Probing the Capability of Future Direct-imaging Missions to Spectrally Constrain the Frequency of Earth-like Planets

Abstract

A critical question in astrobiology is whether exo-Earth candidates (EECs) are Earth-like, in that they originate life that progressively oxygenates their atmospheres similarly to Earth. We propose answering this question statistically by searching for O2 and O3 on EECs with missions such as HabEx or LUVOIR. We explore the ability of these missions to constrain the fraction, f E, of EECs that are Earth-like in the event of a null detection of O2 or O3 on all observed EECs. We use the Planetary Spectrum Generator to simulate observations of EECs with O2 and O3 levels based on Earth’s history. We consider four instrument designs—LUVOIR-A (15 m), LUVOIR-B (8 m), HabEx with a starshade (4 m, “HabEx/SS”), and HabEx without a starshade (4 m, “HabEx/no-SS”)—as well as three estimates of the occurrence rate of EECs (η earth): 24%, 5%, and 0.5%. In the case of a null detection, we find that for η earth = 24%, LUVOIR-A, LUVOIR-B, and HabEx/SS would constrain f E to ≤0.094, ≤0.18, and ≤0.56, respectively. This also indicates that if f E is greater than these upper limits, we are likely to detect O3 on at least one EEC. Conversely, we find that HabEx/no-SS cannot constrain f E, due to the lack of a coronagraph ultraviolet channel. For η earth = 5%, only LUVOIR-A and LUVOIR-B would be able to constrain f E, to ≤0.45 and ≤0.85, respectively. For η earth = 0.5%, none of the missions would allow us to constrain f E, due to the low number of detectable EECs. We conclude that the ability to constrain f E is more robust to uncertainties in η earth for missions with larger aperture mirrors. However, all missions are susceptible to an inconclusive null detection if η earth is sufficiently low.

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

“…We simulated spectra of the inner solar system planets and Moon as observed by a 6 m 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 pc away with the planets at quadrature. Sampling Simulated planet spectra assumed a 1000 s maximum exposure time based on current and planned missions' attempts to minimize the impact of cosmic-ray interference (Giardino et al 2019;Poberezhskiy et al 2021).…”

“…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).…”

Unveiling Nongray Surface of Cloudy Exoplanets: The Influence of Wavelength-dependent Surface Albedo and Cloud Scattering Properties on Retrieval Solutions

“…Next-generation groundbased telescopes such as the TMT, ELT, etc., will be capable of performing high-contrast imaging and high-resolution spectroscopy on exoplanets from gas giants down to sub-Neptunes (Quanz et al 2015;Snellen et al 2015;Mazin et al 2019). Upcoming space-based direct-imaging mission concepts (e.g., the Habitable Exoplanet Observatory, HabEx, and the Large UV/Optical/IR Surveyor, LUVOIR) could reach a contrast ratio of order 10 −10 and would perform low-resolution spectroscopy of terrestrial exoplanets orbiting nearby stars (Fujii et al 2018;Dressing et al 2019;Roberge et al 2019Roberge et al , 2021Checlair et al 2021). Although the above spaceand ground-based telescopes are still years away, it is crucial to investigate what information can be extracted from measurements of reflected starlight from spatially unresolved planets.…”

Hierarchical Bayesian Atmospheric Retrieval Modeling for Population Studies of Exoplanet Atmospheres: A Case Study on the Habitable Zone

“…Since exoplanet atmospheric compositions must be inferred from spectroscopic observations using retrieval models, any trends in atmospheric composition fall into the category of multilevel or hierarchical inference problems. While numerous trends in exoplanet atmospheric composition have been suggested as a means to understand exoplanet habitability (e.g., Turbet et al 2019;Checlair et al 2019;Bixel & Apai 2020;Checlair et al 2021;Bixel & Apai 2021), a consistent framework to tackle the hierarchical atmospheric retrieval problem-going from spectroscopic observations to population-level atmospheric trends-has not been presented.…”