Multiwavelength Mitigation of Stellar Activity in Astrometric Planet Detection

Astrometry, referring to the measurement of stellar positions in the sky, has long been a promising technique for exoplanet detection. As the Earth orbits the Sun, for example, its gravitational pull causes the Sun to wobble with an amplitude of 0.65 milli solar radii. Exoplanets induce analogous reflex motion in their host stars, which can serve as astrometric signals. At its theoretical limits, astrometry would allow for the detection of planets smaller than previously observed by current exoplanet search methods. However, stellar activity may make these theoretical limits unreachable. Magnetic activity in the photosphere of a Sun-like star induces variability of order 0.5 milli solar radii in the position of the photocenter. This astrometric “jitter” creates a fundamental astrophysical noise floor preventing detection of lower mass planets at a single wavelength.

In this thesis, we investigate mitigation of this fundamental astrometric noise for Sun-like stars using correlations across wavelengths. To do so, we perform planet injection-and-recovery tests in five different wavelength passbands using astrometry simulations calibrated from solar data. We treat this problem in the limit of an ideal telescope to isolate the noise floor set by stellar activity. For a true solar analog and a planet at 1 au semi-major axis, we find the 6σ detection limit to be 0.01 Earth masses in the best single passband. We then show that combining information from pairs of passbands with highly correlated jitter but less motion in the redder band enables higher precision measurements of the common signal from the planet. Using this method improves detectable planet masses at 1 au by up to a factor of 10 at some wavelengths, and it sets the new 6σ detection limit across all passbands to 0.005 Earth masses. Given these results, we recommend that future astrometry missions consider proceeding with two or more passbands to progress towards astrometric planet detection by reducing noise due to stellar activity.