Exoplanets Beyond the Solar Neighbourhood: Galactic Tidal Perturbations
  The majority of Milky Way extrasolar planets likely reside within a few kpc
of the Galactic centre. The Galactic tidal forces acting on planets scale
inversely with radius in the Galaxy and so are much greater in the inner Galaxy
than in the Solar neighbourhood. Within a range of 3.5 to 10 kpc, the vertical
tide from the Galactic disc is predominant. Interior to 3.5 kpc, the effects of
the Galactic bulge cannot be neglected and the in-plane tidal components are as
important as the vertical ones. Here, we quantify the orbital changes induced
by these tides. We find that the greatest perturbations occur when the
planetary orbit is severely misaligned to the parent star's orbit. When both
planes are perpendicular, the eccentricity of the planet is driven to unity,
although the semimajor axis is secularly unaffected. When both planes are
coincident, the effect from Galactic tides is minimized, but remains non-zero.
In these cases, we provide estimates for the survival times, as well as the
minimum baseline eccentricity variation for all Milky Way exoplanets as a
function of Galactic parameters. Inclinations similar to the Solar System's
(about 60 degrees) can easily cause eccentric Neptunes (at about 30 AU) around
host stars deep within the Galactic bulge (within 50 pc) to experience
eccentricity variations of several tenths, and cause the exoplanets with the
widest-known separations (at about 1000 AU) to experience similar variations in
the Galactic disc. These variations occur on timescales of a few Gyr, a
fraction of a typical main sequence lifetime.
