An object at the equator is therefore at the farthest possible distance from the line between the rotational poles. That means that an object at the equator is traveling faster during the sphere’s rotation than any other point on the sphere. The faster the movement, the greater outward force, hence the bulge.

2005 YU55 is not spherical, not even well-rounded. It’s quite lumpy. This is exactly what you’d expect from a 400 meter-wide asteroid.

There’s an excellent write-up about in on the “Bad Astronomy” blog:

http://blogs.discovermagazine.com/badastronomy/2011/11/07/nasa-primer-on-yu55/

Joseph: You might try testing the shape and size of the planet you found yourself on by observing whether or not the angle of the stars above you change as you change your location on the planet. This was an experiment originally performed by Eratosthenes (Google him!) in the 3rd Century B.C.E.

Eratosthenes measured changes in the apparent location of the Sun at midday on the Summer Solstice from two locations hundreds of miles apart, and he then used that information to not only conclude that the Earth was round, he also calculated the circumference of Earth with remarkable precision.

Xavi: Everything has gravity, including wisps of gas drifting in space. It just takes a lot of gas and a lot of time for them to fall in on themselves through mutual gravitational attraction.

Amanda: A large enough amount of mass in space will begin to fall in on itself through mutual gravitational attraction.

There is virtually no chance that the movement of any one in-falling bit of matter is always exactly matched and cancelled out by an equal and opposite movement by another in-falling bit of matter. This means that by the time all the in-falling matter gets close to itself there is some small dominant direction motion.

Look at the problem this way…

Think of dumping a million pennies from a high-altitude balloon – there’s virtually no chance you’ll get exactly 500,000 “heads” and exactly 500,000 “tails.” Instead, you’re almost always going to get a few more heads than tails, or vice-versa. It may be a tiny imbalance favoring one side over the other, but you’re pretty well guaranteed to always end up with more of one than the other.

The same thing holds true to tiny bits of dust and gas experiencing mutual gravitational attraction in the emptiness of space. There will always be some slight imbalance of motion as gravity pulls matter together, which means that eventually the whole in-falling collection of matter begins to spin. The spin is very slow at first, but then gets faster as gravity pulls the matter in closer and closer together. It is exactly the same thing you see when a spinning figure skater spins faster as she pulls her arms in close to her body. (The phenomenon is called Conservation of Angular Momentum, if you want to look it up.)

As the collection of matter begins to spin faster and faster, centrifugal forces begin to make the collection bulge outward in the direction perpendicular to the spin axis, and flatten along the plane of the spin. The same thing happens to spinning pizza dough when it’s being tossed to make a pizza crust. Spinning flattens the pizza dough into a disk. (Yummy!)

That’s why matter in any solar system, especially when you get close to the central star, tends to be largely flattened into a disk. It’s also why a planet’s moons tend to orbit on a plane that’s very close to the planet’s equatorial plane, and why spiral galaxies are disc-shaped.

Imagine a box measuring a mile long, a mile wide and a mile high. Now fill that box with rocks – everything from sand to boulders. That’s one cubic mile of rock.

Now imagine one million of these rock-filled boxes.

Now empty your collection of one million cubic mile-sized boxes of rock in roughly the same place in space. After some number of years (any number between a few, to many thousands, depending on the original distribution of the rocks) you would end up with a roughly spherical ball of rocks about 120 miles in diameter.

That amount of rock has just barely enough mass to pull itself together into a sphere, but not enough mass for the gravity-induced pressure at the center of sphere the heat the interior to its melting point to create a differentiated interior like Earth has (core, mantel, lithosphere, etc.)

Gravity on this little rock-ball would be extremely weak – about one-half of one percent the gravity we feel here on Earth. A person who weighs 160 pounds on Earth would only feel like they only weighed about 13 ounces on your little rock-ball world. With so little gravity it would take a very long time for the pile of rock to assemble itself into a sphere. It would definitely not happen instantaneously.

And yet your little 120 mile-wide ball of rock would still have a mass of more than nine million-billion tons.

When you figure out a rocket engine that can move that kind of mass around, I’ll post a discussion of what would happen to your spherical rock-ball when it starts to move.