Is the recently proposed Mars-sized perturber at $65-80~\textrm{AU}$   ruled out by the Cassini ranging data?
  Recently, the existence of a pointlike pertuber PX with $1~m_\textrm{Mars}\lesssim m_\textrm{X}\lesssim 2.4~m_\oplus$ supposedly moving at $65-80~\textrm{AU}$ along a moderately inclined orbit has been hypothesized in order to explain certain features of the midplane of the Kuiper Belt Objects (KBOs). We preliminarily selected two possible scenarios for such a PX, and numerically simulated its effect on the Earth-Saturn range $\rho(t)$ by varying some of its orbital parameters over a certain time span; then, we compared our results with some existing actual range residuals. By assuming $m_\textrm{X} = 1~m_\textrm{Mars}$ and a circular orbit, such a putative new member of our Solar System would nominally perturb $\rho(t)$ by a few km over $\Delta t = 12~\textrm{yr}~(2004-2016)$. However, the Cassini spaceraft accurately measured $\rho(t)$ to the level of $\sigma_\rho\simeq 100~\textrm{m}$. Nonetheless, such a scenario should not be considered as necessarily ruled out since the Cassini data were reduced so far without explicitly modeling any PX. Indeed, a NASA JPL team recently demonstrated that an extra-signature as large as 4 km affecting the Kronian range would be almost completely absorbed in fitting incomplete dynamical models, i.e. without PX itself, to such simulated data, thus not showing up in the standard post-fit range residuals. Larger anomalous signatures would instead occur for $m_\textrm{X} > 1~m_\textrm{Mars}$. Their nominal} amplitude could be as large as $50-150~\textrm{km}$ for $m_\textrm{X} = 2.4~m_\oplus$, thus making less plausible their existence. 
Lorenzo Iorio
gr-qc astro-ph.EP physics.space-ph