Scientific Background Understanding stars is central to much of modern astrophysics. Stars are the fundamental entities providing light and energy in the universe and they have produced most of the elements (except hydrogen and helium) from which the Earth is made. In this respect, they are the very source of life on Earth. Stars also provide vital information about the history and the structure of the Universe, being the only objects for which we have the possibility to determine reliable ages. But we are still very far from a detailed physical understanding of stars, as most of our knowledge is based on limited measurements of the light emitted from the stellar surfaces from which we rely on theoretical models to derive their internal properties. Looking Inside Stars During the past 25 years, members of SONG (Stellar Oscillations Network Group) have been among the world leaders in the development and application of an advanced technique that allows a determination of the physical parameters (temperature, pressure, density) in the interior of the Sun. This technique uses oscillations, or star-quakes, on the surface of the Sun to “see” beyond the surface. These oscillations are caused by processes inside the Sun and change the surface periodically on a time-scale of about 5 minutes. By observing these starquakes we can learn a lot about the solar interior in much the same way as seismologists use earthquakes to learn about the structure of our planet. In both cases this is done by detailed comparison between observations and complex theoretical models. This technique for looking inside the Sun is called Helioseismology. However, although this has provided a very detailed understanding of the solar interior, the Sun is still just one of many stars. The stars evolve and their properties change in time. In order to improve our understanding of the stars in general, it is essential to test the theoretical models of stars with different mass, radius and age. Asteroseismology, the study of stellar oscillations in stars other than the Sun, offers just that possibility. Particularly rich information is available from observations of oscillations like those in the Sun. Two diagrams showing asteroseismic measurements of the nearby star Alpha Centauri A (4.3 light years away), obtained by the SONG group, in comparison to the Sun. As the sizes and internal properties of these two stars are different from each other, their oscillations also show different patterns on the two diagrams below. Nevertheless this is much more challenging, simply because the stars are much more distant. But the current measurement precision has now reached a level which makes it possible to obtain data from other stars in a quality similar to the measurements of the Sun. This is the first of SONG’s scientific goals. Worlds Away Stars are also the hosts of planetary systems. To date (Sept. 2005) 140 planetary systems around other stars are known. Of these 18 have more than one planet, which brings the total number of planets detected to 162. The first discovery of a planet orbiting another star was made only 10 years ago. The discovery was a surprise, as the planet was found being very close to its central star at a distance corresponding to only 1/20 of the distance between the Earth and the Sun. This discovery spurred a strong interest in the study of such planetary systems with the goal to understand the formation of planets. The gas giant Jupiter in comparison to Earth But so far most of the known exoplanets detected are of the hot Jupiter type. This means that they have masses similar to that of Jupiter, while they are located very close to their central star at distances typically corresponding to 1/10 of the Earth-Sun distance. In comparison to this the planet Jupiter in our own solar system is five times farther from the Sun than the Earth. Such close exoplanets will naturally be quite hot, due to the close distance to their central star. Apart from this most of the known exoplanets are large gas giants like Jupiter, while only a single candidate for a planet with a solid surface has been found so far. Currently there are several competing scenarios describing how planets are formed in the early stages of a planetary system. For the two most favoured scenarios, the direct collapse scenario and the core accretion scenario, it is very important to have a consensus of the occurrence of planets with masses below the mass of Jupiter. The direct collapse scenario suggests that planets are formed via a collapse of a gas cloud under the force of gravity. The core accretion scenario predicts that planets are formed around already existing cores, which attract and accumulate matter from their surroundings. It is expected that such planetary systems could harbour a few large planets, similar to Jupiter, as well as several smaller ones approaching the Earth-mass range. In case of the five exoplanets found with masses corresponding to 10 times the Earth’s mass, four of these are located in systems which also contain larger planets. To test such theories it is very important to obtain more knowledge about the distribution of planet masses, which can only be done by discovering more planets. Unfortunately the smaller an exoplanet is, the harder it becomes to detect. A short history of helio- and asteroseismology and exoplanet research 1952 Otto Struve suggests high-precision stellar radial velocities and transits may enable us to detect planets around other stars. 1961 First evidence for periodic variation in the surface velocity of selected areas of the Sun. 1979 Solar full-disk observations reveal global oscillations. 1981 The BiSON network starts limited operations. This network is still operating. 1986 The IRIS 7-station network starts operation. Operation ends 2001. 1991 The first evidence for solar-like oscillations in another star (Procyon) is published. Controversial at the time, but later confirmed. 1995 First exoplanet discovered around the star 51 Pegasi. 1995 The GONG network for solar oscillation observations starts full operations. Operations are still ongoing. 1995 The first detection of individual oscillation modes is published for the star Eta Bootis. Controversial at the time, but later (2003) confirmed. 2001 First clear detection of excess power in another star (Beta Hyi). 2001 First definite solar-like oscillation measurements in Alpha Cen A 2004 A planet with 14 times Earth mass is found around the star Mu Arae. 2004 First detection of l=3 modes and measurement of mode lifetime in another star (Alpha Cen A). 2005 Most precise measurements of stellar radial velocities are made with the ESO VLT and the UVES spectrograph (Alpha Cen B). 2005 Lowest mass planet so far found with a minimum mass of 7.5 Earth masses. The total number of planets with a mass near 10 times Earth mass is now 5. The total number of exoplanets is 162 (Sept. 2005).