# Conjoined Planets?

I was wondering if two bodies could be physically connected to each other such as the ones in the art I have attached. Is there any realism to it?

Original art by Dave Melvin

• Possible Duplicate of Peanut Shaped Planet where some of the answers may offer you some perspective on your specific variation as well. – Tim B II Feb 5 '18 at 4:30
• Please credit artworks you are using, and only use ones with compatible licence. If this is your original artwork, great, don't be shy to sign it. – Mołot Feb 5 '18 at 9:25

The two bodies are not joined. It looks like someone has stuck a smaller planet into the bulk of the larger one. The small planet is now effectively a humongously huge mountain. It will be collapsing under its own mass. All mountains are in the process of slowly collapsing. This is on geological time scales. This set-up will collapse somewhat faster due to its extreme instability.

Both planets will fuse together into a single body. Becoming eventually a spherical object in hydrostatic equilibrium. This is inevitable. It would be like a gigantic, continuous landslide with extra lava. Most definitely a more than Biblical catastrophe.

The embedded, smaller planet is far closer than the Roche limit where gravitational forces it tear astronomical bodies apart that get too close to larger ones.

If such a set-up could be created there would be some seriously interesting geological and especially seismological consequences. Seriously no-one in their right mind would want to live there.

• Most definitely a more than Biblical catastrophe. Not to worry. The initial collision would have wiped out most large lifeforms. Nobody will be around to complain about the subsequent disasters, that knew anything different. – nzaman Feb 5 '18 at 13:05

# Not for a planet, but for a smaller body.

Massive bodies are in hydrostatic equilibrium. Gravity pulls them into spheres, eliminating most non-negligible variations from perfect roundness. Obviously, protrusions like mountains exist, but they're minute compared to the size of a planet.

That said, smaller bodies may not be in hydrostatic equilibrium, and this sort of object can form. 67P/Churyumov–Gerasimenko, for example, is a comet that has a barbell-shaped nucleus. You could absolutely get a minor body shaped like this.

Image from the ESA, under the Creative Commons Attribution-ShareAlike 3.0 IGO License.

• Excellent real life example! thanks for letting us know about this cool comet :). I think this answer may be improved by adding its dimensions as a reference: (from wikipedia) "The comet consists of two lobes connected by a narrower neck, with the larger lobe measuring about 4.1×3.3×1.8 km (2.5×2.1×1.1 mi) and the smaller one about 2.6×2.3×1.8 km (1.6×1.4×1.1 mi)." Also, if there's a known maximum theoretical size for such an object, that'll be a great addition (in other words, this example tells us that an object roughly bounded by a 4x3x3km box is possible - what about a 10x10x10?). – G0BLiN Feb 5 '18 at 16:13

# There is no way to merge while remaining solid

Obviously, this applies to large bodies only. But for the Earth, anything that large that hits it will melt much of the planet.

The escape velocity of Earth is about 11 km/s. That means anything that falls into the Earth will be going at least 11 km/s. Lets say the moon ($7.4\times10^{22}$ kg) 'fell' into the Earth. This impact would release about $4.5\times10^{30}$ J of kinetic energy. The Earth's mass is $6.0\times10^{24}$ kg. Lets assume a specific heat of perhaps 1000 J/kg/K.

This impact is enough to heat the entire planet by 746 C. Ouch! Surely there would be lots of boiled off oceans, and rocks ejected into space and whatever as well.

The moral of the story is: no, your planet cannot look like that. If a moon sized object hit and Earth sized object, at the very least both objects would end up molten. Once they were molten, it would be an easy matter for hydrostatic equilibrium to pull them into a sphere. Either your moon impactor would 'bounce' and create a ring or moon around the planet, or it would merge together in one big sphere of hot lava. Also, definitely no oceans as pictured, those would be long boiled off.

Probably not, because the larger planet would rotate and make the smaller one rounded out and not a hump on it.

• I wonder if you could elaborate a little bit. It would be nice if you could explain in more detail why the rotation will lead to smoothing out of the surface. – Olga Feb 5 '18 at 7:39
• Any body larger than 200km in diameter will shape itself into a sphere. – Made_in_2004 Feb 6 '18 at 1:19