What if two planets stuck together?


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OK, so...

my dad and i were watching star trek: into darkness for the first time last night. at the end of the movie, while the credits are running there is a montage of planets, some with very odd things happening to them, like having the entire inside blown away.

this lead him to ask the question above. by this he meant two planets that fit a couple of parameters, simply for the sake of discussion. they are relatively the same size, and relatively earth-like. the way he explained it is almost like if Pluto and Charon touched. having another parameter be that they would not annihilate, again, for the sake of discussion. a few of the questions were things like.

how would water behave on the surface?
how would gravity feel with both bodies?
could life exist(beyond microbes)?
how would magnetic fields interact?
how would molten cores interact outside of magnetism?

this spurred an interesting conversion between the two of us, and i hope it sparks one here. we really enjoy thinking about weird hypotheticals like this, i hope you do too.

The Exchange

You are talking about two planets forming a 'peanut'.

I don't know where water would settle. If it settles at the point of greatest gravity that would be at the ends opposite one another...the force of two planets of gravity pulling the water 'down' only occurring at the very ends of the peanut. At a certain Point their gravities are working against one another.


Of course, in real life they would crash and annihilate one-another, and eventually after millions or billions of years, they might coalesce into a single sphere. The crashing part being just like in Melancholia (a film co-starring Kirsten Dunst).

But for the sake of discussion, I'd like to point out the movie Upside Down, with Jim Sturgess and, again, Kirsten Dunst. While not the greatest flick ever, it actually revolves around the premise of two worlds, locked gravitationally, just a couple of miles at most, from each other, and the interactions of people who go from one world to the other.

It doesn't deal with the science of such things, instead substituting some McGuffins of its own. But it does have some pretty cool scenes of characters going back and forth.

As to your questions, there are many possibilities, depending on the nature of the two worlds. Not every world, for instance, has such a protective magnetic field as does our own Earth. If one of those worlds were giving off dangerous radiation, you might expect it to possibly strip the atmosphere from the other one, causing some amazing auroras on the way to doing so.


all kinds a cool stuff happens.


One of the core definitions of the word "planet" is that it is a body that has sufficient gravitational mass to form itself into a spherical shape. So if two such objects were to "stick together" they would quickly form into a larger sphere containing the mass of the two planets. This process actually would involve some quite complex physics and physical chemistry, but the best guess is that lighter elements (silicon, oxygen, hydrogen, carbon, etc.) would tend to "float up" as they did in the early earth so there would be a reasonable chance that once the heat of the gravitational compression radiated into space, you could potentially have oceans and continents and stuff. But the surface gravity would be greater than either of the two original planets.

If they HIT each other, all kinds of wonderful things can happen, such as part of the resulting mass can be jettisoned out and go into orbit, forming another "planet". This is exactly what planetary scientists think happened to the early earth when it was hit by a Mars sized planet and the moon was formed (this also explains why the moon has less iron or heavy elements than earth).

There is no plausible way in our own universe for a planet sized body to adopt a non-spherical shape. Of course in the Pathfinder universe I suppose anything is possible.

Shadow Lodge

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Bruunwald wrote:
I have a crush on Kirsten Dunst


Well, first off there's a distance within which neither planet can orbit as the gravity/mass of one or both will tear said one or both apart. Not sure of the name 'Scheartzelt'?

So... that puts a limit on one end of things. Then, you'd just have to work out the speed at which both bodies orbit their central point.

Just some thoughts.

Liberty's Edge

Pathfinder Adventure Path, Rulebook, Starfinder Roleplaying Game Subscriber

The Roche limit; basically, the difference in orbital velocities between the nearest and furthest parts of the orbiting body is enough to shear the body apart.

Silver Crusade

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Rocheworld by Robert L Forward.

RPG Superstar 2013 Top 8

Yeah, read Rocheworld. There were two planets rapidly orbiting each other at a close distance, and at times all the water from one would move to the other one based on where they were relative to the sun. (IIRC, it's been decades.)


Awesome! Learned something AND got references for good reads. :)


The Rocheworld model is also unstable and would rapidly become a single spherical mass as well. Maybe a perfectly balanced arrangement of two planets in such a close orbit could survive for a thousand years or so, but I doubt it. Even if the normal physics effects don't make it unstable long before that, relativistic effects would intervene soon enough.

I did read Rocheworld, it is a decent read.

Sovereign Court

melancholia was the most beautifully depressing movie ever.


akolbi wrote:

OK, so..

my dad and i were watching star trek: into darkness for the first time last night. at the end of the movie, while the credits are running there is a montage of planets, some with very odd things happening to them, like having the entire inside blown away.

this lead him to ask the question above. by this he meant two planets that fit a couple of parameters, simply for the sake of discussion. they are relatively the same size, and relatively earth-like. the way he explained it is almost like if Pluto and Charon touched. having another parameter be that they would not annihilate, again, for the sake of discussion. a few of the questions were things like.

how would water behave on the surface?
how would gravity feel with both bodies?
could life exist(beyond microbes)?
how would magnetic fields interact?
how would molten cores interact outside of magnetism?

this spurred an interesting conversion between the two of us, and i hope it sparks one here. we really enjoy thinking about weird hypotheticals like this, i hope you do too.

See "Thundarr the Barbarian" for the answer to all your questions.

Grand Lodge

Well rogue planets are real, so it's a possibility that earth could strike another planet, but the likelihood of it is astronomically slim. I wold imagine that the light (flaky) crust would be devastated (as well as liquefied) as the dense cores would move towards one another and merge. Life wouldn't stand a chance (perhaps some extremophiles), and the water would evaporate but gravity would keep most of it locked in the new atmosphere until the newly formed world would cool down. It could change the planets chemical composition, air pressure, gravity, electromagnetic field, orbit around the sun and spin, the moon, and likely more. It's hard to say if life would be able to once again take hold on the newly formed world.

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