Hubble Space Telescope time-series photometry of the planetary transit of HD 189733: no moon, no rings, starspots

Abstract

We monitored three transits of the giant gas planet around the nearby K dwarf HD 189733 with the ACS camera on the Hubble Space Telescope. The resulting very-high accuracy lightcurve ( signal-to-noise ratio near 15 000 on individual measurements, 35 000 on 10-min averages) allows a direct geometric measurement of the orbital inclination, radius ratio and scale of the system: i = 85.68 +/- 0.04, R-pl/R-* = 0.1572 +/- 0.0004, a/R-* = 8.92 +/- 0.09. We derive improved values for the stellar and planetary radius, R-* = 0.755 +/- 0.011 R-circle dot, R-pl = 1.154 +/- 0.017 R-J, and the transit ephemerides, T-tr = 2453931.12048 +/- 0.00002+ n center dot 2.218581 +/- 0.000002. The HST data also reveal clear evidence of the planet occulting spots on the surface of the star. At least one large spot complex (> 80 000 km) is required to explain the observed flux residuals and their colour evolution. This feature is compatible in amplitude and phase with the variability observed simultaneously from the ground. No evidence for satellites or rings around HD 189733b is seen in the HST lightcurve. This allows us to exlude with a high probability the presence of Earth-sized moons and Saturn-type debris rings around this planet. The timing of the three transits sampled is stable to the level of a few seconds, excluding a massive second planet in outer 2: 1 resonance.