Space Telescopes Might Be Seeing More Than They Bargained For

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(1) Prabal Saxena, CRESST II/University of Maryland, College Park, Maryland 20742, USA and NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA ([email protected]).

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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

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 meter 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 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 in the spectra. A model of the Earth observed by a 6 meter 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.

Efforts to harness the power of direct imaging to explore habitable worlds is underlied by the belief that Earth is not only under our feet, but may also exist over our heads in other regions of space. Contextual information that requires both observational and theoretical reconnaissance of promising planets and their systems is critical to ensure a suspected habitable world deserves the resources required to characterize it. Obtaining this contextual information must consider capabilities and limits of planned future telescopes at the forefront of exploring these high priority worlds. In this study we discuss how the diffraction limit of these future telescopes may influence observation strategy and outcomes. A significant manner in which diffraction limits may do this is alluded to in the title, where ’photobombing’ refers to an image that contains the unexpected appearance of an unintended object in the camera’s field of view as the image was taken. These objects may include other planets and moons, which can influence obtained spectra if they are unresolvable from the target planets’ point-spread function (PSF). Understanding where/how such blends may occur is important in development of future telescopes aimed at detecting habitable exoplanets, such as the recently recommended infrared/optical/ultraviolet telescope in the 2020 Astrophysical Decadal Survey (?). Recent frameworks for assessing potential biosignatures (Catling et al. 2018; Green et al. 2021) have stressed the need to first extricate a true signature potentially indicative of life and then vet potential complicating scenarios that may confound accurate interpretation of a signature, and we will show that photobombing by additional planets or moons may complicate both those requirements.

\ Exploration of this effect deserves a thorough treatment as there will likely be variation due to range of properties relevant to specific observations, including system inclination, planetary orbital phase, intrinsic spatial variations in each relevant body and other system and planetary characteristics. However, this letter is an initial exploratory study focusing on four topics that guide further study and constitute the paper’s sections. First, we examine the angular size of the diffraction limit of several telescope diameter/observational wavelength combinations as a function of distance to targeted systems. This includes comparison to key distances within potential planetary systems that may exist around stars of different stellar types - the habitable zone width (HZ) and size of the Hill radius for a mid-HZ Earth-twin. Second, we use our solar system as a model for the type of photobombing effect that may be observed for planetary systems with multiple small, rocky planets in/near the habitable zone. We examine the likelihood that additional planets appear in the Earth’s PSF as a function of time, if our solar system was observed using a 6 meter telescope. Third, we model consequences such contamination of the Earth’s PSF would have on snapshot visible and near-infrared spectra obtained for the Earth using the telescope from 10 parsecs away. Finally, we discuss other potential photobombing scenarios relevant to future observations and potential means of extricating the true Earth-twin spectrum from potential contamination.

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:::info This paper is available on arxiv under ATTRIBUTION-SHAREALIKE 4.0 INTERNATIONAL license.

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