Exoplanetary Systems in Polarized Light

The detection of planets and planetary systems outside of our own solar system has been one of the major astronomical milestones of the last century. While much of the current effort is spent on detecting planetary systems, my interest is in characterizing known extrasolar planetary systems in a variety of evolutionary states from the early stages of planet formation to mature planetary systems.

To make significant progress in our understanding of the formation, evolution, and structure of extra-solar planetary systems, direct imaging and spectroscopy of the circumstellar environment is required. However, circumstellar environments with exoplanets and dust and debris disks are many orders of magnitude fainter than their central stars in the visible and near-infrared. The light from the star tends to completely dominate the signal due to diffraction and scattering by telescope optics and due to the turbulence in the Earth's atmosphere (“seeing”) that affects ground-based telescopes. Even with the best telescopes using adaptive optics systems, which partially compensate the seeing, additional methods are required to extract the circumstellar light from the central star's light. Coronagraphs and nulling interferometers can remove a large part of the central star’s light, but measuring the polarization and not just the intensity of the light provides the ultimate mean to remove the unwanted light coming from the star.

Unpolarized light (the light wave has no preferred plane in which it oscillates) from the central star becomes partially polarized (the light has a preferred plane in which the light wave oscillates) when it is scattered off circumstellar material such as exoplanets and disks (Seager et al. 2000, Baba and Murakami 2003, Stam et al. 2004). Our precision measurements of the polarization will therefore be able to clearly discriminate between light from circumstellar environments and scattered light from the central star.

Here in Utrecht, we are working on our own instrument, the Extreme Polarimeter (ExPo) for the William Herschel Telescope, the SPHERE-ZIMPOL instrument for the Very Large Telescope (VLT), and EPICS for the European Extremely Large Telescope (E-ELT). The latter two are in collaboration with ETH Zurich in Switzerland. More information on these projects is available under the Projects link.

Current Exoplanetary Systems Projects

EPICS	Exo-Planet Imaging Camera and Spectrograph Phase A Study EPICS is an instrument for the direct imaging and characterization of extra-solar planets with the European ELT (E-ELT). EPICS will be optimized for observations in the visible and the near-IR and will have photometric, spectroscopic and polarimetric capabilities. Funding provided by ESO, NOVA
ExPo	Extreme Polarimeter An imaging polarimeter for the 4.2-m William Herschel Telescope at La Palma to image circumstellar environments (protoplanetary and debris disks and exoplanets). Funding provided by NWO/VICI
HARPS Polarimeter	HARPS Polarimeter The HARPS instrument at La Silla, Chile is famous for its planet detections through the radial velocity method. The ultrastable spectrograph system will be upgraded to a precise spectropolarimeter by replacing the obsolete Iodine cell in the Cassegrain adapter with a dual-beam polarimetry unit. This compact unit encompasses two separate modulators, one for circular polarimetry and one for linear polarimetry. Each modulator consists of a superachromatic wave-plate that doesn't introduce (polarized) spectral fringes and a Foster prism that acts as a polarizing beam splitter. The two beams out of the Foster prism are imaged onto the two fibers of the HARPS fiber head. The new instrument mode in combination with the ultrastable spectrograph enables unprecedented observations of stellar magnetism, e.g. through Zeeman-Doppler imaging. Funding provided by
SPHERE	Spectro-Polarimetric High-contrast Exoplanet REsearch The prime objective of the Spectro-Polarimetric High-contrast Exoplanet Research (SPHERE) instrument for the VLT is the discovery and study of new extra-solar giant planets orbiting nearby stars by direct imaging of their circumstellar environment. Funding provided by NWO, NOVA