Hans Kjeldsen, Professor, Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Denmark
One of the fastest developing fields of astrophysics is the study of planets orbiting other stars than our Sun, extra solar planets or exoplanets for short. These alien worlds display some surprising characteristics, unknown from our own solar system. Some exoplanets orbit their stars in only a few days, others travel in extremely elliptical orbits, and some orbit over stellar poles. However, not only the orbital characteristics, also the planets themselves have surprised us. The atmospheres of some exoplanets reflect as little light as coal, and some planets have densities as low as cork. We have discovered planets with sizes intermediate between Earth and Neptune ('Super-Earth' planets), a class of planets absent in the Solar System. Currently, the planets about which we can learn the most are those, which transit their host stars. Their orbits are seen nearly edge on, and we observe a periodical dimming, when they move between us and their star allowing for detailed investigations of their orbital characteristics, internal structure and atmospheres. In this lecture, I will discuss how we search for and study the properties of exoplanets using telescopes on ground and in space.