Extrasolar Planets

After the start of high-precision exoplanet (extrasolar planets) search programs in the early 1990ies and more than several hundred exoplanets found so far (see Extrasolar Planets Encyclopaedia), one can now compare the observational results with planet formation and evolution theories in order to understand the formation of known planetary systems. However, the observational results today are strongly biased by selection and observational effects. For example, the radial velocity technique (RV) just yields the minimum mass of an exoplanet candidate due to the unknown orbital inclination. Furthermore, classical transit measurements are mainly sensitive for very short planetary periods and only possible for nearly edge-on orbits, thus an inclination angle of about 90 degree. Saturn or Neptune like planets at a distance of ten or more AU (Astronomical Unit) around a solar like star are currently not detectable with these techniques, but can be detected by astrometric observations which are most sensitive to larger planetary periods and deliver the true (dynamically determined) mass of an exoplanet.

The final aim should be to obtain unbiased observational data by using and merging several observation methods with different sensitivities, like the RV, transit, imaging, and astrometric technique.

The diagram above shows the (minimum) mass over the planetary semi-major axis of all exoplanets detected so far (based on Extrasolar Planets Encyclopaedia until August 2011) with the radial velocity technique (red circles) and transit observations (green circles). The size of the circles represents the mass of the exoplanet host-star.

The colored lines indicate the radial velocity signal (red lines), the transit signal (green lines) and the astrometric signal (blue lines) of an exoplanet orbiting a solar like star in a circular orbit. In addition, some planets from our own solar system are displayed. The vertical gray dashed lines are selected orbital periods for an exoplanet around a solar like star. Furthermore, the probability to detect a transit signal of an exoplanet orbiting a solar like star as well as the maximum allowed difference of the planetary inclination from an edge-on orbit are shown in gray at the bottom of the diagram.



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