Orbital semi-major axis vs. eccentricity for inactive (black dots) and active (stars) centaurs (as of July 2017). Continuous curves show the loci of orbits whose perihelia are equal to the semi-major axes of the four giant planets. Jupiter-family comets (small grey dots) are also included in the plot. Among inactive centaurs, the red-circled black dots show centaurs in the potential activity region (see text). The blue-circled black dot is Centaur 523676 (2013 UL10).