P^{2} = a^{3}

P^{2} = \frac{4π^{2}}{k^{2}(M_{Sun} + M_{Earth})}a^{3}

P^{2} = \frac{4π^{2}}{k^{2}(M_{tot})}a^{3}

k^{2} = \frac{4π^{2}}{P^{2}(M_{tot})}a^{3}