 In their paper, Stern and Levison sought an algorithm to determine which "planetary bodies control the region surrounding them".They defined "Λ" (lambda), a measure of a body's ability to scatter smaller masses out of its orbital region over a period of time equal to the age of the Universe (Hubble time).Λ is a dimensionless number defined as

where "m" is the mass of the body, "a" is the body's semi-major axis, and "k" is a function of the orbital elements of the small body being scattered and the degree to which it must be scattered.