1. It must orbit a star.
2. It must be big enough to have enough gravity to force it into a spherical shape.
3. It must be big enough that its gravity cleared away any other objects of a similar size near its orbit around the Sun.

Both objects are planets, or planetary-mass objects, that orbit around a common center of mass, rather than one object orbiting around the other. Pluto and it's largest moon, Charon, are the only double planetary system in our solar system. The same surfaces of Charon and Pluto always face each other, a phenomenon called mutual tidal locking.

Uranus has an unusual orientation, it is about 90 degrees sideways compared to other planets. This tilt causes the most extreme weather conditions because during half of Uranus' year (21 Earth years) the sun shines directly over the north or south pole of the planet.