Planetary mass-radius relations across the galaxy
  Planet formation theory suggests that planet bulk compositions are likely to
reflect the chemical abundance ratios of their host star's photosphere.
Variations in the abundance of particular chemical species in stellar
photospheres between different galactic stellar populations demonstrate that
there are differences among the expected solid planet bulk compositions. We aim
to present planetary mass-radius relations of solid planets for kinematically
differentiated stellar populations, namely, the thin disc, thick disc, and
halo. Using two separate internal structure models, we generated synthetic
planets using bulk composition inputs derived from stellar abundances. We
explored two scenarios, specifically iron-silicate planets at 0.1 AU and
silicate-iron-water planets at 4 AU. We show that there is a persistent
statistical difference in the expected mass-radius relations of solid planets
among the different galactic stellar populations. At 0.1 AU for silicate-iron
planets, there is a 1.51 to 2.04\% mean planetary radius difference between the
thick and thin disc stellar populations, whilst for silicate-iron-water planets
past the ice line at 4 AU, we calculate a 2.93 to 3.26\% difference depending
on the models. Between the halo and thick disc, we retrieve at 0.1 AU a 0.53 to
0.69\% mean planetary radius difference, and at 4 AU we find a 1.24 to 1.49\%
difference depending on the model. Future telescopes (such as PLATO) will be
able to precisely characterize solid exoplanets and demonstrate the possible
existence of planetary mass-radius relationship variability between galactic
stellar populations.
