Earth-like planets:

But there is no real chance of two planets being EXACTLY equal. Any difference in gravitation will place the gravitational tipping point inside the smaller planet. All the mass from this point to the smaller planet's surface will then fall away from the smaller planet to the larger one, causing a chain reaction that causes the tipping point to move further into the small planet until it is consumed.

 

[DrStupid]

Any difference in gravitation will place the gravitational tipping point inside the smaller planet. All the mass from this point to the smaller planet's surface will then fall away from the smaller planet to the larger one, causing a chain reaction that causes the tipping point to move further into the small planet until it is consumed.

I don't think that it is so easy. During the mass transfer the radius of the small planet decreases faster than the radius of the large planet increases. In the result the matter bridge and therefore the mass transfer would be interrupted if the distance remains constant. With the increasing mass and therefore the increased gravity of the larger planet, the orbit of the smaller planet would become elliptical with a decreased periapsis distance if its orbital velocity remains constant. Due to the coriolis forces acting on the mass moving inside during the mass transfer the orbital velocity of the small planet will increase. I don't know which of these effects would dominate.

Edit:

I just repeated my calculation above with an Earth-like planet and another planet with half the mass. It appears that the system remains stable as long as the surfaces do not touch. This is the result for 1 km distance:

When they touch and the mass transfer starts, the system turns into this:

That seems to confirm that there is a runaway mass transfer if the planets touch and that it stops at a certain distance. However, this calculation is just a very rough estimation with many simplifications. A full simulation of the process would be much more complicate.

 

[Algr]

If a planet lost mass to its partner it would cease to be tidally locked. It's rotation would remain the same, but the speed of its orbit would speed up due to orbiting a heavier planet.

 

[DrStupid]

If a planet lost mass to its partner it would cease to be tidally locked. It's rotation would remain the same, but the speed of its orbit would speed up due to orbiting a heavier planet.

That's one of the assumptions used for the calculation above. But it is just an assumtion. It would require a complex simulation to check if this really happens.