Conditions for a planet to become spherical I have a question, from which size/mass will a body in space adopt a spherical shape? Over 500 kilometers wide and/or 1/4 the mass of Pluto?
Something like that, I always had this doubt.
 A: The minimum size has been called the potato radius, as anything too small will look more like a potato than a sphere. The potato radius depends on composition, hereafter assumed uniform. Eq. (9) (ibid.) gives the radius as proportional to $\rho^{-1}\sqrt{\sigma_y/G}$, with $\rho$ the density and $\sigma_y$ the compressive strength. This is to be expected from dimensional analysis. The authors estimate a $300 \text{km}$ value for representative values of $\rho,\,\sigma_y$.
A: For a body to become a sphere, it must have sufficient self-gravity to pull itself into the shape of one. However, because the self-gravity of an object depends on its mass rather than size, it means that a body made of a denser material would become spherical at smaller radii than that of less dense material. Additionally, there are other factors such as how easy the materials are to mould into a sphere which gives further deviations. (The second reason tends to win).
Hence, for bodies made of rock, the minimum size to become a self-gravitating sphere is about 600km in diameter, whereas, for ice, the minimum size is only 400km.
