The Best Stars for Directly Imaging Mature Exoplanets from Space

Nearby G-K dwarfs will be prime targets for the discovery of exoplanets in reflected light with future space-based imaging observations such as the Roman Space Telescope, the Habitable Worlds Observatory (HWO) and the Large Interferometer For Exoplanets (LIFE). The detection and measurement of the atmospheric composition of cool planets around mature stars will require integration times exceeding hundreds of hours, making target selection a critical task. Three important considerations are (i) orbital inclinations, which affect the probability of detecting planets through direct imaging and can be inferred from the stellar line of sight inclinations, (ii) the ages of the host stars, which are required to interpret detections of possible atmospheric biomarkers, and (iii) the timescales and amplitudes of stellar variability, which affect our capability of finding candidate planets through extreme precision radial velocity surveys. We will discuss how a combination of time-domain space-based photometry and ground-based extreme precision radial velocity campaigns will be able to solve this problem through the systematic application of high-resolution spectroscopy, asteroseismology and gyrochronology. In particular, we will show how the novel 20-second cadence data from TESS provides unprecedented photometric precision for asteroseismology and rotation period measurements of nearby stars, and present the first asteroseismic detections in K dwarfs enabled by extreme precision radial velocities from the Keck Planet Finder and ESPRESSO. These precursor observations require dedicated long-term campaigns that should be designed now in order to help achieve the goals of next-generation space-based direct-imaging facilities.