It is also not a sphere ; scientists believe Proteus is about as big as a body of its density can be without being pulled into a spherical shape by its own gravity . The shape of Proteus is close to a sphere with the radius of about 210 km , although deviations from the spherical shape are largeâ '' up to 20 km ; scientists believe it is about as large as a body of its density can be without being pulled into a perfect spherical shape by its own gravity .