Good answer. Aspects I never thought about before. but you explanation makes complete sense.

I posted a link below where a team ran some simulations and saw that it's possible for a terrestrial binary planet system to form. Granted they wouldn't be touching one another, they would become tidally locked and share a center of gravity with the distance between them not being much more than the diameter of 1 of the planets. Given that, I think it would be possible for them to eventually impact in a way that wouldn't destroy the structure of both planets.. at first. Over time, gravity will most likely merge both planets into 1.

I also haven't looked into their actual research. They may have been plugging in unrealistic scenarios to fit their agenda..

@FatherOfNyx -- A binary planetary system is certainly possible. The Earth/moon system is approaching that. However, there is no way the two bodies could get close enough to make contact before they were torn apart by tidal forces. There is no way to avoid these mechanical restrictions.

[tobyrsmith.github.io]

However, there is an interesting case that occurs when the two objects are of equal density. The Roche limit is then 1.26xR in which case both objects will have made contact before the limit comes into play. That is to say, there is a point at which the densities of the bodies in question will cause them to kiss at the exact point when the Roche limit is reached and the amount of contact will increase until the densities of the two are equal. That point is obviously when the calculation results in a distance of 2R. The density of the primary exceeds the secondary body by just enough to reach that point. As the densities of the two approach equilibrium, the distance will decrease until the limit of 1.26R is reached.

@evidentialist The second part is kind of confusing. Because you started off saying the 2 objects would have to have equal density, but then end by saying the amount of contact would increase as their densities equalize.

But that's kind of what I was referring to. Planets that are basically equal in almost all characteristics. I know that this wouldn't be possible with 2 planets with differing mass/density. 1, if not both, would get shredded for sure. I can't seem to find it now, but earlier when I was looking, I found a forum with someone asking the same thing. There was someone who said pretty much the same thing with a Roche limit of 1.26.

@FatherOfNyx -- Yep, that's the whole deal, and I don't always achieve the clarity I want in my descriptions. I understand the confusion there. Suffice to say that when the density ratio results in a distance of 2R for objects of equal radii and density, the limit has been reached. Two objects, say of Earth mass, will collide and both will be destroyed as they meld. The result will be a molten ball approaching the mass of the two objects, but there will be considerable mass thrown off from the collision. Some of that mass might achieve escape velocity, but most of it will likely form rings around the new body. Elements of the rings inside the Roche limit will eventually fall back to the surface. Elements of the rings beyond the Roche limit could, in time, coalesce into a moon or moons depending upon the makeup of the rings.

I mean an impact that doesn't rip a large chunk out. If you've seen photos of Ultima Thule, you'll see it's 2 objects that slowly impacted and fused with minimal destruction. If 2 planets of similar mass slowly collided, they might be able to fuse without destroying one another. The moon was most likely made before the Earth hardened, I'm talking about 2 fully formed planets that slowly drift together.

@John_Tyrrell Never say never. 2 planets orbiting the same direction where 1 is basically chasing the other. The planet leading could have enough momentum to lessen the impact from their gravitational pull.

@John_Tyrrell [m.phys.org]

This is talking about the possibility of binary planets forming. Based on their simulations, it's possible for 2 planets to form a binary system where the distance between them is about the diameter of one of the planets. If 2 planets form a binary system where they become tidally locked and share a center of gravity, I still don't see it as being impossible that they both couldn't slowly drift to their shared center of gravity. Sure, over a long period of time their gravities will most likely make the planets merge into 1, but that could take thousands or millions of years.