In a post yesterday, I talked about the Moon orbiting the Earth, and the Earth's gravitational sphere of influence, called its Hill sphere. If you have an object orbiting the Sun (like a planet, say), anything inside that object's Hill sphere is more affected by that object than it is by the Sun.
I knew about this tidbit of physics, but was reminded of it by Dr. Alan Stern, the head guy of the New Horizons Pluto probe. Alan and I have mixed it up a bit before on what the definition of a planet is. He likes the idea that one criterion of planethood for an object is that the object is massive enough to modify its shape into a sphere. In other words, its gravity can overcome the tensile strength of the material making up the object, and it molds itself into a rough ball. I will admit ...
