How to get the Half-Major axis of a 2D collider ?

Hi, I'm programming a space game, so gravity is a major part of it. I'm trying to calculate the sphere of influence of each planet: SOI = a(M/m)^(2/5) where a is the Half-Major Axis of the smaller celestial body (here my spaceship). So how can I obtain this from a GameObject, collider, rigidbody, or just with scripting ?