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Exoplanetary science is increasingly prioritizing efforts toward direct imaging of planetary systems, with emphasis on those that may enable the detection and characterization of potentially habitable exoplanets. The recent 2020 Astronomy and Astrophysics decadal survey recommended the development of a space-based direct imaging mission that has subsequently been referred to as the Habitable Worlds Observatory (HWO). A fundamental challenge in the preparatory work for the HWO search for exo-Earths is the selection of suitable stellar targets. Much of the prior efforts regarding the HWO targets has occurred within the context of exoplanet surveys that have characterized the stellar properties for the nearest stars. The preliminary input catalog for HWO consists of 164 stars, of which 30 are known exoplanet hosts to 70 planets. Here, we provide a dynamical analysis for these 30 systems, injecting a terrestrial planet mass into the habitable zone (HZ) and determining the constraints on stable orbit locations due to the influence of the known planets. For each system, we calculate the percentage of the HZ that is dynamically viable for the potential presence of a terrestrial planet, providing an additional metric for inclusion of the stars within the HWO target list. Our analysis shows that, for 11 of the systems, less than 50% of the HZ is dynamically viable, primarily due to the presence of giant planets whose orbits pass near or through the HZ. These results demonstrate the impact that known system architectures can have on direct imaging target selection and overall system habitability.

Stellar X-ray and UV radiation can significantly affect the survival, composition, and long-term evolution of the atmospheres of planets in or near their host star’s habitable zone (HZ). Especially interesting are planetary systems in the solar neighborhood that may host temperate and potentially habitable surface conditions, which may be analyzed by future ground- and space-based direct-imaging surveys for signatures of habitability and life. To advance our understanding of the radiation environment in these systems, we leverage ∼3 Ms of XMM-Newton and Chandra observations in order to measure three fundamental stellar properties at X-ray energies for 57 nearby FGKM stellar systems: the shape of the stellar X-ray spectrum, the luminosity, and the timescales over which the stars vary (e.g., due to flares). These systems possess HZs that will be directly imageable to next-generation telescopes such as the Habitable Worlds Observatory and ground-based Extremely Large Telescopes. We identify 29 stellar systems with L X/L bol ratios similar to (or less than) that of the Sun; any potential planets in the HZs of these stars therefore reside in present-day X-ray radiation environments similar to (or less hostile than) modern Earth, though a broader set of these targets could host habitable planets. An additional 19 stellar systems have been observed with the Swift X-ray Telescope; in total, only ∼30% of potential direct imaging target stars has been observed with XMM-Newton, Chandra, or Swift. The data products from this work (X-ray light curves and spectra) are available via a public Zenodo repository (doi:10.5281/zenodo.11490574).

We present an analysis of XMM-Newton and Chandra observations of 85 nearby main-sequence FGK stars with age estimates ranging from 0.2 to 12 Gyr. We measure quiescent 0.3–10 keV luminosities, variability metrics, and multitemperature thermal plasma spectral parameters. Quiescent spectra are typically described by three characteristic plasma components (kT ≈ 0.1, 0.4, 0.8 keV); the fraction of flux from T ≥ 7 MK rises with X-ray surface flux, reaching ∼50% for FX ≳ 106 erg cm−2 s−1. We derive relations between emission measure-weighted coronal temperature and both LX and FX, enabling temperature-informed count rate conversions for faint sources. We quantify how bandpass conversions (ROSAT 0.1–2.4 keV versus XMM-Newton 0.3–10 keV) depend on temperature and show that inferred ROSAT-band LX broadly follows the canonical t−1.5 decay, while the harder band exhibits increased scatter at >4 Gyr. Several stars show excess activity suggestive of age errors, inclination effects, or unresolved companions. Some of these “outlier” stars are potential direct imaging targets for the Habitable Worlds Observatory, and detailed characterization of these stars is needed to inform their likely influence on the atmospheric evolution of orbiting planets.

The transit method of exoplanet discovery and characterization has enabled numerous breakthroughs in exoplanetary science. These include measurements of planetary radii, mass-radius relationships, stellar obliquities, bulk density constraints on interior models, and transmission spectroscopy as a means to study planetary atmospheres. The Transiting Exoplanet Survey Satellite (TESS) has added to the exoplanet inventory by observing a significant fraction of the celestial sphere, including many stars already known to host exoplanets. Here we describe the science extraction from TESS observations of known exoplanet hosts during the primary mission. These include transit detection of known exoplanets, discovery of additional exoplanets, detection of phase signatures and secondary eclipses, transit ephemeris refinement, and asteroseismology as a means to improve stellar and planetary parameters. We provide the statistics of TESS known host observations during Cycle 1 and 2, and present several examples of TESS photometry for known host stars observed with a long baseline. We outline the major discoveries from observations of known hosts during the primary mission. Finally, we describe the case for further observations of known exoplanet hosts during the TESS extended mission and the expected science yield.

Radial velocity (RV) surveys have detected hundreds of exoplanets through their gravitational interactions with their host stars. Some will be transiting, but most lack sufficient follow-up observations to confidently detect (or rule out) transits. We use published stellar, orbital, and planetary parameters to estimate the transit probabilities for nearly all exoplanets that have been discovered via the RV method. From these probabilities, we predict that 25.5 − 0.7 + 0.7 of the known RV exoplanets should transit their host stars. This prediction is more than double the amount of RV exoplanets that are currently known to transit. The Transiting Exoplanet Survey Satellite (TESS) presents a valuable opportunity to explore the transiting nature of many of the known RV exoplanet systems. Based on the anticipated pointing of TESS during its two-year primary mission, we identify the known RV exoplanets that it will observe and predict that 11.7 − 0.3 + 0.3 of them will have transits detected by TESS. However, we only expect the discovery of transits for ∼3 of these exoplanets to be novel (i.e., not previously known). We predict that the TESS photometry will yield dispositive null results for the transits of ∼125 RV exoplanets. This will represent a substantial increase in the effort to refine ephemerides of known RV exoplanets. We demonstrate that these results are robust to changes in the ecliptic longitudes of future TESS observing sectors. Finally, we consider how several potential TESS extended mission scenarios affect the number of transiting RV exoplanets we expect TESS to observe.

ABSTRACT As part of NASA's mission to explore habitable planets orbiting nearby stars, this article explores the detection and characterization capabilities of a 4 m space telescope plus 50 m starshade located at the Earth-Sun L2 point, known as the New Worlds Observer (NWO). Our calculations include the true spectral types and distribution of stars on the sky, an iterative target selection protocol designed to maximize efficiency based on prior detections, and realistic mission constraints. We conduct simulated observing runs for a wide range in exozodiacal background levels (ε = 1-100 times the local zodi brightness) and overall prevalence of Earth-like terrestrial planets (η⊕ = 0.1-1). We find that even without any return visits, the NWO baseline architecture (IWA = 65 mas, limiting FPB = 4 × 10-11) can achieve a 95% probability of detecting and spectrally characterizing at least one habitable Earth-like planet and an expectation value of ∼3 planets found, within the mission lifetime and ΔV budgets, even in the worst-case scenario (η⊕ = 0.1 and ε = 100 zodis for every target). This achievement requires about 1 yr of integration time spread over the 5 yr mission, leaving the remainder of the telescope time for UV-NIR general astrophysics. Cost and technical feasibility considerations point to a \"sweet spot\" in starshade design near a 50 m starshade effective diameter, with 12 or 16 petals, at a distance of 70,000-100,000 km from the telescope.

A set of simulations with a 3D global climate model are performed to investigate the roles of obliquity and rotation period in the habitability of Earthlike exoplanets. The simulations cover the obliquity–rotation parameter space, from 0° to 90° in obliquity and 1–128 days in rotation period. The simulated global mean temperatures are warmest at 45° obliquity with fast rotations, due to the modification of the greenhouse effect from the spatial redistribution of clouds and water vapor. The slow-moving insolation–cloud mechanism, previously found in simulations with slow rotations and zero obliquity, also produces a cooling trend from intermediate obliquity to high obliquity, with the coldest climate occurring at 90° obliquity for all rotation periods. At low obliquities and fast rotation, persistent snow and sea ice can form, producing cooler temperatures. A Climate Habitability metric is defined, based on temperature and precipitation, which compares well with observations when applied to a simulation using Earth’s obliquity and rotation. Over a wider range of obliquity and rotation period, the Climate Habitability ranges from 10% to 70% of the terrestrial area. Overall, the simulated global mean surface temperature shows a much larger spread across the range of simulated rotation periods at 45° obliquity compared to 0° obliquity. Therefore, we conclude that 3D exoplanet simulations using intermediate obliquities (e.g., 45°) instead of 0° will reveal a wider range of possible climate conditions for specific orbital configurations. In addition, Earth’s climate habitability can increase by 25% if the obliquity increases from 23.°5 to 45°.

The Terran world Spectral Simulator (TSS) is a flexible software package for modeling direct detection of reflected stellar radiation from Earth-similar terrestrial exoplanets. Exoplanets similar to Earth require more sophisticated simulation tools than planets with optically thick atmospheres, because both the atmospheric composition and spatial distribution of clouds and surface materials will impact the integrated reflected radiation. Thus, accurate simulations need to employ a three-dimensional approach where the exoplanet surface and cloud field are explicitly modeled. Our modeling framework is designed using a modular approach which splits the explicit radiative transfer calculations from the geometric calculations to produce a disk-integrated reflectance. The modular layout allows different radiative transfer models to be used, and their outputs can be efficiently re-used in larger simulations of orbital phase or rotational light curves. The model is designed to compute unpolarized disk-integrated reflectance spectra. These simulated spectra can help inform preparatory science activities for future direct-imaging missions, such as the WFIRST CGI, HabEx, and LUVOIR. This work highlights several case studies using the TSS: simulation of rotational light curves for a modern Earth twin, simulation of the EPOXI Earth observations, and simulation of a past Earth from a paleoclimate simulation. These case studies illustrate that the TSS simulations agree well with the EPOXI Earth observations, and illustrates how the TSS can be used to support exoplanet research.

We present two state-of-the-art models of the solar system, one corresponding to the present day and one to the Archean Eon 3.5 billion years ago. Each model contains spatial and spectral information for the star, the planets, and the interplanetary dust, extending to 50 au from the Sun and covering the wavelength range 0.3–2.5 μm. In addition, we created a spectral image cube representative of the astronomical backgrounds that will be seen behind deep observations of extrasolar planetary systems, including galaxies and Milky Way stars. These models are intended as inputs to high-fidelity simulations of direct observations of exoplanetary systems using telescopes equipped with high-contrast capability. They will help improve the realism of observation and instrument parameters that are required inputs to statistical observatory yield calculations, as well as guide development of post-processing algorithms for telescopes capable of directly imaging Earth-like planets.

The Terran world Spectral Simulator (TSS) is a flexible software package for modeling direct detection of reflected stellar radiation from Earth-similar terrestrial exoplanets. Exoplanets similar to Earth require more sophisticated simulation tools than planets with optically thick atmospheres, because both the atmospheric composition and spatial distribution of clouds and surface materials will impact the integrated reflected radiation. Thus, accurate simulations need to employ a three-dimensional approach where the exoplanet surface and cloud field are explicitly modeled. Our modeling framework is designed using a modular approach which splits the explicit radiative transfer calculations from the geometric calculations to produce a disk-integrated reflectance. The modular layout allows different radiative transfer models to be used, and their outputs can be efficiently re-used in larger simulations of orbital phase or rotational light curves. The model is designed to compute unpolarized disk-integrated reflectance spectra. These simulated spectra can help inform preparatory science activities for future direct-imaging missions, such as the WFIRST CGI, HabEx, and LUVOIR. This work highlights several case studies using the TSS: simulation of rotational light curves for a modern Earth twin, simulation of the EPOXI Earth observations, and simulation of a past Earth from a paleoclimate simulation. These case studies illustrate that the TSS simulations agree well with the EPOXI Earth observations, and illustrates how the TSS can be used to support exoplanet research.