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 15000 on individual measurements , 35000 on 10-minute averages ) allows a direct geometric measurement of the orbital inclination , radius ratio and scale of the system : i = 85.68 \pm 0.04 , R _ { pl } / R _ { * } = 0.1572 \pm 0.0004 , a / R _ { * } = 8.92 \pm 0.09 .
We derive improved values for the stellar and planetary radius , R _ { * } = 0.755 \pm 0.011 Â R _ { \odot } , R _ { pl } = 1.154 \pm 0.017 Â R _ { J } , and the transit ephemerides , T _ { tr } = 2453931.12048 \pm 0.00002 + n \cdot 2.218581 \pm 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 ( > 80000 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 189733 b 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 .