The Oblique Orbit of WASP-107b from K2 Photometry

• Published in: The Astronomical journal 2017-05, Vol.153 (5), p.205
• Main Authors: Dai, Fei, Winn, Joshua N.
• Format: Article
• Language: English
• Subjects: ALIGNMENT; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; EXCITATION; MASS; ORBITS; PHOTOMETRY; planetary systems; planets and satellites: individual (WASP-107); ROTATION; SATELLITES; SPIN; STARS; stars: individual (WASP-107); STARSPOTS
• ISSN: 0004-6256; 1538-3881; 1538-3881
• DOI: 10.3847/1538-3881/aa65d1

Observations of nine transits of WASP-107 during the 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°, 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 with a high obliquity. Most such stars have been observed to have low obliquities; all of the exceptions, including WASP-107, involve planets with relatively wide orbits ("warm Jupiters," with ). This demonstrates a connection between stellar obliquity and planet properties, in contradiction to some theories for obliquity excitation.