Short- and Long-Period Planets around Evolved Stars

Since the discovery of the first exoplanet orbiting a main-sequence star in 1995 we have seen a tremendous increase in the number of known exoplanets and the pace at which these systems are discovered. For the first years most systems were discovered using the Radial Velocity Method, which infers the presence of planetary companions by measuring the reflex motion of the host star via the Doppler shift of the stellar spectral lines. However, with the advent of the Kepler space telescope in 2009 the picture changed dramatically as the number of known systems went up by over two thousand over the course of the next years. Kepler’s success is based on the employed Transit Technique which identifies potential planets by the small reduction of the brightness of the host star when the planet passes through the line of sight. While the RV method is limited to observing individual stars one by one, the transit technique allows to monitor large areas of the sky simultaneously and is thus very efficient. However, the technique inherently biases the planet detections towards short orbital periods (because the chance for a transit is higher and requires shorter time baselines) and towards main-sequence stars (because evolved stars are several times larger and thus the planet blocks a smaller fraction of the stellar emission). Because of these biases a large number of short-period planets are known around main-sequence stars and while these are incredibly interesting, we are still missing a large part of Nature’s diversity in the composition and configuration of planetary systems.

This project focuses on evolved stars, i.e. stars that have left the main sequence and evolved into giant stars. For these stars only around 100 planets are known, almost all of which were found using the Radial Velocity Method. As these stars are the successors of sun-like stars, the search for planetary systems around them can give us a picture of the Earth’s fate in the dying stages of our Sun. Moreover, because massive main-sequence stars are not suited for Radial Velocity measurements (due to the paucity of lines in their spectra and fast stellar rotation), monitoring giant stars gives a means to investigate the planet population in a higher stellar mass regime.

In contrast to main-sequence stars, only very few planetary systems are known around giant stars with short orbital periods, i.e. periods with less than 100 days. The explanation for this lack of short-period planets is still being debated. Tidal interaction of the planet with its host star during the stellar evolution certainly plays an important role, as simulations show that planets close to the star can be engulfed or thrown out of the system during the stellar expansion. However, because the sample of giant stars hosting planets is on average more massive than the known main-sequence hosts, another explanation is that close in planets are not formed around more massive stars. This could be explained with a shorter lifetime of the protoplanetary disks around massive stars and thus less time for migration of massive planets towards the inner regions of the respective system. This project attempts to investigate this lack of short-period planets by searching for transiting candidates using NASA’s Transiting Exoplanet Survey Satellite (TESS), which has a very large Field of view and the required photometric precision to find planets around evolved stars. After RV follow-up of these systems we hope to increase the number of known short-period planets around evolved stars to a statistically meaningful sample which allows occurrence rate studies.

On the other end of the period range we also know little about planets with very long periods, i.e. periods longer than 10 years. For evolved stars, just one planet is known which meets this criterion. The lack of these planets can possibly be attributed to selection effects as these planets require long temporal baselines and thus a very dedicated long-term RV program. Moreover the far out planets lead to a smaller reflex motion of their host stars than their short-period counterparts. However, already looking at our own solar system we find all of the four gas giants in this period range and thus it seems obvious that we miss a large part of the planet population. The Exoplanet Group of the Landessternwarte (LSW) Heidelberg has conducted a long-term RV survey of 373 G and K giant stars, starting in 1999 at the Lick observatory. Over 20 years later we now have the unique chance to continue this survey with LSW’s own Waltz Telescope, which has been equipped with a high-resolution Echelle Spectrograph. Several stars in the sample show long term trends, which might be caused by long-period planetary companions. This follow-up can reveal the far out regions of evolved stellar systems that have never been observed before.