The geometric origin of the thermal eccentricity law

The distribution $f(e)=2e$ is commonly referred to as the `thermal eccentricity distribution’ of Keplerian binaries. However, the result does not require thermal equilibrium, and its usual derivations obscure why the distribution is linear in eccentricity. We seek a simple geometrical interpretation of this result and its relation to more general eccentricity distributions. We consider bound Kepler orbits in a Euclidean space of dimension $D\ge 2$, with a phase-space distribution depending only on energy, and derive the eccentricity distribution from the Liouville measure. We obtain $f_D(e)=(D-1)e(1-e^2{)}^{(D-3)\mathrm{/}2}$, so that, within this family, $D=3$ is the only case for which the distribution function is linear in $e$. This follows from the $D-1$ transverse momentum dimensions and the Kepler period degeneracy. We further show that two known generalisations – namely a power-law weighting of the normalised angular momentum and a constant velocity anisotropy – are, in fact, two representations of the same one-parameter family of eccentricity distributions.