Use of Light Coherence for Exoplanet Detection and Characterization

Citation

Xin, Yeyuan (Yinzi) (2025) Use of Light Coherence for Exoplanet Detection and Characterization. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/jaay-9953. https://resolver.caltech.edu/CaltechTHESIS:05302025-142334562

Abstract

Since the first detection of an exoplanet in 1992, over 5,000 exoplanets have now been found through a variety of methods, both indirect (such as the radial velocity or transit method) and direct (such as with imaging, coronagraphy, and interferometry). The direct imaging and spectroscopy of exoplanets in particular plays a key role in characterizing their atmospheres, which can help distinguish between different models of planet formation and detect molecular signatures associated with life. However, directly observing exoplanets is extremely difficult: the small angular separations between the star and the planet require large telescopes to resolve, and the flux ratios between a planet and its star range from 10⁻⁴ in the infrared for hot, young, massive planets to 10⁻¹⁰ in the optical for mature Earth-like planets. Photon noise from the star drowns out the planet signal in conventional imagers or spectrographs, so dedicated instruments are needed to remove the majority of the starlight before it reaches the detector. Additional wavefront sensing and control methods are also needed to compensate for aberrations in the system --- from fast varying atmospheric fluctuations to slower quasi-static drifts in the instrument and telescope.

This thesis presents advances in instrumentation for directly characterizing exoplanets, focusing on exploiting the coherence properties of light to increase sensitivity. It presents the invention of the Photonic Lantern Nuller, which uses a multimode-to-single-mode demultiplexing waveguide to cancel out starlight while maintaining planet light, allowing for the direct characterization of planets at a telescope’s diffraction limit. The PLN was experimentally characterized in the lab, enhanced using common-path wavefront sensing control techniques, and demonstrated on sky at the Subaru Telescope. This thesis also presents work on the Keck Planet Imager and Characterizer, a fiber-fed high-resolution spectrograph --- namely, the on-sky demonstration of the speckle nulling technique to destructively interfere residual starlight. Future directions in optimal stellar suppression and instrument-informed data analysis techniques are also discussed.

The advances in instrumentation and methodology from this work have applications to giant planets on existing ten-meter class ground-based telescopes, Earth-like exoplanets on the planned Habitable Worlds Observatory space telescope, and many planets of interest on future thirty-meter class telescopes.

Thesis Files