The Oblique Orbit of WASP-107b from K2 Photometry

Observations of nine transits of WASP-107 during the {\it K2} mission reveal three separate occasions when the planet crossed in front of a starspot. The data confirm the stellar rotation period to be 17 days --- approximately three times the planet's orbital period --- and suggest that large spots persist for at least one full rotation. If the star had a low obliquity, at least two additional spot crossings should have been observed. They were not observed, giving evidence for a high obliquity. We use a simple geometric model to show that the obliquity is likely in the range 40-140$^\circ$, i.e., both spin-orbit alignment and anti-alignment can be ruled out. WASP-107 thereby joins the small collection of relatively low-mass stars hosting a giant planet with a high obliquity. Most such stars have been observed to have low obliquities; all the exceptions, including WASP-107, involve planets with relatively wide orbits ("warm Jupiters", with $a_{\rm min}/R_\star \gtrsim 8$). This demonstrates a connection between stellar obliquity and planet properties, in contradiction to some theories for obliquity excitation.