Keele Research Repository

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