Asked by Rab Manderson
For a body to become a sphere, it has to have sufficient self-gravity to pull it into a spherical shape. This depends on what the body is made of, for two reasons. The first reason is because the strength of self-gravity depends on the mass of the object rather than its size, implying that bodies made of denser materials become spherical at smaller radii.
The second reason is that some materials are easier to mould into a sphere than others, implying that less strong gravity is needed to push some materials into a spherical shape. The second reason tends to win out. Therefore, for bodies made mainly of rock, the minimum size to become a self-gravitating sphere is about 600km diameter; but, for bodies mainly made of ice, the minimum size is about 400km diameter.
Answered by Megan Whewell, Education Team Presenter for the National Space Centre
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