Science: Finding the brightest exoplanets in the sky

The MASCARA target population as function of magnitude and stellar type

Extrasolar planets (planets that orbit other stars than the sun) are being discovered at a very high pace. Most have been found using the radial velocity method, which measures the reflex motion of the host star around the planet-star barycentre. Also, a large fraction is discovered through their transit, mostly via dedicated ground-based (SuperWASP, HATNet) and space-based (CoRoT, Kepler) surveys. The radial velocity and transit surveys are rapidly deciphering the exoplanet population. The discovery of several rocky super-Earths shows that the hunt for the first real Earth-like planet is on. Exoplanet demography suggests that there should be at least 50 billion planets in the Milky Way, including 30,000 planets in the habitable zones of their host stars already within 1000 light-years from Earth (Borucki -kepler.nasa.gov).

The expected transit population as function of magnitude and stellar type

Astronomers have now reached a new phase in exoplanetology with the goal to find the nearest planets - the most valuable for planet characterization. Not much can yet be learned from the planets discovered with the radial velocity method, since they require significantly better direct imaging techniques to separate the light of these cool (old) planets from that of their host stars. In strong contrast, planets that transit their host stars can readily be characterized. The planet's radius, mass and density can be directly determined from the transit, and in the case of very bright systems the planet's atmosphere can be characterized via transmission and eclipse photometry, and orbital phase variations. The hundreds of CoRoT and Kepler transiting planets and candidates are unfortunately very faint (V>11-12) allowing only their basic parameters to be determined. The also very successful ground-based surveys, such as HATNet and SuperWASP, target brighter stars (V=8-12) for which more detailed atmospheric studies are possible on the hottest systems. However, more than 80% of the scientific papers on atmospheric characterization of exoplanet atmospheres feature the two bright transiting planet systems, HD189733 and HD209458 (both of V~7.7), detailing their atmospheric constituencies, temperature profiles and even their atmospheric circulation. This demonstrates the enormous importance of bright transiting planet systems.