The exozodi spectral effect: Residual habitable zone dust may bias exoEarth characterization

Abstract

All exoplanetary systems are expected to host exozodiacal dust, or exozodi, originating from planetesimals. For many stars, exozodi will likely be the largest source of astrophysical noise in direct observations of terrestrial exoplanets. Nearby Sun-like systems likely have $\gtrsim3$x more habitable zone (HZ) dust than our solar system, which must be removed from direct images and spectra to reveal exoplanet signals. The albedo of this micron-sized dust varies smoothly over VIS -- NIR wavelengths, but exozodi can be composed of different materials that can impact its color. If left unsubtracted, exozodiacal light will add cloud-like continuum emission to extracted spectra, potentially biasing characterization studies by reducing the apparent absorption depth of spectral features. To quantify these effects, we simulate exoEarth systems with a range of exozodi densities and compositions, and apply an atmospheric retrieval tool to synthetic Habitable Worlds Observatory (HWO) spectra. We find that exozodi at levels similar to the solar system (i.e., 1 zodi) can reduce the apparent depth of visible wavelength molecular absorption features by up to 50\%, an effect that worsens at longer wavelengths. To measure molecular abundances, significant post-processing may be required to remove exozodi to a fractional residual that tightens with dust density. However, targeting a binary detection result for an absorbing species instead relaxes this requirement by an order of magnitude, especially at higher spectral resolution. Understanding and mitigating the effects of exozodi in extracted exoEarth spectra is critical to characterize HZ exoplanet environments with HWO and ultimately to search for signs of habitability and life.