Orbital period versus semimajor axis (log-log scales) of possible satellites of P9 with two different satellite-to primary- (planet) mass ratios m_s/m_p: 10^-9 for a small satellite and 1 for a possible twin planet system, while the total mass of the system is kept: 10 × Earth masses. The radii of the Hill stability domain (blue dash-dotted vertical lines) and of Szebehely’s domain (red dashed vertical lines) are marked by thin lines for the small satellite case and by thick lines for a possible twin planet case.