How Nearby Planets Impact the Spectra of Earth-like Exoplanets

Table of Links

Abstract and 1. Introduction

2. Diffraction Limit Comparisons for Future Telescopes
3. Photobombing in our Solar System
4. Consequences for Obtained Spectra
5. Mitigation Strategies and Discussion, and References

4. CONSEQUENCES FOR OBTAINED SPECTRA

The possibility of Exo-Earth PSF contamination by other bodies necessitates understanding of the potential effect on obtained spectra. We extend our Earth as an exoplanet analogue to probe this effect by simulating photobombing spectra scenarios using the Planetary Spectrum Generator (PSG) (Villanueva et al. 2018). We simulated spectra of the inner solar system planets and Moon as observed by a 6m space telescope using a telescope/instrument configuration similar to the LUVOIR ECLIPS mode. We adapted models and observing parameters from Checlair et al. (2021) to simulate spectra of the system as viewed edge on (system inclination = 90◦ ) from 10 parsecs away with the planets at quadrature. Sampling techniques and radiative transfer details are given in Saxena et al. (2021) and observation geometry, planet, and instrument properties are in config files (see supplemental).

Simulated planet spectra assumed a 1000 second maximum exposure time based on current and planned missions’ attempts to minimize impact of cosmic ray interference (Poberezhskiy et al. 2021; Giardino et al. 2019). Total observation time was set to be equal for all planets to probe the photobombing effects and set to 1000 hours, the approximate time required to distinguish key features in the Earth’s visible spectra to ≈ 5σ. This is also the approximate time of a single continuous starshade observability window assumed for the HabEx mission (Gaudi et al. 2020) and roughly half the time of the x-axis spacing in figure 2.

Results are in figure 3 for the visible/near-infrared (left/right) wavelength ranges of ECLIPS. We plot individual planet spectra and spectra (with noise) of Earth alone and the Earth with three other photobombing scenarios in the visible, and Earth spectra (with noise) and two other photobombing scenarios for the near-infrared. The most notable feature of photobombing scenarios in the visible (Earth + Moon, Earth + Mars and Earth + Mars + Moon) is that in each case the additional source in the PSF results in additional flux and/or noise that complicates interpretation of spectra with respect to extricating molecular signatures. In the Earth + Moon scenario, overall continuum flux level is barely changed relative to the Earth only case, but oxygen feature depth is reduced and there is an increase in noise that reduces the significance of multiple features. In other visible spectra photobombing scenarios, the continuum flux and noise goes up significantly. While this reduces significance of some molecular features, there is also an additional spurious increase in the oxygen signature absorption depth, which may result in an erroneously interpreted greater

oxygen abundance. In the infrared spectra, photobombing cases have some minor effects on continuum and molecular signature flux (with the Earth + Mars case having larger effects), but the greatest effect is significant increase in noise. This increase reduces the significance of molecular signatures (particularly in the Earth + Mars case) such that robust detection may not be possible in such observing scenarios. Note these simplified scenarios provide a snapshot of effects on spectra, without phase and inclination effects taken into account, and no consideration of other factors that may cause time varying spectra. They are also dependent on observational assumptions listed above.