Status and Prospects of Planetary Transit Searches: Hot Jupiters Galore

The first transiting extrasolar planet, orbiting HD 209458, was a Doppler wobble planet before its transits were discovered with a 10 cm CCD camera. Wide-angle CCD cameras, by monitoring in parallel the light curves of tens of thousands of stars, should find hot Jupiter transits much faster than the Doppler wobble method. The discovery rate could easily rise by a factor 10. The sky holds perhaps 1000 hot Jupiters transiting stars brighter than V = 13. These are bright enough for follow-up radial velocity studies to measure planet masses to go along with the radii from the transit light curves. I derive scaling laws for the discovery potential of ground-based transit searches, and use these to assess over two dozen planetary transit surveys currently underway. The main challenge lies in calibrating small systematic errors that limit the accuracy of CCD photometry at milli-magnitude levels. Promising transit candidates have been reported by several groups, and many more are sure to follow. 1. Three Waves of Extrasolar Planet Discovery 1.1. Doppler Wobble Planets In the first wave of extrasolar planet discovery, Doppler wobble surveys have sustained a discovery rate of 1 or 2 planets per month since the debut of 51 Peg (Mayor & Queloz 1995). The extrasolar planet catalog now holds over 100 Doppler wobble planets, filling in the top-left quadrant of the planet mass m vs. orbit size a discovery space (Figure 1). The Doppler wobble planets have a roughly uniform distribution on the logm–log a plane, with two clear and interesting boundaries, a maximum planet mass m ∼< 10 mJ , and minimum orbit size a ∼> 0.03 AU (P > 3 d). In their second decade, Doppler surveys will extend the catalog to larger orbits and push down to somewhat lower masses as precision radial velocities improve from 3 to 1 m s−1. 1.2. Hot Jupiter Transits A second wave, the hot Jupiter discovery era, is fast approaching. The Doppler wobble surveys establish that hot Jupiters, with P ∼ 4 d and a ∼ 0.05 AU, are orbiting around ∼ 1% of nearby sun-like stars. A fraction of these, Pt ≈ R⋆/a ∼ 10%, will have orbital inclinations close enough to edge-on so that the planet transits in front of its star. We may therefore expect that 1 star