coplanarity

noun

1. (countable, uncountable)The state or characteristic of being within the same plane.
2. (countable, uncountable)The state or characteristic of orbiting a central celestial object within the same orbital plane.

   “Gliese 876 affords us by far our most detailed and accurate map of any extrasolar planetary system in the solar neighborhood, and […] it is reassuring to see a kinship in the most fundamental property of system coplanarity.”