High-Precision and High-Accuracy Polarimetry of Exoplanets

Detecting polarization of the light reflected from an exoplanet requires extremely high-precision polarimeters and high-accuracy calibration techniques. The polarimetric precision of a few parts per million (ppm), approaching the photon noise, was demonstrated for the Sun and bright distant stars by several groups and instruments. However, the accuracy of absolute polarimetric calibration strongly depends on the polarimeter design and observing conditions, which results in largely unknown systematic errors hindering the exoplanet polarization detection. Here we discuss some of the crucial aspects of exoplanet polarimetric data acquisition, e.g., effects of seeing, sky polarization, telescope polarization, etc. We simulate examples of polarimetric measurements with various levels of random and systematic errors. They demonstrate that sparse measurements (ten or less) and unknown systematic errors can hinder exoplanet signal detection even when the signal is significantly larger than the polarimetric precision. We discuss various approaches which help improve random errors (precision) and mitigate systematic errors (accuracy) caused by various effects. We also discuss the performance of polarimeters with different designs and indicate their strengths and weaknesses in terms of precision and accuracy.