Hubble Space Telescope times-series photometry of the planetary transit of HD189733: no moon, no rings, starspots

TL;DR

This study used Hubble Space Telescope photometry to precisely measure the properties of the HD 189733 system, detect starspots, and rule out the presence of moons or rings around the planet.

Contribution

It provides highly accurate measurements of the system's parameters, detects starspots during transits, and excludes the existence of moons or rings around HD 189733b.

Findings

Precise orbital and physical parameters of HD 189733 system.

Detection of starspots on the host star during transits.

No evidence found for moons or rings around HD 189733b.

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 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 +- 0.04, Rpl/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 Rsol, Rpl=1.154 +- 0.017 RJ, and the transit ephemerides, Ttr=2453931.12048 +- 0.00002 + n 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 (>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 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.