How is it possible for a collision to be responsible for Uranus's axial tilt? I realize that the collision explanation is in any case highly (purely?) speculative, but I'm curious how it would work.  If Uranus is a ball of gas, why wouldn't any colliding object just pass through (or perhaps be destroyed/significantly damaged by the heat of friction)?  What would there be for the colliding object to hit?  Did Uranus have a significant solid mass at one time?  Is this a function of the speeds that would have been involved (sort of akin to skipping rocks on a pond, if that's a relevant analogy)?
For objects on the scale of a planet, the state of matter doesn't really matter much.  A colliding planet or planetesimal would not &quot;just pass through&quot;  The amount and density of the gas would prevent that.
Some easy experiments:  Put your hand outside a car as you are driving along, you'll feel that &quot;gas&quot; (ie air) has real substance and pushes against your hand quite hard.  Now imagine that you are driving at 20 km/s.... What happens when a piece of rock from space hits the atmosphere? It doesn't just pass through, it burns up in the air as a meteor.
Moreover the pressure and density in Uranus is huge.  Not so far below the surface are layers of water,ice, ammonia, methane.  And the pressure is so great that the density is more that that of water.
The collision of two gas planets could result in a combined planet that had a rotation axis inclinded at 90 degrees or more, and tidal effects and equatorial bulge effects would pull the moons into orbits around the equator.