Exoplanet research at the RIU
The RIU, Department of Planetary Research, was one of the detection teams of the CoRoT mission (CO-I contribution by Martin Paetzold). Its task was the detection of planetary candidates that were followed up with Radial Velocity.
It cooperates in Germany with:
- Observatory in Tautenburg
- DLR Berlin
Further foci at RIU-PF are:
- Transit Time Variation
- Tidal evolution of the orbits of extrasolar planets (Paetzold, M. and H. Rauer, 2002; Paetzold, M., L. Carone and H. Rauer, 2004; Paetzold, M. 2014)
Since 2004, the RIU, Department of Planetary Research, has been developing software to detect extrasolar planets from a light curve. A light curve is overlain by disturbances that make the detection of transits difficult, and, therefore, a software that can filter the disturbances is essential. Such a software that also automatically detects planetary candidates has been developed at RIU-PF by Gahr (2006) and Grziwa et al. (2012). During the six-year mission, the RIU-PF has worked on the detection of planetary candidates and was responsible for the selection of candidates for ground-based follow-up. The detection pipeline is continuously developed through the experience achieved during the mission:
In 2010 the detection pipeline received a software that simulates combinations of binary stars and compares them with the transit in the light curves (Grziwa 2010). With this software transits of binary stars can be identified and the confusion with planetary transits excluded. Stellar variations and other disturbances in light curves are reduced with new developed wavelet-based filtering methods (VARLET and PHALET (Grziwa 2016):
VARLET completely separates variations and jumps out of the light curve without removing the transit. Thus is simplifies the search for transits of small planets. VARLET filtered light curves of CoRoT were included by the CNES in the data in 2014.
PHALET separates periodic signals of known frequency like transits or binaries from a light curve. This makes the detection of planetary systems and planets in binary star systems easier.
Since the availability of the Kepler data, RIU-PF has also used the detection pipeline to search these light curves for extrasolar planets and has detected thousands of potential extrasolar planets. A comparison with the Kepler team lists (Candidates of Interest) yields an agreement of 95 %. Other previously unknown candidates for investigation were found. Currently, RIU-PF is developing new detection algorithms and specialized filters to remove instrumental noise. Further aims included determining as many planet parameters as precisely as possible directly from the light curve.
Due to a failure of two reaction wheels Kepler went out of service in 2013. The mission (now called K2) was continued with a different observing strategy and a lower pointing accuracy. RIU-PF developed an additional correction method to partially compensate this inaccuracy (tracking and correction of the movement and rotation of the CCD). As part of the KEST collaboration the RIU-Pf detected many new planets in K2 light curves which were confirmed by follow-up observation (Grziwa et al. 2016, Marshall et al. 2016).
Determining Transit Time Variations (TTV) and Transit Shape Variations (TSV) should support the process. These methods should ensure that there is an exact characterization of extrasolar planets in future missions.
Transit Time Variation:
Planets in a system mutually exert gravitational forces on each other, influencing their motion in their orbits around the star.
The consequence of the orbit disturbances of the transiting planet is that the current period is not constant but varies periodically around a mean period. This is called Transit Time Variation (TTV). More contributions to the orbit disturbance of the transiting planet in addition to the gravitational interaction are:
- The oblate (flattened) figure of the star
- The tidal forces between star and transiting planet
- The movement of transiting planets in the gravitational potential of the star (theory of relativity)
Measurements of the transit times and the deviations from a mean period may indicate the presence of other planetary bodies disturbing the orbit of the transiting planet. Thus an unknown and/or not transiting planets can be detected.
The orbit parameters of the planetary candidates that show TTV can be estimated through the simulation of the planetary system. These orbit parameters are: orbital period, semimajor axis, mass of the planet, orbit inclination and eccentricity. Thus the planetary constellation that can produce the measured TTV can be estimated. Comparing the measured TTV and the simulated TTV allows confirmation of whether a further planet (with the calculated parameters) disturbs the orbit of the transiting planet.
A consequence of the analysis of the transit times is the detection of multi-planet systems.