Polarimetry and High-Contrast Imaging of Planetary Systems
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2023
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Doctoral Thesis
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Abstract
The discovery of the first exoplanets less than three decades ago and the subsequent confirmation of more than 5500 has opened up a new era in astronomy. Knowing that most stars have planets and that there is a huge variety in their characteristics brings us within reach of detecting signs of life outside of our planet Earth. The majority of exoplanets discovered have been found using indirect methods, and only a small fraction have been imaged directly. All of the directly imaged planets are found with thermal emission in the near-IR or even longer wavelengths. Detecting reflected, scattered starlight from an exoplanet is much more difficult because the contrast between the bright star and the faint planet is even more challenging.
The aim of the thesis is to search for reflected, polarised light from exoplanets and to achieve a very deep contrast as a proof of concept for future missions. I am also investigating the short-wavelength thermal emission, described by Wien’s law, from known exoplanets. Planets are born shortly after stars in protoplanetary disks, and as a starting point for the thesis I am analysing the faint light from these disks.
In chapter 1 I introduce the short history of exoplanets and explain the different detection methods. I illustrate the importance of instrumentation as a prerequisite for recent discoveries and motivate the importance of better understanding the properties of dust in protoplanetary disks as a starting point for planet formation. I end the chapter with a short excursion into post-processing methods in high-contrast imaging and the Stokes parameter and polarisation as used in this thesis.
In chapter 2 I quantitatively measure basic properties of the prominent protoplanetary disk of HD 169142, in two filter bands in the optical to near-IR wavelengths. These disks are hundreds of light-years away, so we need to obtain information with telescopes at different wavelengths. Precise measurements of polarised reflectivities are made possible by forward modelling of polarisation cancellation effects, despite rapidly changing adaptive optics performance. In addition, the total intensity of the disk could be extracted and compared with the longer wavelengths in the SED.
In chapter 3 on RX J1604, I show my adaptation of the procedure in chapter 2 to this second disk, and emphasise that careful data reduction is the key to accurate conclusions.
In chapter 4 I search for the reflected light from the known RV planet candidate Epsilon Eridani b. Despite almost 40 hours of dedicated VLT observations and improved, advanced calibrations and post-processing, the planet could not be detected. The search for warm, faint dust did not yield a clear detection of a dust ring, but the sensitivity rules out certain scenarios that would have been plausible from the IR excess. The integrated, positively polarised reflectivity could indicate scattering from circumstellar dust and would be of the right order of magnitude.
In chapter 5 I present the search for sub-Jupiter planets in the habitable zone of the very nearby star Alpha Centauri A using direct imaging with reflected light. The detection of a cold, small planet in the habitable zone of one of the nearest stars would make scientific history. The contrast achieved in terms of intensity and polarised light is unprecedented.
In chapter 6 I analyse archival SPHERE/ZIMPOL data of the famous multiplanetary system HR 8799. The detection of a faint signal from the thermal emission of the young and hot planets would help to constrain the effective temperatures and atmospheric modelling.
In chapter 7 I propose future ZIMPOL observations of the star Beta Pictoris to study the faint thermal emission of the famous planet b at shorter wavelengths. An analysis of archival data could not reveal the planet, as it was at a much closer separation at the time of the observations, and the choice of filter was not optimal.
In chapter 8 I summarise my thesis and make some concluding remarks, including an outlook for the next few years.
This thesis demonstrates the capabilities of polarimetry as a niche differential technique for protoplanetary dust disks, debris disks and exoplanets. Using the Very Large Telescope, one of the world’s leading ground-based telescopes, and the SPHERE/ZIMPOL imaging polarimeter, the best polarimetric and intensity contrasts ever obtained in the field of high-contrast imaging were achieved. The thesis paves the way for even larger surveys to directly image the polarised reflected light from exoplanets.
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ETH Zurich
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High-Contrast Imaging; Polarimetry; Exoplanets; Remote Sensing
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08824 - Schmid, H. M. (Tit.-Prof.) (emeritus) / Schmid, H. M. (Tit.-Prof.) (emeritus)
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Funding
181983 - Search for reflecting extra-solar planets with SPHERE-ZIMPOL (SNF)