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I've just come here for some help drawing a map for this fictional planet where two roughly similar planets have been smushed into kind of pear shape like this:

two planets merging into pear-shaped megaplanet

So this planet spins on an axis which runs through the center of the original planets, and their magnetic fields merge, as in the drawing:

magnetic polarity of pear planet

Would the ridge I've depicted where they meet remain an exposed mountain range, or would it fall beneath the oceans?

Also I want my alien race, zefusians, to live happily on this planet in a number of biomes such as snowy mountains, sparse forests, semiarid deserts, tropical islands and few volcanic hot spots without constantly getting natural disasters.

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    $\begingroup$ Hi Leonard, welcome to Worldbuilding! You’ve got an interesting question here but we’ll need some more information - the most pressing of which is “how much water do your planets have?” in order to determine whether they stay above sea level. $\endgroup$ – Dubukay May 8 at 3:16
  • $\begingroup$ @Dubukay I'm still thinking about ideal land-water ratio of this planet, something that doesn't cause too much drought or disastrous floods. Also I need to figure out some ideal axial tilt that northern side doesn't get overshadowed too much by southern side. $\endgroup$ – Leonard Greenland May 8 at 4:04
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    $\begingroup$ sweet! Sounds like you’re putting a lot of thought into this. May I recommend our Sandbox to you? It’s ideal for questions that are still looking for feedback on scope and you can get a general sense for some of the planetary numbers there. Once you’ve got the question narrowed down, you can migrate it back to Main and will likely get a more positive response $\endgroup$ – Dubukay May 8 at 4:11
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    $\begingroup$ I was under the general impression that if planets were to collide and merge together, they would basically heat each other up to the point where they are molten and reform into a sphere shape once everything has settled. $\endgroup$ – Shadowzee May 8 at 7:48
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    $\begingroup$ Shadowzee and Trotski are correct. The problem is that no matter how gently you 'smoosh' the planets together, if they're large enough to have anything remotely close to earth gravity, that alone will generate sufficient pressure and heat to cause the two to merge into a spherical shape. You can't maintain a non-spherical shape in anything much larger than 20km in diameter. $\endgroup$ – Morris The Cat May 8 at 12:09
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The easy answer is "yup! mountains!" the truth is more complex

Sometimes we get caught up too much in "reality." In "reality," this planet cannot exist as you've described. But what's the fun in that? Therefore, I'm going to utterly ignore all aspects of physics that would either (a) tear this planet apart or (b) force it to eventually become spherical. Instead, I'm going to answer the question from the perspective of your world and its rules.

All that mass must go somewhere

Your drawing (nice, BTW) shows about 1/3 of the small planet and 1/5 of the large planet being involved in the collision. All that mass had to go somewhere. A bit of it would be converted to energy during the fissionable component of the collision, but most of it is still there.

Where is it?

  • Some of it is pushed toward the center of both original masses, actually causing a bit of a dimple on the far side of each mass. So, you have some mountainous regions at both poles due to the collision.

  • Some of it creates a high-density region extending more-or-less to the center of both original masses. The region is sphericalish. This creates some small (probably not particularly noticable) variations in gravity where it's weaker where you might not expect it, like the southern half of the larger sphere.

  • Most of it becomes a serious ring of mountains right where you've drawn them. Sink under the oceans? I seriously doubt it! They'd be chain of mountains the circumference of the collision point that would likely challenge Everest for height. Lots-o-force involved with that collision.

But what does that mean for my oceans?

That ring of mountains is not a universal, impassable wall. Clouds will move around and through it. That means rain. Rain means erosion. And erosion means you'll eventually have mountain passes. If the planet is old enough, it's plausible to have "cracks" in that ring of mountains that would connect the waters above with the waters below. However, for the most part, the waters will be segregated.

And my biomes?

The northern and southern biomes will remain fairly segregated for a long time. Birds will get around that, and winds carrying seeds will, too, eventually. The older the planet, the more merged the biomes will become. But they will not be 100% segregated. As a wise man in a dinosaur movie once said, "life finds a way." But it finds it slowly.

Could anything have actually survived the collision?

Nope. At least it's mighty hard to believe. The earthquakes, dust, heat, yada, yada, yada... That was what I consider an extinction-level event. Cockroaches and dandelions might have survived, though.

And what about that magnetic field?

That's not actually how magnetic fields work. They don't follow the contour of solid mass unless that mass is entirely magnetic itself. And even then, they tend to smooth out. What I'm saying is that neat little tuck you show at the seam wouldn't exist. It's a small detail, though, and not very important.

What is important is that magnetospheres are thought to be created due in part to the churning liquid core — which was compromised in the collision. You can't have two independent liquid cores and over time you won't have a core shaped like your surface. Erosion, it would all smooth out inside, possibly even cooling to a thicker mantle at the north pole.

In the worst case, you'll end up with a spherical magnetosphere centered on the original larger mass that does NOT extend far enough beyond the northern pole. Lots of cancer at the north pole! Amazing Aurora Borealis, though.

In the best case the "center of the planet" shifts to something around the original tropic of Cancer1 for the original larger mass. It might very well be weaker (a consequence of that density increase I was talking about, the core is not as fluid as it once was... at least not yet. There's that age thing again).

But, it's your planet, so you might simply define the magnetosphere as you wish it to be!

So could life live there?

Sure! If it evolved after the collision or colonized after the collision. But whatever was on the planet before the collision is burnt popcorn (unless you declare it to be otherwise, it's your world).

TL;DR

The smushed belt of mountains will remain mountains forever. Eventually, erosion would smooth them out and allow some passes to form, but they would never be consumed by the sea. Too much mass involved in the collision.


1The tropics of Cancer and Capricorn aren't simply the 1/3 points of the sphere, they're defined by the solstice events, which are a consequence of the Earth's axial tilt. I'm using the Earth-reference as a literal map reference for convenience to help you imagine the point I'm talking about. Heck if I know what the original mass' tropics were, or even if it had them.

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Alright, I'm not an expert, but I've watched a few documentaries on planet stuff.... here's my best shot:

The center of gravity will move toward the point between the two planets, and the water will always drift toward the center of gravity. The mountains between the two spheres might not be totally submerged, but they will be volcanic and subject to frequent earthquakes/eruptions, so they will be growing as the planets merge together over time and become more spherical. This relatively rapid change in shape ("relatively" rapid, but still very slow to observers), will result in extreme weather patterns across the whole planet; your storms will be really hardcore, and the waves on the shores of that big ocean will be huge all the time. Sailing on that ocean will be very dangerous, and using a boat to get from one planet to the next will likely require finding some small passageway in the volcanic ring.

Any water on the far-end of the planets will only remain there if it is collected in basins, preventing it from flowing toward the joining-point. The air on the far ends of the planets will be very thin ("high altitude"), so the far ends of the planets might be icy, too, and that will prevent all the water from totally escaping. The massive ring of volcanos between your planets will warm the water there, and release a lot of steam, so depending on whether or not you have a moon and how it affects your wind currents, the areas nearby the coast (which get wind from the ocean) might be very warm and humid year-round, while other areas near the coast which don't get the warm wind, but still get shrouded in steam, might be very cold for lack of sunlight. The middle area between the joining-point and the far-end of each planet may enjoy a wide range of climates, so you could easily find all of your biomes here.

Any bodies of water on the surface of the planet will lean toward the center of gravity, so the middle region between the ends and the joining area will not likely have large bodies of water, unless they are inside large craters, or leaning up against mountains which were already on the planets. Likewise, people traveling from the middle to the end of the planets will feel like they're going uphill for almost the whole journey.

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  • $\begingroup$ the same forces acting on the water and air are working on the rock, which is why sucha shape can't persist long enough to have water. $\endgroup$ – John May 8 at 18:51
  • $\begingroup$ @John You do remember... we're developing a fictional planet where the conditions described by the OP exist -- not an actual planet where the conditions don't exist? $\endgroup$ – boxcartenant May 8 at 18:58
  • $\begingroup$ and there is not way to have such a planet without magic at which point the answer is whatever they want because magic. The question is unanswerable since the forces that control the position of land masses would not allow such a planet to exist. $\endgroup$ – John May 8 at 19:02
  • $\begingroup$ @John "Because magic" makes for boring fiction. Great fiction gives partial (plausible) explanations for things. That's why this stack exchange exists, because people want plausible, partial explanations for things. Otherwise everyone would just say, "magic", and all the sciency hypothetical questions would be in science exchanges. --- also, I'm not convinced that you're right about the limitations on planet shape. $\endgroup$ – boxcartenant May 8 at 19:04
  • $\begingroup$ And sometimes that answer is "there isn't one" This is an example of a question in which there is no way to assess the correctness of an answer. This is a question that needs ot go through the sandbox first so some basic facts can be established to make it answerable. $\endgroup$ – John May 8 at 19:11

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