How could Venus and Uranus have a horrific collision and end up being round?

Question posed by Randy, in response to this post. You might like to read that first and then come back here.

Randy's full message was actually:

"If two objects had a violent crash, they likely would be destroyed, would they not? There are other odd-shaped objects in the Solar System such as Ceres.  How could Venus and Uranus have a horrific collision and end up being round? There is nothing in space to smooth them over. Simply spinning around would not do it. I believe things are as they were created. I believe God put Uranus on it's [sic] side and spun Venus backwards as a big slap in the face to Atheism and "man's" knowledge."

There was a bit more to it, but you get the idea. I must confess that Randy's question clearly wasn't suggesting that Venus and Uranus have collided into one another at some point in their history, but I did think the wording may attract a few more people to the post...

Two planets collide (artist's impression)
Image by Neypomuk-Studios from Pixabay 

The post that sparked Randy's ire, for those who didn't wander over and check it out first, concerns Venus and Uranus being two oddballs in our Solar System in that they don't spin around their axes in the same direction as the vast majority of the larger bodies orbiting their Sun: if you're in a godlike position and looking down upon our Solar System from "above"* most of the planets (and, indeed, the Sun) rotate anticlockwise but Venus spins clockwise, and Uranus spins on its side. It suggests that this may well be due to some planet-sized collisions.

I'll tackle each of Randy's questions/points in order:

If two objects had a violent crash, they likely would be destroyed, would they not?

That depends. With a decent pair of binoculars you can look at the Moon on a clear night and see evidence of any number of violent collisions in its craters. These violent crashes undoubtedly destroyed one of the two objects, but the Moon, whilst scarred, continued on. Increasing the size of the impactor, however, would understandably leave larger and larger scars until those scars became so large as to knock noticeable chunks of the impactee, or other things that happen to large amounts of rock subjected to the massively high pressures and temperatures associated with a planet-sized smash. Indeed, there is a fair amount of evidence to support the idea that the Moon itself was formed in just such a cataclysmic collision between a very early Earth and something around the size of Mars (see this post if you're interested in reading more).

Another important consideration that collisions of any size have an effect not just on the structural integrity of the bodies involved, but also on the motions: every lump of rock that has slammed into the Moon has altered its orbit or spin just very slightly. Even as a falling rock is obliterated by its impact onto the surface of the Moon, the energy of that impact transfers momentum to the larger body. If the difference in mass of the two bodies is large, then the change in the motion of the larger body is correspondingly tiny, and the almost random direction from which collisions come over time all but cancels out their effect. Larger bodies, though, will impart greater momentum on their target, and a good whack from a large body may well end up making a visible difference to the target even if the smaller body is completely destroyed in the process.

But maybe. Two things that are large enough to be considered planets colliding with each other might well result in them both being reduced to gooey collections of molten rock.

There are other odd-shaped objects in the Solar System such as Ceres

Indeed there are, but Ceres is an odd choice for the point being made here as it's actually pretty impressively round for its size**. There are an enormous number of lumpy, bumpy bodies orbiting the Sun (or planets), not least comets which are often kind of peanutty and potatoey in shape. Even Ceres, though, is pretty small compared to Venus and Uranus, and when we're discussing the shape of astronomical objects, size matters***.

How could Venus and Uranus have a horrific collision and end up being round? There is nothing in space to smooth them over

This is both true and not true. It is true in that there is no (known) Cosmic Sander, roaming the universe with the most enormous Black & Decker**** you've ever seen.

It's not true in the sense that there are actually things in space that can smooth things over. There's a lot of stuff floating around in space, from microscopic dust particles up to the largest stars, and collections of matter representing every possible size in between. Just as on Earth a trickling stream, over time, uses tiny particles of rock to buff-up pebbles into polished marvels, a consistent rain of particles smooths out the edges of Moon craters. But that's not the main action at work when it comes to turning recently-walloped planets back into almost-spheres.

This is down to two things that we're all fairly familiar with: gravity and surface tension.

Gravity is a relatively weak force, but relentless. It is what we have to blame for our very existence, with clouds of gas and dust being drawn together by it over time to form the Solar System (and galaxy) in which we live*****. That gooey collection of molten rock that we mentioned earlier is held together largely by gravity, and a number of effects (not least friction) causes that blob to collapse in on itself over time.

Surface tension is familiar to everyone who has blown a bubble: It's what gives a bubble its spherical shape. Why that happens is a bit beyond the scope of this post, but the basic idea is that a sphere is actually a pretty efficient shape. Lumps and bumps on a bubble would require some other input to keep them in place and it is precisely the lack of this input that ensures an unmolested bubble settles into a nice, comfortable sphere.

We can see the same thing in videos of astronauts drinking on the International Space Station: they squirt some liquid out of a pouch and, assuming they play with it a bit first, we see it naturally settling into a spherical shape.

A gooey collection of molten rock acts, by definition, like a liquid. On Earth we see recently erupted lava cooling over time, if left to its own devices, into a smooth, flat(ish) rock. In space, a massive lump of water, kept as a lump by gravity, would act much like a blob of water: Left to its own devices, the twin forces of gravity and surface tension mould it into something vaguely spherical. Over time the molten rock cools and solidifies, but this happens much slower than the forces shaping it.

This is what made stars, planets and (some) moons spherical in the first place. The lumpy ones are lumpy not (just) because they've had violent collisions but because there's simply not enough stuff making them up for its collective gravity to draw it together. The energy involved in collisions between the smaller, lumpier bodies may not even be great enough to melt them in the first place: many comets and asteroids are really collections of much smaller rocks loosely held together by gravity and ice.

I believe... [etc]

You can, of course, believe what you like. Whilst such things as described above do not preclude the existence of a god or gods, there are well-researched,  explained and explainable mechanisms which certainly do not require them to be invoked.

Further Reading

• What shape are comets?
• Why does Venus Rotate on its Side?
• Don't forget you can ask your own questions here!

* That doesn't really mean anything, I know, but at the same time you know what I mean, don't you?

** It's a pretty interesting space-object in its own right. Remind me to dedicate a post to it sometime.

*** Actually, mass matters, but then I'd've missed the opportunity for a sort of pun.

**** Other brands of power-sanding equipment are available

***** That's a really, really short overview of our solar system and its evolution. There's much more to it than that.