Larger and faster: revised properties and a shorter orbital period for the WASP-57 planetary system from a pro-am collaboration

Abstract

Transits in the WASP-57 planetary system have been found to occur half an hour earlier than
\nexpected. We present 10 transit light curves from amateur telescopes, on which this discovery
\nwas based, 13 transit light curves from professional facilities which confirm and refine this
\nfinding, and high-resolution imaging which show no evidence for nearby companions. We
\nuse these data to determine a new and precise orbital ephemeris, and measure the physical
\nproperties of the system. Our revised orbital period is 4.5 s shorter than found from the
\ndiscovery data alone, which explains the early occurrence of the transits. We also find both
\nthe star and planet to be larger and less massive than previously thought. The measured mass
\nand radius of the planet are now consistent with theoretical models of gas giants containing
\nno heavy-element core, as expected for the subsolar metallicity of the host star. Two transits
\nwere observed simultaneously in four passbands. We use the resulting light curves to measure
\nthe planet’s radius as a function of wavelength, finding that our data are sufficient in principle
\nbut not in practise to constrain its atmospheric properties. We conclude with a discussion of
\nthe current and future status of transmission photometry studies for probing the atmospheres
\nof gas-giant transiting planets.