Planets In Habitable Zone Research Articles

Multi-Observatory Research of Young Stellar Energetic Flares (MORYSEF): X-Ray-flare-related Phenomena and Multi-epoch Behavior

The most powerful stellar flares driven by magnetic energy occur during the early pre-main-sequence (PMS) phase. The Orion Nebula represents the nearest region populated by young stars, showing the greatest number of flares accessible to a single pointing of Chandra. This study is part of a multi-observatory project to explore stellar surface magnetic fields (with the Hobby–Eberly Telescope Habitable-zone Planet Finder, HET-HPF), particle ejections (with the Very Long Baseline Array, VLBA), and disk ionization (with the Atacama Large Millimeter/submillimeter Array, ALMA) immediately following the detection of PMS superflares with Chandra. In 2023 December, we successfully conducted such a multi-telescope campaign. Additionally, by analyzing Chandra data from 2003, 2012, and 2016, we examine the multi-epoch behavior of PMS X-ray emission related to PMS magnetic cyclic activity and ubiquitous versus sample-confined megaflaring. Our findings are as follows. (1) We report detailed stellar quiescent and flare X-ray properties for numerous HET/ALMA/VLBA targets, facilitating ongoing multiwavelength analyses. (2) For numerous moderately energetic flares, we report correlations (or lack thereof) between flare energies and stellar mass/size (presence/absence of disks) for the first time. The former is attributed to the correlation between convection-driven dynamo and stellar volume, while the latter suggests the operation of solar-type flare mechanisms in PMS stars. (3) We find that most PMS stars exhibit minor long-term baseline variations, indicating the absence of intrinsic magnetic dynamo cycles or observational mitigation of cycles by saturated PMS X-rays. (4) We conclude that X-ray megaflares are ubiquitous phenomena in PMS stars, which suggests that all protoplanetary disks and nascent planets are subject to violent high-energy emission and particle irradiation events.

Searching for GEMS: Characterizing Six Giant Planets Around Cool Dwarfs

Transiting giant exoplanets around M-dwarf stars (GEMS) are rare, owing to the low-mass host stars. However, the all-sky coverage of TESS has enabled the detection of an increasingly large number of them to enable statistical surveys like the Searching for GEMS survey. As part of this endeavor, we describe the observations of six transiting giant planets, which include precise mass measurements for two GEMS (K2-419Ab, TOI-6034b) and statistical validation for four systems, which includes validation and mass upper limits for three of them (TOI-5218b, TOI-5616b, TOI-5634Ab), while the fourth one—TOI-5414b is classified as a “likely planet.” Our observations include radial velocities from the Habitable-zone Planet Finder on the Hobby–Eberly Telescope, and MAROON-X on Gemini-North, along with photometry and high-contrast imaging from multiple ground-based facilities. In addition to TESS photometry, K2-419Ab was also observed and statistically validated as part of the K2 mission in Campaigns 5 and 18, which provide precise orbital and planetary constraints despite the faint host star and long orbital period of ∼20.4 days. With an equilibrium temperature of only 380 K, K2-419Ab is one of the coolest known well-characterized transiting planets. TOI-6034 has a late F-type companion about 40″ away, making it the first GEMS host star to have an earlier main-sequence binary companion. These confirmations add to the existing small sample of confirmed transiting GEMS.

Tunable 30 GHz laser frequency comb for astronomical spectrograph characterization and calibration.

The search for Earth-like exoplanets with the Doppler radial velocity (RV) technique is an extremely challenging and multifaceted precision spectroscopy problem. Currently, one of the limiting instrumental factors in reaching the required long-term 10-10 level of radial velocity precision is the defect-driven subpixel quantum efficiency (QE) variations in the large-format detector arrays used by precision echelle spectrographs. Tunable frequency comb calibration sources that can fully map the point spread function (PSF) across a spectrograph's entire bandwidth are necessary for quantifying and correcting these detector artifacts. In this work, we demonstrate a combination of laser frequency and mode spacing control that allows full and deterministic tunability of a 30 GHz electro-optic comb together with its filter cavity. After supercontinuum generation, this gives access to any optical frequency across 700-1300 nm. Our specific implementation is intended for the comb deployed at the Habitable-Zone Planet Finder (HPF) spectrograph and its near-infrared Hawaii-2RG array, but the techniques apply to all laser frequency combs (LFCs) used for precision astronomical spectrograph calibration and other applications that require broadband tuning.

How Landmass Distribution Influences the Atmospheric Dynamics of Tidally Locked Terrestrial Exoplanets

Interpretation of the ongoing efforts to simulate the atmospheres of potentially habitable terrestrial exoplanets requires that we understand the underlying dynamics and chemistry of such objects to a much greater degree than 1D or even simple 3D models enable. Here, for the tidally locked habitable-zone planet TRAPPIST-1e, we explore one effect which can shape the dynamics and chemistry of terrestrial planets: the inclusion of an Earth-like land–ocean distribution with orography. To do this we use the Earth-system model WACCM6/CESM2 to run a pair of TRAPPIST-1e models with N2–O2 atmospheres and with the substellar point fixed over either land or ocean. The presence of orography shapes atmospheric transport, and in the case of Earth-like orography, breaks the symmetry between the Northern and Southern Hemispheres which was previously found in slab ocean models. For example, peak zonal jet speeds in the Southern Hemisphere are 50%–100% faster than similar jets in the Northern Hemisphere. This also affects the meridional circulation, transporting equatorial material toward the south pole. As a result we also find significant changes in the atmospheric chemistry, including the accumulation of potentially lethal quantities of ozone at both the south pole and the surface. Future studies which investigate the effects of landmass distribution on the dynamics of exoplanetary atmospheres should pay close attention to both the dayside land fraction as well as the orography of the land. Simply modeling a flat landmass will not give a complete picture of its dynamical impact.

The erosion of large primary atmospheres typically leaves behind substantial secondary atmospheres on temperate rocky planets

Exoplanet exploration has revealed that many—perhaps most—terrestrial exoplanets formed with substantial H2-rich envelopes, seemingly in contrast to solar system terrestrials, for which there is scant evidence of long-lived primary atmospheres. It is not known how a long-lived primary atmosphere might affect the subsequent habitability prospects of terrestrial exoplanets. Here, we present a new, self-consistent evolutionary model of the transition from primary to secondary atmospheres. The model incorporates all Fe-C-O-H-bearing species and simulates magma ocean solidification, radiative-convective climate, thermal escape, and mantle redox evolution. For our illustrative example TRAPPIST-1, our model strongly favors atmosphere retention for the habitable zone planet TRAPPIST-1e. In contrast, the same model predicts a comparatively thin atmosphere for the Venus-analog TRAPPIST-1b, which would be vulnerable to complete erosion via non-thermal escape and is consistent with JWST observations. More broadly, we conclude that the erosion of primary atmospheres typically does not preclude surface habitability, and frequently results in large surface water inventories due to the reduction of FeO by H2.

Statistics and Habitability of F-type Star–Planet Systems

F-type star–planet systems represent an intriguing case for habitability studies. Although F-type stars spend considerably less time on the main sequence (MS) than G-, K-, and M-type stars, they still offer a unique set of features, allowing for the principal possibility of exolife. Examples of these features include the increased widths of stellar habitable zones as well as the presence of enhanced UV flux, which in moderation may have added to the origin of life in the Universe. In this study, we pursue a detailed statistical analysis of the currently known planet-hosting F-type stars by making use of the NASA Exoplanet Archive. After disregarding systems with little or no information on the planet(s), we identify 206 systems of interest. We also evaluate whether the stars are on the MS based on various criteria. In one approach, we use the stellar evolution code MESA. Depending on the adopted criterion, about 60–80 stars have been identified as MS stars. In 18 systems, the planet spends at least part of its orbit within the stellar habitable zone. In one case, i.e., HD 111998, known as 38 Vir, the planet is situated in the habitable zone at all times. Our work may serve as a basis for future studies, including studies on the existence of Earth-mass planets in F-type systems, as well as investigations of possibly habitable exomoons hosted by exo-Jupiters, as the lowest-mass habitable zone planet currently identified has a mass estimate of 143 Earth masses.

Astrometry and Precise Radial Velocities Yield a Complete Orbital Solution for the Nearby Eccentric Brown Dwarf LHS 1610 b

The LHS 1610 system consists of a nearby (d = 9.7 pc) M5 dwarf hosting a candidate brown dwarf companion in a 10.6 days, eccentric (e ∼ 0.37) orbit. We confirm this brown dwarf designation and estimate its mass ( 49.5−3.5+4.3 M Jup) and inclination (114.5° −10.0+7.4 ) by combining discovery radial velocities (RVs) from the Tillinghast Reflector Echelle Spectrograph and new RVs from the Habitable-zone Planet Finder with the available Gaia astrometric two-body solution. We highlight a discrepancy between the measurement of the eccentricity from the Gaia two-body solution (e = 0.52 ± 0.03) and the RV-only solution (e = 0.3702 ± 0.0003). We discuss possible reasons for this discrepancy, which can be further probed when the Gaia astrometric time series become available as part of Gaia Data Release 4. As a nearby mid-M star hosting a massive short-period companion with a well-characterized orbit, LHS 1610 b is a promising target to look for evidence of sub-Alfvénic interactions and/or auroral emission at optical and radio wavelengths. LHS 1610 has a flare rate (0.28 ± 0.07 flares per day) on the higher end for its rotation period (84 ± 8 days), similar to other mid-M dwarf systems such as Proxima Cen and YZ Ceti that have recent radio detections compatible with star–planet interactions. While available Transiting Exoplanet Survey Satellite photometry is insufficient to determine an orbital phase dependence of the flares, our complete orbital characterization of this system makes it attractive to probe star–companion interactions with additional photometric and radio observations.

TOI-2015 b: A Warm Neptune with Transit Timing Variations Orbiting an Active Mid-type M Dwarf

We report the discovery of a close-in (P orb = 3.349 days) warm Neptune with clear transit timing variations (TTVs) orbiting the nearby (d = 47.3 pc) active M4 star, TOI-2015. We characterize the planet's properties using Transiting Exoplanet Survey Satellite (TESS) photometry, precise near-infrared radial velocities (RVs) with the Habitable-zone Planet Finder Spectrograph, ground-based photometry, and high-contrast imaging. A joint photometry and RV fit yields a radius Rp=3.37−0.20+0.15R⊕ , mass mp=16.4−4.1+4.1M⊕ , and density ρp=2.32−0.37+0.38gcm−3 for TOI-2015 b, suggesting a likely volatile-rich planet. The young, active host star has a rotation period of P rot = 8.7 ± 0.9 days and associated rotation-based age estimate of 1.1 ± 0.1 Gyr. Though no other transiting planets are seen in the TESS data, the system shows clear TTVs of super-period Psup≈430days and amplitude ∼100 minutes. After considering multiple likely period-ratio models, we show an outer planet candidate near a 2:1 resonance can explain the observed TTVs while offering a dynamically stable solution. However, other possible two-planet solutions—including 3:2 and 4:3 resonances—cannot be conclusively excluded without further observations. Assuming a 2:1 resonance in the joint TTV-RV modeling suggests a mass of mb=13.3−4.5+4.7M⊕ for TOI-2015 b and mc=6.8−2.3+3.5M⊕ for the outer candidate. Additional transit and RV observations will be beneficial to explicitly identify the resonance and further characterize the properties of the system.

A review for Kepler-453 binary system and its giant planet

Kepler 453 is a binary with a G star and a red dwarf found by Kepler telescope in 2015, and it has a giant planet named Kepler 453b detected by eclipsing in light curves with the period of 240.5 days, which will be showed in part 2. Based on the researches for circumbinary systems, this paper also discusses the radiation and plasma environment and some possibility for other celestial bodies like Trojan, earth-like planets in habitable zone. The result shows that Trojan has a narrow appropriate zone while the earth-size planets will be ejected by the current giant in Kepler 453.

Closeby Habitable Exoplanet Survey (CHES). I. Astrometric Noise and Planetary Detection Efficiency Due to Stellar Spots and Faculae

The Closeby Habitable Exoplanet Survey (CHES) is dedicated to the astrometric exploration for habitable-zone Earth-like planets orbiting solar-type stars in close proximity, achieving unprecedented microarcsecond precision. Given the elevated precision, meticulous consideration of photocenter jitters induced by stellar activity becomes imperative. This study endeavors to model the stellar activity of solar-type stars, compute astrometric noise, and delineate the detection limits of habitable planets within the astrometric domain. Simulations were conducted for identified primary targets of CHES, involving the generation of simulated observed data for astrometry and photometry, accounting for the impact of stellar activity. Estimation of activity levels in our sample was achieved through chromospheric activity indices, revealing that over 90% of the stars exhibited photocenter jitters below 1 μas. Notably, certain proximate stars, such as α Cen A and B, displayed more discernible noise arising from stellar activity. Subsequent tests were performed to evaluate detection performance, unveiling that stellar activity tends to have a less pronounced impact on planetary detectability for the majority of the stars. Approximately 95% of the targets demonstrated a detection efficiency exceeding 80%. However, for several cold stars, e.g., HD 32450 and HD 21531, with the habitable zones close to the stars, a reduction in detection efficiency was observed. These findings offer invaluable insights into the intricate interplay between stellar activity and astrometric precision, significantly advancing our understanding in the search for habitable planets.

The Epoch of Giant Planet Migration Planet Search Program. II. A Young Hot Jupiter Candidate around the AB Dor Member HS Psc* *Based on observations obtained with the Hobby–Eberly Telescope (HET), which is a joint project of the University of Texas at Austin, the Pennsylvania State University, Ludwig-Maximillians-Universitaet Muenchen, and Georg-August Universitaet Goettingen. The HET is named in honor of

We report the discovery of a hot Jupiter candidate orbiting HS Psc, a K7 (≈0.7 M ⊙) member of the ≈130 Myr AB Doradus moving group. Using radial velocities over 4 yr from the Habitable-zone Planet Finder spectrograph at the Hobby–Eberly Telescope, we find a periodic signal of Pb=3.986−0.003+0.044 days. A joint Keplerian and Gaussian process stellar activity model fit to the radial velocities yields a minimum mass of mpsini=1.5−0.4+0.6 M Jup. The stellar rotation period is well constrained by the Transiting Exoplanet Survey Satellite light curve (P rot = 1.086 ± 0.003 days) and is not an integer harmonic nor alias of the orbital period, supporting the planetary nature of the observed periodicity. HS Psc b joins a small population of young, close-in giant planet candidates with robust age and mass constraints and demonstrates that giant planets can either migrate to their close-in orbital separations by 130 Myr or form in situ. Given its membership in a young moving group, HS Psc represents an excellent target for follow-up observations to characterize this young hot Jupiter further, refine its orbital properties, and search for additional planets in the system.

A Large and Variable Leading Tail of Helium in a Hot Saturn Undergoing Runaway Inflation

Atmospheric escape shapes the fate of exoplanets, with statistical evidence for transformative mass loss imprinted across the mass–radius–insolation distribution. Here, we present transit spectroscopy of the highly irradiated, low-gravity, inflated hot Saturn HAT-P-67 b. The Habitable Zone Planet Finder spectra show a detection of up to 10% absorption depth of the 10833 Å helium triplet. The 13.8 hr of on-sky integration time over 39 nights sample the entire planet orbit, uncovering excess helium absorption preceding the transit by up to 130 planetary radii in a large leading tail. This configuration can be understood as the escaping material overflowing its small Roche lobe and advecting most of the gas into the stellar—and not planetary—rest frame, consistent with the Doppler velocity structure seen in the helium line profiles. The prominent leading tail serves as direct evidence for dayside mass loss with a strong day-/nightside asymmetry. We see some transit-to-transit variability in the line profile, consistent with the interplay of stellar and planetary winds. We employ one-dimensional Parker wind models to estimate the mass-loss rate, finding values on the order of 2 × 1013 g s−1, with large uncertainties owing to the unknown X-ray and ultraviolet (XUV) flux of the F host star. The large mass loss in HAT-P-67 b represents a valuable example of an inflated hot Saturn, a class of planets recently identified to be rare, as their atmospheres are predicted to evaporate quickly. We contrast two physical mechanisms for runaway evaporation: ohmic dissipation and XUV irradiation, slightly favoring the latter.

The Strength and Variability of the Helium 10830 Å Triplet in Young Stars, with Implications for Exosphere Detection

Young exoplanets trace planetary evolution, in particular the atmospheric mass loss that is most dynamic in youth. However, the high activity level of young stars can mask or mimic the spectroscopic signals of atmospheric mass loss. This includes the activity-sensitive He 10830 Å triplet, which is an increasingly important exospheric probe. To characterize the He-10830 triplet at young ages, we present time-series NIR spectra for young transiting planet hosts taken with the Habitable-zone Planet Finder. The He-10830 absorption strength is similar across our sample, except at the fastest and slowest rotations, indicating that young chromospheres are dense and populate metastable helium via collisions. Photoionization and recombination by coronal radiation only dominates metastable helium population at the active and inactive extremes. Volatile stellar activity, such as flares and changing surface features, drives variability in the He-10830 triplet. Variability is largest at the youngest ages before decreasing to ≲5–10 mÅ (or 3%) at ages above 300 Myr, with six of eight stars in this age range agreeing with there being no intrinsic variability. He-10830 triplet variability is smallest and age-independent at the shortest timescales. Intrinsic stellar variability should not preclude detection of young exospheres, except at the youngest ages. We recommend out-of-transit comparison observations taken directly surrounding transit and observation of multiple transits to minimize activity’s effect. Regardless, caution is necessary when interpreting transit observations in the context of stellar activity, as many scenarios can lead to enhanced stellar variability even on timescales of an hour.

Ocean, atmosphere, and cloud quantity on the surface conditions of tidally-locked habitable zone planets

The study of tidally-locked planets has become an active field of research in recent years due to their interesting properties and abundance in the universe. As one hemisphere of these planets is always facing a star and the other is covered in perpetual darkness, habitable conditions on such extreme worlds were generally considered impossible. However, recent studies have shown that under specific planetary atmospheric and oceanic conditions, temperatures could stay mild enough for the planet to be habitable. In this paper, we created an energy balance model to determine the atmospheric and surface temperatures of tidally-locked worlds as a function of several planetary factors. We hypothesized that these factors—atmospheric and oceanic heat circulation and clouds—are crucial for habitable conditions to exist on these planets. We then assessed what specific set of values of these factors would make habitable conditions possible. We found that global heat transportation cycles act as a major determining factor on the planetary climate and, therefore, habitability. Apart from being in the habitable zone, Earth-like oceans and atmospheric coverage could also be essential for the habitability of tidally-locked planets, which establishes viable criteria in future searches for extraterrestrial life and livable worlds.

TOI-5344 b: A Saturn-like Planet Orbiting a Super-solar Metallicity M0 Dwarf

We confirm the planetary nature of TOI-5344 b as a transiting giant exoplanet around an M0-dwarf star. TOI-5344 b was discovered with the Transiting Exoplanet Survey Satellite photometry and confirmed with ground-based photometry (the Red Buttes Observatory 0.6 m telescope), radial velocity (the Habitable-zone Planet Finder), and speckle imaging (the NN-Explore Exoplanet Stellar Speckle Imager). TOI-5344 b is a Saturn-like giant planet (ρ = 0.80 g cm−3) with a planetary radius of 9.7 ± 0.5 R ⊕ (0.87 ± 0.04 R Jup) and a planetary mass of (0.42 ). It has an orbital period of days and an orbital eccentricity of . We measure a high metallicity for TOI-5344 of [Fe/H] = 0.48 ± 0.12, where the high metallicity is consistent with expectations from formation through core accretion. We compare the metallicity of the M-dwarf hosts of giant exoplanets to that of M-dwarf hosts of nongiants (≲8 R ⊕). While the two populations appear to show different metallicity distributions, quantitative tests are prohibited by various sample caveats.

Astrophysical investigation on the virtual planet pandora in Avatar film

The story of AVATAR happened in an imaginary planet Pandora. It is in the nearest stellar system from us, which is a triple star system located in the constellation Centauri. Currently, only one Earth-size planet has been detected around Proxima Centauri in 2016 by radial velocity technique, and any Jupiter-size planet has been ruled out around all the three stars. The result is in conflict with the scenario in AVATAR film, where Pandora is an Earth-size moon of Polyphemus, a Jupiter-size planet around Centauri B. Earth-size planets in habitable zones around Centauri A and Centauri B are still possible but have not been detected in the precision of current observations. Considering the dynamical complexity of planets in triple star system, which may induce extreme climate change in the possible habitable planets to prohibit development of civilization, we take long-term orbital revolution simulation to explore the stability of Earth-size planet in habitable zones around stars. We focus on habitable planets around Centauri A and Centauri B, as they are close, the average distance between them is small than the size of Solar system. We found that habitable planets with orbits aligned with the binary orbit are long-term stable. However, the orbital eccentricity and inclination changes a lot, if the planetary orbits significantly are tilted with the binary orbit. In comparison with the Milankovitch cycles observed in paleoclimate, we conclude that the titled planets in habitable zone around Centauri A and Centauri B cannot support development of intelligent life.

Planetary entropy production as a thermodynamic constraint for exoplanet habitability

ABSTRACT Any biosphere emerges, lives, and grows producing entropy. Entropy production is a thermodynamic function crucial in the framework of non-equilibrium thermodynamics as it is directly related to the dynamical behaviour of far-from equilibrium systems. The extent of entropy production is proportional to the ability of such systems to dissipate free energy and thus to ‘live’, to evolve, to grow in complexity. Generally, a certain threshold of entropy production must be exceeded for the emergence of complex self-organizing structures. Thus, the entropy production can be considered as the thermodynamic thrust that drives life emergence and evolution. In this perspective, we propose that the value of the planetary entropy production (PEP) can provide a first order estimate of the thermodynamic potential of planetary environment to sustain a complex biosphere. Here we use a simplified approach to evaluate the upper limit to the PEP and to the corresponding free energy as function of stellar temperature and orbital parameters of the planet. We found that only Earth-like planets in the circumstellar habitable zone (CHZ) of G and F stars can have a PEP value higher than the Earth value. Further significant thermodynamic differences exist between the inner and outer edge of the CHZ, with the inner edge being thermodynamically more advantageous for the development of complex biospheres. Interestingly, among the recently proposed habitable exoplanets, the ones belonging to the Hycean planets appear the thermodynamically best candidates.

The Tianlin Mission: A 6 m UV/Opt/IR Space Telescope to Explore Habitable Worlds and the Universe

It is expected that ongoing and future space-borne planet survey missions including Transiting Exoplanet Survey Satellite (TESS), PLATO and Earth 2.0 will detect thousands of small to medium-sized planets via the transit technique, including over a hundred habitable terrestrial rocky planets. To conduct a detailed study of these terrestrial planets, particularly the cool ones with wide orbits, the exoplanet community has proposed various follow-up missions. The currently proposed European Space Agency mission Ariel is the first step for this purpose, and it is capable of characterization of planets down to warm super-Earths mainly using transmission spectroscopy. The NASA Large Ultraviolet/Optical/Infrared Surveyor mission proposed in the Astro2020 Decadal Survey white paper will endeavor to further identify habitable rocky planets, and is expected to launch around 2045. In the meanwhile, China is funding a concept study of a 6 m class space telescope named Tianlin (a UV/Opt/NIR large aperture space telescope) that aims to start its operation within the next 10–15 yr and last for 5+ yr. Tianlin will be primarily aimed at the discovery and characterization of rocky planets in the habitable zones around nearby stars and to search for potential biosignatures mainly using the direct imaging method. Transmission and emission spectroscopy at moderate to high resolution will be carried out as well on a population of exoplanets to strengthen the understanding of the formation and evolution of exoplanets. It will also be utilized to perform in-depth studies of the cosmic web and early galaxies, and constrain the nature of dark matter and dark energy. We describe briefly the primary scientific motivations and main technical considerations based on our preliminary simulation results. We find that a monolithic off-axis space telescope with primary mirror diameter larger than 6 m equipped with a high contrast coronagraph can identify water in the atmosphere of a habitable-zone Earth-like planet around a Sun-like star. More simulations for the detectability of other key biosignatures including O3, O2, CH4 and chlorophyll are coming.

Impact of M-dwarf stellar wind and photoevaporation on the atmospheric evolution of small planets

ABSTRACT The evolution of a planet’s atmosphere depends strongly on its host star’s properties. When their host stars are younger, planets can experience stronger winds and extreme ultraviolet radiation (EUV) emissions. This is particularly true for planets orbiting M dwarfs due to their close proximity to the host star. To determine if these planets retain an atmosphere, we consider the impacts from stellar wind and EUV fluxes in driving atmospheric escape throughout the planet’s lifetime. For this, we determined the atmospheric mass-loss due to stellar wind and photoevaporation on four planets in close orbit and 34 in their star’s habitable zone (HZ). The M-dwarf host stars’ wind velocity, density, and EUV flux were calculated through rotation period and X-ray flux scaling over time. The mass-loss rate due to stellar wind and photoevaporation was then computed as a function of time and accumulated throughout the planet’s age to determine the total atmospheric mass-loss of the planet’s initial H/He envelope. We find that for HZ planets at orbits <0.1 au, stellar wind can only remove ${\le} 1~{{ \rm per\ cent}}$ of the H/He envelope, while photoevaporation is essential for completely removing the H/He envelope of most targets. Moreover, due to either mechanism, most planets orbiting at >0.1 au do not have their primordial envelope stripped. Overall, out of the 38 planets studied, 13 were predicted to have lost the primordial envelope due to photoevaporation, while two planets lost the envelope due to both stellar wind and photoevaporation.

Chasing rainbows and ocean glints: Inner working angle constraints for the Habitable Worlds Observatory

ABSTRACT NASA is engaged in planning for a Habitable Worlds Observatory (HabWorlds ), a coronagraphic space mission to detect rocky planets in habitable zones and establish their habitability. Surface liquid water is central to the definition of planetary habitability. Photometric and polarimetric phase curves of starlight reflected by an exoplanet can reveal ocean glint, rainbows, and other phenomena caused by scattering by clouds or atmospheric gas. Direct imaging missions are optimized for planets near quadrature, but HabWorlds ’ coronagraph may obscure the phase angles where such optical features are strongest. The range of accessible phase angles for a given exoplanet will depend on the planet’s orbital inclination and/or the coronagraph’s inner working angle (IWA). We use a recently created catalog relevant to HabWorlds of 164 stars to estimate the number of exo-Earths that could be searched for ocean glint, rainbows, and polarization effects due to Rayleigh scattering. We find that the polarimetric Rayleigh scattering peak is accessible in most of the exo-Earth planetary systems. The rainbow due to water clouds at phase angles of ∼20○ − 60○ would be accessible with HabWorlds for a planet with an Earth equivalent instellation in ∼46 systems, while the ocean glint signature at phase angles of ∼130○ − 170○ would be accessible in ∼16 systems, assuming an IWA = 62 mas (3λ/D). Improving the IWA = 41 mas (2λ/D) increases accessibility to rainbows and glints by factors of approximately 2 and 3, respectively. By observing these scattering features, HabWorlds could detect a surface ocean and water cycle, key indicators of habitability.