Polarimetry as a tool for characterising planets and exoplanets
|Date||22 January 2020|
Polarisation is a little known property of light. Maybe because human cannot really see it with their eyes, polarisation appears to have a hard time getting into our hearts and remote-sensing instrumentation.
Direct light of the sun and sun-like stars is basically unpolarised, i.e. the electromagnetic waves have no preferential vibrational direction, but when this unpolarised light is scattered by atmospheric particles and/or when it is reflected by a surface, it will usually become polarised. The state of polarisation of this light is very sensitive to the composition and structure of the atmosphere and/or the surface, and polarisation measurements have been shown to provide information that cannot not be retrieved from brightness measurements alone, such as the composition of the clouds of our neighbouring planet Venus. Polarimetry also promises to be a strong tool for the detection and characterisation of exoplanets, i.e. planets around other stars, because it enhances the star-planet contrast, provides a direct confirmation of the planetary nature of the stellar companion, and could reveal information on the atmosphere and surface of the exoplanet.
I will provide examples of the application of polarimetry for solar system planets, but will focus on exoplanets and especially terrestrial-type exoplanets: what unique information could polarimetry help to retrieve?
I will also present LOUPE, the Lunar Observatory for Unresolved Polarimetry of Earth, a small spectropolarimeter that is being developed in a Leiden-TU Delft project to observe the Earth from the lunar surface, as if it were an exoplanet.
Daphne Stam studied Physics and Astronomy at the Vrije Universiteit in Amsterdam. Her PhD-research, on spectral variations in the polarisation of sunlight that is reflected by the Earth, was performed at the KNMI and the Vrije Universiteit, with prof. Joop Hovenier as advisor. As a postdoctoral researcher at Cornell University, Daphne worked on the analysis of observations of clouds and hazes on Saturn, Uranus and Neptune. Back in the Netherlands, she trained for a year to become a Clinical Physicist at the Antoni van Leeuwenhoekziekenhuis, but missed planetary research too much and returned with a Veni-grant to work on polarisation signals of exoplanets at the UvA. With a Vidi-grant on the same topic, Daphne started the research group Planetary and Exoplanetary Atmospheres at SRON, where she initiated with Leiden astronomers Frans Snik and Christoph Keller the development of SPEX, a small spectropolarimeter for planetary remote-sensing. In 2012, Daphne took up a position at the rapidly expanding Planetary Exploration group at Aerospace Engineering at TU Delft, where she is currently Associate Professor of Planetary Sciences, still working on SPEX-like instruments (such as LOUPE), and atmospheres of planets around the sun and beyond.