[4][verification needed] Much of Kirkpatrick's research has concentrated on the discovery and characterization of the lowest mass stars and brown dwarfs.

Along with Neill Reid, he uncovered many objects similar to and colder than GD 165B, leading to the definition of the L dwarf[7] spectral class.

Along with Adam Burgasser and Chris Gelino, Kirkpatrick maintains a listing of all known L, T, and Y dwarf discoveries at the DwarfArchives[13] website.

Kirkpatrick is credited with first using the name "Tyche" to refer to a heretofore unseen, low-mass Solar companion in the Oort Cloud.

He chose this term to be in direct contrast[14] to "Nemesis", an unseen Solar companion hypothesized to lie in a highly elliptical orbit and to be responsible for periodic mass extinctions on the Earth.