Photobombing Earth 2.0: Diffraction-limit-related Contamination and Uncertainty in Habitable Planet Spectra

Abstract

Observing habitable exoplanets that may resemble Earth is a key priority in astronomy that is dependent on not only detecting such worlds but also ascertaining that apparent signatures of habitability are not due to other sources. Space telescopes designed to observe such worlds, such as that recommended by NASA’s 2020 Astrophysics Decadal Survey, have a diffraction-limited resolution that effectively spreads light from a source in a region around the source point. In this Letter, we show that the diffraction limit of a 6 m space telescope results in a point-spread function of an Earth-like planet that may contain additional unanticipated bodies for systems at distances relevant to the proposed searches. These unexpected additional objects, such as other planets and moons, can influence obtained spectra for a putative habitable planet by producing spurious features and adding additional uncertainty to the spectra. A model of Earth observed by a 6 m space telescope as though it was an exoplanet shows that the light from the Earth would be blended with the Moon, Mercury, Venus, and Mars in various combinations and at different times for numerous combinations of distance to the system and wavelength. Given the importance of extricating the true spectra of a potentially habitable planet in order to search for biosignatures, we highlight the need to account for this effect during the development of relevant telescopes and suggest some potential means of accounting for this photobombing effect.