Polar planets: birthplace and dynamics

N. CUELLO; C. A. GIUPPONE

Antofagasta

Conferencia; XVI Latin American Regional IAU Meeting; 2019

IAU - Sochias

Dynamical studies suggest that most of the circumbinary discs (CBDs) should be coplanar. However, some theoretical works show that under certain conditions a CBD can become polar and recent observations showed that polar CBDs exist in Nature (e.g. HD~98800). We test the predictions of CBD alignment around eccentric binaries based on linear theory. In particular, we compare prograde and retrograde CBD configurations. Then, assuming planets form in these systems, we thoroughly characterise the orbital behaviour and stability of misaligned (P-type) particles. This is done for massless and massive particles. The evolution of the CBD alignment for various configurations is modelled through three-dimensional hydrodynamical simulations. For the orbital characterisation and the analysis stability, we relied on long-term N-body integrations and structure and chaos indicators, such as $\Delta e$ and MEGNO. We confirm previous analytical predictions on CBD alignment, but find an unexpected symmetry breaking between prograde and retrograde configurations. This phenomenon is more likely to occur around eccentric equal-mass binaries. Regarding the stability of circumbinary P-type planets (aka Tatooines), polar orbits are stable over a wide range of binary parameters. In particular, for binary eccentricities below 0.4, the orbits are stable for any value of the binary mass ratio. Interestingly, the stability of these planets is guaranteed for planetary masses below $10^{-5}\,M_{\odot}$. This means that polar circumbinary planets are likely rocky or at most ice-giants. Finally, we predict that {\it Polar Tatooines} should be searched around mildly eccentric equal-mass binaries