Underdetermination and decomposition in Kepler's Astronomia Nova

•Past philosophical analyses of the problem of the three world systems are inconclusive.•I propose a new way of understanding Kepler's resolution of the problem.•I characterize Kepler's method as involving decomposition and identification.•The method is compared with other views of scientific inference such as bootstrapping.

This paper examines the underdetermination between the Ptolemaic, Copernican, and the Tychonic theories of planetary motions and its attempted resolution by Kepler. I argue that past philosophical analyses of the problem of the planetary motions have not adequately grasped a method through which the underdetermination might have been resolved. This method involves a procedure of what I characterize as decomposition and identification. I show that this procedure is used by Kepler in the first half of the Astronomia Nova, where he ultimately claims to have refuted the Ptolemaic theory, thus partially overcoming the underdetermination. Finally, I compare this method with other views of scientific inference such as bootstrapping.