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The search for habitable planets has revealed many planets that can vary greatly from an Earth analog environment. These include highly eccentric orbits, giant planets, different bulk densities, relatively active stars, and evolved stars. This work catalogs all planets found to reside in the habitable zone (HZ) and provides HZ boundaries, orbit characterization, and the potential for spectroscopic follow-up observations. Demographics of the HZ planets are compared with a full catalog of exoplanets. Extreme planets within the HZ are highlighted, and how their unique properties may affect their potential habitability is discussed. Kepler-296 f is the most eccentric ≤2 R ⊕ planet that spends 100% of its orbit in the HZ. HD 106270 b and HD 38529 c are the most massive planets (≤13 M J) that orbit within the HZ, and are ideal targets for determining the properties of potential hosts of HZ exomoons. These planets, along with the others highlighted, will serve as special edge cases to the Earth-based scenario, and observations of these targets will help test the resilience of habitability outside the standard model. The most promising observational HZ target that is known to transit is GJ 414 A b. Of the transiting, ≤2 R ⊕ HZ planets, LHS 1140 b, TRAPPIST-1 d, and K2-3 d are the most favorable. Of the nontransiting HZ planets, HD 102365 b and 55 Cnc f are the most promising, and the best nontransiting candidates that have ≤2 R ⊕ are GJ 667 C c, Wolf 1061 c, Ross 508 b, Teegarden’s Star b, and Proxima Cen b.

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.

Understanding the physical characteristics of Venus, including its atmosphere, interior, and its evolutionary pathway with respect to Earth, remains a vital component for terrestrial planet evolution models and the emergence and/or decline of planetary habitability. A statistical strategy for evaluating the evolutionary pathways of terrestrial planets lies in the atmospheric characterization of exoplanets, where the sample size provides sufficient means for determining required runaway greenhouse conditions. Observations of potential exo-Venuses can help confirm hypotheses about Venus’s past, as well as the occurrence rate of Venus-like planets in other systems. Additionally, the data from future Venus missions, such as DAVINCI, EnVision, and VERITAS, will provide valuable information regarding Venus, and the study of exo-Venuses will be complimentary to these missions. To facilitate studies of exo-Venus candidates, we provide a catalog of all confirmed terrestrial planets in the Venus zone, including transiting and nontransiting cases, and quantify their potential for follow-up observations. We examine the demographics of the exo-Venus population with relation to stellar and planetary properties, such as the planetary radius gap. We highlight specific high-priority exo-Venus targets for follow-up observations, including TOI-2285 b, LTT 1445 A c, TOI-1266 c, LHS 1140 c, and L98–59 d. We also discuss follow-up observations that may yield further insight into the Venus/Earth divergence in atmospheric properties.

Given the vast number of stars that exist within binary systems, it remains important to explore the effect of binary star environments on the formation and evolution of exoplanetary systems. Nearby binaries provide opportunities to characterize their properties and orbits through a combination of radial velocities, astrometry, and direct imaging. Eta Cassiopeiae is a bright, well-known binary system for which recent observations have provided greatly improved stellar masses and orbital parameters. We present additional radial velocity data that are used to perform an injection-recovery analysis for potential planetary signatures. We further provide a detailed dynamical study that explores the viability of planetary orbits throughout the system. Our combined analysis shows that giant planets are significantly ruled out for the system, and indeed no planetary orbits are viable beyond ∼8 au of the primary star. However, terrestrial planets may yet exist within the habitable zone where orbits can remain long-term stable. We discuss the implications of these results, highlighting the effect of wide binary companions on giant planet formation, and the consequences for occurrence rates and planetary habitability.

The discovery of planetary systems beyond the solar system has revealed a diversity of architectures, most of which differ significantly from our system. The initial detection of an exoplanet is often followed by subsequent discoveries within the same system as observations continue, measurement precision is improved, or additional techniques are employed. The HD 104067 system is known to consist of a bright K-dwarf host star and a giant planet in a ∼55 days period eccentric orbit. Here we report the discovery of an additional planet within the HD 104067 system, detected through the combined analysis of radial velocity (RV) data from the High Resolution Echelle Spectrometer and High Accuracy Radial velocity Planet Searcher instruments. The new planet has a mass similar to Uranus and is in an eccentric ∼14 days orbit. Our injection-recovery analysis of the RV data exclude Saturn-mass and Jupiter-mass planets out to 3 au and 8 au, respectively. We further present Transiting Exoplanet Survey Satellite observations that reveal a terrestrial planet candidate (R p = 1.30 ± 0.12 R ⊕) in a ∼2.2 days period orbit. Our dynamical analysis of the three planet model shows that the two outer planets produce significant eccentricity excitation of the inner planet, resulting in tidally induced surface temperatures as high as ∼2600 K for an emissivity of unity. The terrestrial planet candidate may therefore be caught in a tidal storm, potentially resulting in its surface radiating at optical wavelengths.

The origin and evolution of planetary rings and moons remains an active area of study, particularly as they relate to the impact history and volatile inventory of the outer solar system. The Uranian system contains a complex system of rings that are coplanar with the highly inclined planetary equator relative to the orbital plane. Uranus also harbors five primary regular moons that play an important role in the distribution of material that surrounds the planet. Here we present the results of a dynamical simulation suite for the Uranian system, intended to explore the interaction between the five primary regular moons and particles within the system. We identify regions of extreme mass loss within 40 planetary radii of Uranus, including eccentricity excitation of particle orbits at resonance locations that can promote moonlet formation within the rings. We calculate a total dynamical particle mass-loss rate of 35% within 0.5 × 106 yr and 40% mass loss within 107 yr. We discuss the implications for postimpact material, including dynamical truncation of stable ring locations and/or locations of moon formation promoted by dynamical excitation of ring material.

Multiplanet systems exhibit a diversity of architectures that diverge from the solar system and contribute to the topic of exoplanet demographics. Radial velocity (RV) surveys form a crucial component of exoplanet surveys, as their long observational baselines allow for searches for more distant planetary orbits. This work provides a significantly revised architecture for the multiplanet system HD 134606 using both HARPS and UCLES RVs. We confirm the presence of previously reported planets b, c, and d with periods of 12.0897−0.0018+0.0019 , 58.947−0.054+0.056 , and 958.7−5.9+6.3 days and masses of 9.14−0.63+0.65 , 11.0 ± 1, and 44.5 ± 2.9 Earth masses, respectively, with the planet d orbit significantly revised to over double that originally reported. We report two newly detected super-Earths, e and f, with periods of 4.31943−0.00068+0.00075 and 26.9−0.017+0.019 days and masses of 2.31−0.35+0.36 and 5.52−0.73+0.74 Earth masses, respectively. In addition, we identify a linear trend in the RV time series, and the cause of this acceleration is deemed to be a newly detected massive companion with a very long orbital period. HD 134606 now displays four low-mass planets in a compact region near the star, one gas giant further out in the habitable zone, an additional companion in the outer regime, and a low-mass M dwarf stellar companion at large separation, making it an intriguing target for system formation/evolution studies. The location of planet d in the habitable zone proves to be an exciting candidate for future space-based direct imaging missions, whereas continued RV observations of this system are recommended for understanding the nature of the massive, long-period companion.

The unprecedented contrast ratio of future direct imaging (DI) instruments offers a new pathway to discover and characterize exoplanets around nearby stars. The Nancy Grace Roman Space Telescope, set for launch before May 2027, will conduct a science demonstration mission with its coronagraph instrument to test the new technologies for high-contrast direct imaging of exoplanets. This mission serves as a stepping stone to the Habitable Worlds Observatory, which aims to directly image and characterize Earth-like planets in the habitable zones of nearby stars in the 2040s. To ensure the success of these highly anticipated missions, extensive precursor science is required. This dissertation addresses critical questions for the DI community and demonstrates the role of the radial velocity (RV) method in supporting these goals. I will showcase the RV method as not only a primary discovery tool but also as a unique means to reveal hidden details about planetary systems that other methods cannot access. Then, I will present a method that synergizes RV and DI and explores the parameter space within which planetary signatures are hidden in the RV signal noise due to precision limit but could be detected by DI. The information returned by complementing DI with RV will be crucial to an effective DI target selection process. Next, I will present two planetary discoveries along with a full characterization of a nearby system harnessing the full of power the RV. The result of which provides additional valuable candidates to the DI community. Lastly, I will investigate how observational factors influence the refinement of orbital periods of long-period planets and offer strategies for efficient ground-based RV precursor observations to the DI candidates. These strategies are designed to maximize reductions in planetary orbital location uncertainty using RV within limited time frames, such that DI candidates can be observed at the optimal epochs. The results contained within this dissertation provides key insights and guidelines for precursor science to maximize the science return of future DI missions.

As long as astronomers have searched for exoplanets, the intrinsic variability of host stars has interfered with the ability to reliably detect and confirm exoplanets. One particular source of false positives is the presence of stellar magnetic or chromospheric activity that can mimic the radial velocity reflex motion of a planet. Here we present the results of a photometric data analysis for the known planet-hosting star BD –06°1339, observed by the Transiting Exoplanet Survey Satellite during Sector 6 at a cadence of 2 minutes. We discuss evidence that suggests that the observed 3.9-day periodic radial velocity signature may be caused by stellar activity rather than a planetary companion, since variability detected in the photometric data is consistent with the periodic signal. We conclude that the previously reported planetary signature is likely the result of a false-positive signal resulting from stellar activity, and we discuss the need for more data to confirm this conclusion.

Planetary systems with multiple giant planets provide important opportunities to study planetary formation and evolution. The HD 45364 system hosts two giant planets that reside within the habitable zone (HZ) of their host star and was the first system discovered with a 3:2 mean motion resonance (MMR). Several competing migration theories with different predictions have previously provided explanations regarding the observed resonance through dynamical simulations that utilized limited data. Here, over ten years since the original discovery, we revisit the system with a substantially increased radial velocity (RV) sample from High Accuracy Radial Velocity Planet Searcher spectrograph and High Resolution Echelle Spectrometer that significantly extends the observational baseline. We present the revised orbital solutions for the two planets using both Keplerian and dynamical models. Our RV models suggest orbits that are more circular and separated than those previously reported. As a result, the predicted strong planet–planet interactions were not detected. The system dynamics were reanalyzed, and the planet pair was found to exhibit apsidal behavior of both libration and circulation, indicating a quasi-resonance state rather than being truly in MMR. The new orbital solution and dynamical state of the system confirm migration models that predicted near-circular orbits as the preferred scenario. We also study the habitability prospects of this system and found that an additional Earth-mass planet and exomoons in the HZ are possible. This work showcases the importance of continued RV observations and its impact on our knowledge of the system’s dynamical history. HD 45364 continues to be an interesting target for both planetary formation and habitability studies.