 Steven Soter proposed an observationally based measure "µ" (mu), which he called the ""planetary discriminant"", to separate bodies orbiting stars into planets and non-planets.He defines mu as 
where µ is a dimensionless parameter, M is the mass of the candidate planet, and m is the mass of all other bodies that share an "orbital zone", that is all bodies whose orbits cross a common radial distance from the primary, and whose non-resonant periods differ by less than an order of magnitude.The order-of-magnitude similarity in period requirement excludes comets from the calculation, but the combined mass of the comets turns out to be negligible compared to the other small Solar System bodies, so their inclusion would have little impact on the results.