The Wide Field Infrared Survey Telescope (WFIRST) Coronagraphic Instrument (CGI) technology demonstration and potential science mission, as well as mission concepts with exoplanet imaging capabilities such as HabEx and LUVOIR, all require the identification of the best targets for exoplanet observations. To date, the focus has been primarily on two classes of targets: those with known exoplanets discovered indirectly that may be observable by these imaging missions, and those targets with no known planets that have high completeness values (probabilities of planet detections) under some assumptions of the instrument performance and the overall population of exoplanets. A third class of target, however, has received much less scrutiny: stars with known exoplanets that could not possibly be directly imaged due to the size of their orbits. These are planets that would be guaranteed to spend all of their time inside the inner working angles or outside the outer working angles of all currently proposed coronagraphs. However, these systems could potentially harbor additional planets, either exterior to or interior to the currently known planets, but not yet detectable by indirect means. Here, we discuss how to identify systems from all three categories that would be good targets for WFIRST and other, future, space-based imagers. We present a method for assessing the utility of these targets based on an exploration of the available dynamical phase space of the systems that would result in long-term stable orbits for both the currently known and the potentially discoverable companions, and show how it augments existing methods for assessing target completeness and utility.
Vortex fiber nulling (VFN) is a method that may enable the detection and characterization of exoplanets at small angular separations (0.5-2 λ/D) with ground- and space-based telescopes. Since the field of view is within the inner working angle of most coronagraphs, nulling accesses non-transiting planets that are otherwise too close to their star for spectral characterization by other means, thereby significantly increasing the number of known exoplanets available for direct spectroscopy in the near-infrared. Furthermore, VFN targets planets on closer-in orbits which tend to have more favorable planet-to-star flux ratios in reflected light. Here, we present the theory and applications of VFN, show that the optical performance is approximately equivalent for a variety of implementations and aperture shapes, and discuss the trade-offs between throughput and engineering requirements using numerical simulations. We compare vector and scalar approaches and, finally, show that beam shaping optics may be used to significantly improve the throughput for planet light. Based on theoretical performance, we estimate the number of known planets and theoretical exoEarths accessible with a VFN instrument linked to a high-resolution spectrograph on the future Thirty Meter Telescope.