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Some of the planets in our solar system have been known to mankind for thousands of years and people have speculated for a long time about other planets around distant stars. Thus, one of the most fundamental astrophysical discoveries was the first detection of a planet orbiting around another solar-like star in 1995. Since then many hundreds of such extrasolar planets have been discovered.

The vast majority of the presently known extrasolar planets have been detected by two methods: First, from the reflex motion induced by the planet on their hosts through a careful measurement of the stars' changing radial velocity (the so-called radial velocity method). Second, by an apparent dip in the brightness of a star - which is called a "transit" - when an extrasolar planet passes through the line of sight between us and the host star (the so-called transit method). To detect such a transit, the brightnesses of stars have to be measured with high precision and the resulting lightcurves have to be analyzed carefully. The most accurate brightness measurements of thousands of stars are currently made by two space missions, Kepler (NASA) and CoRoT (ESA), which are specifically designed to detect transits of exoplanets.

Somewhat surprisingly, the majority of extrasolar planets, especially the early discovered planetary systems, have properties drastically different from the planets in our solar systems. For example, there are Jupiter-like planets orbiting their stars within the orbit of Mercury (Hot Jupiters). Such findings directly contradict classical theories of planet formation and models have to be developed to explain their existence. A lot of work is also dedicated to the discovery of a second earth, i.e., a rocky planet in the so-called habitable zone around a star, which is especially hard to detect because of its small size and its large period.

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In contrast to the planets in our solar system, which can be directly imaged and even studied in detail by spacecrafts, the large distances to extrasolar planets imply that they (usually) cannot be observed directly but only indirectly through the light of their host stars. Furthermore, host star and planet(s) are gravitationally bound and form a system with a common history and evolution. For these reasons it is impossible to understand extrasolar planetary systems without an adequate consideration of their host stars. In addition, extrasolar planets - and in particular transiting extrasolar planets - can be used to learn more about the properties of their host stars, e.g., their activity, their limb darkening, and their rotation.

Our group is actively involved in designing and carrying out studies of exoplanetary systems with a special emphasis on the host stars. For example, we developed a new technique using transiting planets to scan stellar surfaces for spots and bright regions. We applied the technique to the lightcurve of the active host-star CoRoT-2 and, using many transits of the planet CoRoT-2b, we could create a map of the evolution of the star's surface region eclipsed by the planet with unprecedented accuracy (see graph above).

Transit of HD209458b in different colors

Another field of research using lightcurves is stellar limb darkening. Since stars cannot be resolved as disks, the measurement of their limb darkeing - the fact that the edge of the stellar disk appears not as bright as the center - is very difficult. For space-based photometry, e.g., Kepler, the quality of lightcurves is good enough that the limb darkening can be measured from transits and compared to theoretical predictions. These studies are important because limb darkening usually is a free parameter when determining the parameters of planets and, thus, wrong limb darkening causes corrupted planetary parameters. The image on the right-hand side illustrates the influence of limb darkening on the shape of a transit.

Spectral information is also extremely valuable for the detection and analysis of exoplanetary systems. The Hamburger Sternwarte is a member of the CARMENES project which will search for small planets in the habitable zone around low mass stars using the radial velocity method. From the beginning of 2014 two spectrographs will observe about 300 M stars with the Calar Alto Observatory's 3.5 m telescope in two different wavelegth intervals measuring their radial velocities with an accuracy down to below 1 m/s.

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Again, spectroscopy can also be used to study the host star. For example, our group used the so-called Rossiter-McLaughlin effect - which is an apparent change in the star's radial velocity during a planetary transit - to scan the outer atmospheric layers of the star CoRoT-2. This first detection of the "chromospheric Rossiter-McLaughlin effect" (see graph on right side) shows the extended chromosphere above the star's photosphere and is one of the very few methods to spacially resolve the outer atmospheres of a star.

The close proximity of the so-called Hot Jupiters to their hosts may also lead to entirely new phenomena. A Hot Jupiter close to an active star is inundated with extreme ultraviolet and X-ray radiation from its host and is likely to lose its atmosphere to hydrodynamic blow-off; such phenomena can be observed in chromospheric and coronal emission lines and our group is also engaged in such studies.

The research regarding extrasolar planetary systems and their host stars is primarily done in the framework of the DFG funded Graduiertenkolleg 1351; the study program includes both observational studies of host stars as well as numerical simulations of stellar and extrasolar planetary atmospheres.