Abstract
Exoplanets have been detected around stars at various stages of their lives, ranging from young stars emerging from formation to the latter stages of evolution, including white dwarfs and ...neutron stars. Post-main-sequence stellar evolution can result in dramatic, and occasionally traumatic, alterations to the planetary system architecture, such as tidal disruption of planets and engulfment by the host star. The
ρ
CrB system is a particularly interesting case of advanced main-sequence evolution, due to the relative late age and brightness of the host star, its similarity to solar properties, and the harboring of four known planets. Here, we use stellar evolution models to estimate the expected trajectory of the stellar properties of
ρ
CrB, especially over the coming 1.0–1.5 billion yr as it evolves off the main sequence. We show that the inner three planets (e, b, and c) are engulfed during the red giant phase and asymptotic giant branch, likely destroying those planets via either evaporation or tidal disruption at the fluid-body Roche limit. The outer planet, planet d, is briefly engulfed by the star several times toward the end of the asymptotic giant branch, but the stellar mass loss and subsequent changing planetary orbit may allow the survival of the planet into the white dwarf phase of the stellar evolution. We discuss the implications of this outcome for similar systems and describe the consequences for planets that may lie within the habitable zone of the system.
Abstract
The search for exoplanets has revealed a diversity of planetary system architectures, the vast majority of which diverge significantly from the template of the solar system. In particular, ...giant planets beyond the snow line are relatively rare, especially for low-mass stars, placing the solar system within a small category of systems with multiple giant planets at large separations. An exoplanetary system of note is that of HD 141399, consisting of a K-dwarf host star that harbors four giant planets with separations extending to ∼4.5 au. The architecture of the system creates a complex pattern of mean motion resonances and gravitationally perturbed regions that may exclude the presence of other planets, including within the habitable zone of the system. Here, we present the results of dynamical simulations that explore the interaction of the known planets of the system, their apsidal trajectories, resonance locations, and dynamical evolution. We further investigate the results of injecting Earth-mass planets and provide the regions of dynamical viability within the habitable zone where terrestrial planets may maintain long-term stability. We discuss these results in the context of the importance of giant planets for volatile delivery and planetary habitability considerations.
Among the numerous discoveries resulting from the Kepler mission are a plethora of compact planetary systems that provide deep insights into planet formation theories. The architecture of such ...compact systems also produces unique opportunities to study orbital dynamics in compact environments and the subsequent evolution of orbital parameters. One of the compact Kepler systems is Kepler-89, a system for which the radial velocity follow-up observations place strong upper limits on the masses of the planets and their Keplerian orbital elements. The potential for noncircular orbits in this system make it a compelling system to study dynamical constraints on the measured orbital parameters. I present a dynamical analysis of the system that demonstrates the stability of the circular model and shows that the eccentric model of the system is not stable. The analysis indicates that planets c and d, although close to the 2:1 secular resonance, do not permanently occupy the 2:1 resonance configuration. I explore regions of orbital parameter space to identify the upper bounds of orbital eccentricity for the planets. I further show how the dynamics in the compact system leads to significant periastron precession of the innermost planets. Finally, I quantify the effect of the periastron precession on the transit times of the planets compared with the cyclic variations expected from transit timing variations.
Among the most highly valued of exoplanetary discoveries are those of terrestrial planets found to reside within the habitable zone (HZ) of the host star. In particular, those HZ planets with ...relatively bright host stars will serve as priority targets for characterization observations, such as those involving mass determinations, transmission spectroscopy, and direct imaging. The properties of the star are greatly affected by the distance measurement to the star, and subsequent changes to the luminosity result in revisions to the extent of the HZ and the properties of the planet. This is particularly relevant in the realm of Gaia, which has released updated stellar parallaxes for the known exoplanet host stars. Here we provide a generalized formulation of the effect of distance on planetary system properties, including the HZ. We apply this methodology to three known systems and show that the recent Gaia Data Release 2 distances have a modest effect for TRAPPIST-1 but a relatively severe effect for Kepler-186 and LHS 1140.
Exoplanet hunting efforts have revealed the prevalence of exotic worlds with diverse properties, including Earth-sized bodies, which has fueled our endeavor to search for life beyond the Solar ...System. Accumulating experiences in astrophysical, chemical, and climatological characterization of uninhabitable planets are paving the way to characterization of potentially habitable planets. In this paper, we review our possibilities and limitations in characterizing temperate terrestrial planets with future observational capabilities through the 2030s and beyond, as a basis of a broad range of discussions on how to advance "astrobiology" with exoplanets. We discuss the observability of not only the proposed biosignature candidates themselves but also of more general planetary properties that provide circumstantial evidence, since the evaluation of any biosignature candidate relies on its context. Characterization of temperate Earth-sized planets in the coming years will focus on those around nearby late-type stars. The James Webb Space Telescope (JWST) and later 30-meter-class ground-based telescopes will empower their chemical investigations. Spectroscopic studies of potentially habitable planets around solar-type stars will likely require a designated spacecraft mission for direct imaging, leveraging technologies that are already being developed and tested as part of the Wide Field InfraRed Survey Telescope (WFIRST) mission. Successful initial characterization of a few nearby targets will be an important touchstone toward a more detailed scrutiny and a larger survey that are envisioned beyond 2030. The broad outlook this paper presents may help develop new observational techniques to detect relevant features as well as frameworks to diagnose planets based on the observables. Key Words: Exoplanets-Biosignatures-Characterization-Planetary atmospheres-Planetary surfaces. Astrobiology 18, 739-778.
The Transiting Exoplanet Survey Satellite (TESS) will be conducting a nearly all-sky photometric survey over two years, with a core mission goal to discover small transiting exoplanets orbiting ...nearby bright stars. It will obtain 30 minute cadence observations of all objects in the TESS fields of view, along with two-minute cadence observations of 200,000-400,000 selected stars. The choice of which stars to observe at the two-minute cadence is driven by the need to detect small transiting planets, which leads to the selection of primarily bright, cool dwarfs. We describe the catalogs assembled and the algorithms used to populate the TESS Input Catalog (TIC), including plans to update the TIC with the incorporation of the Gaia second data release in the near future. We also describe a ranking system for prioritizing stars according to the smallest transiting planet detectable, and assemble a Candidate Target List (CTL) using that ranking. We discuss additional factors that affect the ability to photometrically detect and dynamically confirm small planets, and we note additional stellar populations of interest that may be added to the final target list. The TIC is available on the STScI MAST server, and an enhanced CTL is available through the Filtergraph data visualization portal system at the URL http://filtergraph.com/tess_ctl.
Abstract
Multiplanet systems provide important laboratories for exploring dynamical interactions within the range of known exoplanetary system architectures. One such system is GJ 357, consisting of ...a low-mass host star and three orbiting planets, the outermost (planet d) of which does not transit but lies within the habitable zone (HZ) of the host star. The minimum mass of planet d causes its nature to be unknown, both in terms of whether it is truly terrestrial and if it is a candidate for harboring surface liquid water. Here, we use three sectors of photometry from the Transiting Exoplanet Survey Satellite to show that planets c and d do not transit the host star, and therefore may have masses higher than the derived minimum masses. We present the results for a suite of dynamical simulations that inject an Earth-mass planet within the HZ of the system for three different orbital and mass configurations of planet d. These results show that planet d, rather than being a potentially habitable planet, is likely a source of significant orbital instability for other potential terrestrial planets within the HZ. We find that relatively small eccentricities of planet d cause a majority of the HZ to be unstable for an Earth-mass planet. These results highlight the importance of dynamical stability for systems that are prioritized in the context of planetary habitability.
Exoplanet discoveries over recent years have shown that terrestrial planets are exceptionally common. Many of these planets are in compact systems that result in complex orbital dynamics. A key step ...toward determining the surface conditions of these planets is understanding the latitudinally dependent flux incident at the top of the atmosphere as a function of orbital phase. The two main properties of a planet that influence the time-dependent nature of the flux are the obliquity and orbital eccentricity of the planet. We derive the criterion for which the flux variation due to obliquity is equivalent to the flux variation due to orbital eccentricity. This equivalence is computed for both the maximum and average flux scenarios, the latter of which includes the effects of the diurnal cycle. We apply these calculations to four known multi-planet systems (GJ 163, K2-3, Kepler-186, and Proxima Centauri), where we constrain the eccentricity of terrestrial planets using orbital dynamics considerations and model the effect of obliquity on incident flux. We discuss the implications of these simulations on climate models for terrestrial planets and outline detectable signatures of planetary obliquity.
A Catalog of Habitable Zone Exoplanets Hill, Michelle L.; Bott, Kimberly; Dalba, Paul A. ...
The Astronomical journal,
02/2023, Volume:
165, Issue:
2
Journal Article
Peer reviewed
Open access
Abstract
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.
A key legacy of the recently launched the Transiting Exoplanet Survey Satellite (TESS) mission will be to provide the astronomical community with many of the best transiting exoplanet targets for ...atmospheric characterization. However, time is of the essence to take full advantage of this opportunity. The James Webb Space Telescope (JWST), although delayed, will still complete its nominal five year mission on a timeline that motivates rapid identification, confirmation, and mass measurement of the top atmospheric characterization targets from TESS. Beyond JWST, future dedicated missions for atmospheric studies such as the Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) require the discovery and confirmation of several hundred additional sub-Jovian size planets (Rp < 10 R⊕) orbiting bright stars, beyond those known today, to ensure a successful statistical census of exoplanet atmospheres. Ground-based extremely large telescopes (ELTs) will also contribute to surveying the atmospheres of the transiting planets discovered by TESS. Here we present a set of two straightforward analytic metrics, quantifying the expected signal-to-noise in transmission and thermal emission spectroscopy for a given planet, that will allow the top atmospheric characterization targets to be readily identified among the TESS planet candidates. Targets that meet our proposed threshold values for these metrics would be encouraged for rapid follow-up and confirmation via radial velocity mass measurements. Based on the catalog of simulated TESS detections by Sullivan et al., we determine appropriate cutoff values of the metrics, such that the TESS mission will ultimately yield a sample of ∼300 high-quality atmospheric characterization targets across a range of planet size bins, extending down to Earth-size, potentially habitable worlds.
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