arXiv subjects

Cayman T. Unterborn

Publications and source records attributed to Cayman T. Unterborn.

At least 19 recordsLinked to original sources

arXiv2025

Interior and Climate Modeling of the Venus Zone Planet TOI-2285 b

As the discovery of exoplanets progresses at a rapid pace, the large number of known planets provides a pathway to assess the stellar and planetary properties that govern the climate evolution of terrestrial planets. Of particular interest are those planetary cases that straddle the radius boundary of being terrestrial or gaseous in nature, such as super-Earth and sub-Neptune exoplanets, respectively. The known exoplanet, TOI-2285 b, is one such case, since it lies at the radius boundary of super-Earth and sub-Neptune ($R_p = 1.74$ $R_\oplus$), and receives a relatively high instellation flux since its orbit exists within both the Habitable Zone (HZ) and Venus Zone (VZ). Here, we present an analysis of the planetary interior and climate to determine possible evolutionary pathways for the planet. We provide volatile inventory estimates in terms of the planet's bulk density and interior composition. We performed climate simulations using ROCKE-3D that provide a suite of possible temperate scenarios for the planet for a range of topographical and initial surface water assumptions. Using the outputs of the climate simulations, we modeled JWST transmission and emission spectroscopy for each scenario. Our results demonstrate that there are temperate scenarios consistent with the known planetary properties, despite the planet's estimated steam atmosphere, and its location relative to the VZ.

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arXiv2023

The Carbon-Deficient Evolution of TRAPPIST-1c

Transiting planets orbiting M dwarfs provide the best opportunity to study the atmospheres of rocky planets with current facilities. As JWST enters its second year of science operations, an important initial endeavor is to determine whether these rocky planets have atmospheres at all. M dwarf host stars are thought to pose a major threat to planetary atmospheres due to their high magnetic activity over several billion-year timescales, and might completely strip atmospheres. Several Cycle 1 and 2 GO and GTO programs are testing this hypothesis, observing a series of rocky planets to determine whether they retained their atmospheres. A key case-study is TRAPPIST-1c, which receives almost the same bolometric flux as Venus. We might, therefore, expect TRAPPIST-1c to possess a thick, $\mathrm{CO}_2$-dominated atmosphere. Instead, Zieba et al. (2023) show that TRAPPIST-1c has little to no CO$_2$ in its atmosphere. To interpret these results, we run coupled time-dependent simulations of planetary outgassing and atmospheric escape to model the evolution of TRAPPIST-1c's atmosphere. We find that the stellar wind stripping that is expected to occur on TRAPPIST-1c over its lifetime can only remove up to $\sim 16$ bar of $\mathrm{CO}_2$, less than the modern $\mathrm{CO}_2$ inventory of either Earth or Venus. Therefore, we infer that TRAPPIST-1c either formed volatile-poor, as compared to Earth and Venus, or lost a substantial amount of $\mathrm{CO}_2$ during an early phase of hydrodynamic hydrogen escape. Finally, we scale our results for the other TRAPPIST-1 planets, finding that the more distant TRAPPIST-1 planets may readily retain atmospheres.

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arXiv2023

Fizzy Super-Earths: Impacts of Magma Composition on the Bulk Density and Structure of Lava Worlds

Lava worlds are a potential emerging population of Super-Earths that are on close-in orbits around their host stars with likely partially molten mantles. To date, few studies address the impact of magma on the observed properties of a planet. At ambient conditions magma is less dense than solid rock; however, it is also more compressible with increasing pressure. Therefore, it is unclear how large-scale magma oceans affect planet observables, such as bulk density. We update ExoPlex, a thermodynamically self-consistent planet interior software, to include anhydrous, hydrous (2.2 wt \% H_2O), and carbonated magmas (5.2 wt\% CO_2). We find that Earth-like planets with magma oceans larger than \sim 1.5 R_{\oplus} and \sim 3.2 M_{\oplus} are modestly denser than an equivalent mass solid planet. From our model, three classes of mantle structures emerge for magma ocean planets: (1) mantle magma ocean, (2) surface magma ocean, and (3) one consisting of a surface magma ocean, solid rock layer, and a basal magma ocean. The class of planets in which a basal magma ocean is present may sequester dissolved volatiles on billion-year timescales, in which a 4 M_{\oplus} mass planet can trap more than 130 times the mass of water than in Earth's present-day oceans and 1000 times the carbon in the Earth's surface and crust.

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arXiv2023

Origin of Low-${}^{26}{\rm Al}/{}^{27}{\rm Al}$ Corundum/Hibonite Inclusions in Meteorites

Most meteoritic calcium-rich, aluminum-rich inclusions (CAIs) formed from a reservoir with ${}^{26}{\rm Al}/{}^{27}{\rm Al} \approx 5 \times 10^{-5}$, but some record lower $({}^{26}{\rm Al}/{}^{27}{\rm Al})_0$, demanding they sampled a reservoir without live ${}^{26}{\rm Al}$. This has been interpreted as evidence for "late injection" of supernova material into our protoplanetary disk. We instead interpret the heterogeneity as chemical, demonstrating that these inclusions are strongly associated with the refractory phases corundum or hibonite. We name them "Low-${}^{26}{\rm Al}/{}^{27}{\rm Al}$ Corundum/Hibonite Inclusions" (LAACHIs). We present a detailed astrophysical model for LAACHI formation in which they derive their Al from presolar corundum, spinel or hibonite grains $0.5 - 2 \, μ{\rm m}$ in size with no live ${}^{26}{\rm Al}$; live ${}^{26}{\rm Al}$ is carried on smaller ($<$50 nm) presolar chromium spinel grains from recent nearby Wolf-Rayet stars or supernovae. In hot ($\approx$ 1350-1425 K) regions of the disk these grains, and perovskite grains, would be the only survivors. These negatively charged grains would grow to sizes $1 - 10^3 \, μ{\rm m}$, even incorporating positively charged perovskite grains, but not the small, negatively charged ${}^{26}{\rm Al}$-bearing grains. Chemical and isotopic fractionations due to grain charging was a significant process in hot regions of the disk. Our model explains the sizes, compositions, oxygen isotopic signatures, and the large, correlated ${}^{48}{\rm Ca}$ and ${}^{50}{\rm Ti}$ anomalies (if carried by presolar perovskite) of LAACHIs, and especially how they incorporated no ${}^{26}{\rm Al}$ in a solar nebula with uniform, canonical ${}^{26}{\rm Al}/{}^{27}{\rm Al}$. A late injection of supernova material is obviated, although formation of the Sun in a high-mass star-forming region is demanded.

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arXiv2023

The Demographics of Terrestrial Planets in the Venus Zone

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 exoVenuses can help confirm hypotheses about Venus' 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 exoVenuses will be complimentary to these missions. To facilitate studies of exoVenus candidates, we provide a catalog of all confirmed terrestrial planets in the Venus Zone, including transiting and non-transiting cases, and quantify their potential for follow-up observations. We examine the demographics of the exoVenus population with relation to stellar and planetary properties, such as the planetary radius gap. We highlight specific high-priority exoVenus 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.

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arXiv2022

The Nominal Range of Rocky Planet Masses, Radii, Surface Gravities and Bulk Densities

The two primary observable quantities of an exoplanet--its mass and radius--alone are not sufficient to probe a rocky exoplanet's interior composition and mineralogy. To overcome this, host-star abundances of the primary planet-building elements (Mg, Si, Fe) are typically used as a proxy for the planet's bulk composition. The majority of small exoplanet hosts, however, do not have available abundance data. Here we present the open-source ExoPlex mass-radius-composition solver. Unlike previous open-source mass-radius solvers, ExoPlex calculates the core chemistry and equilibrium mantle mineralogy for a bulk composition, including effects of mantle FeO content, core light elements and surface water/ice. We utilize ExoPlex to calculate the planetary radii, surface gravities and bulk densities for 10$^6$ model planets up to 2 R$_\oplus$ across these geochemistries, adopting the distribution of FGK stellar abundances to estimate of the range of bulk exoplanet compositions. We outline the $99.7\%$ distribution of radii, surface gravity and bulk densities that define planets as "nominally rocky." Planets outside this range require compositions outside those expected from stellar abundance data, likely making them either Fe-enriched super-Mercuries, or volatile-enriched mini-Neptunes. We apply our classification scheme to a sample of 85 well-resolved exoplanets without available host-star abundances. We estimate only 9 planets are within the "nominally rocky planet zone" at $>70\%$ confidence, while $\sim20\%$ and $\sim30\%$ of this sample can be reasonably classified as super-Mercuries or volatile-rich, respectively. Our results provide observers with a self-consistent way to broadly classify a planet as likely rocky, Mercury-like or volatile-enriched, using mass and radius measurements alone.

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arXiv2022

Mantle Degassing Lifetimes through Galactic Time and the Maximum Age Stagnant-lid Rocky Exoplanets can Support Temperate Climates

The ideal exoplanets to search for life are those within a star's habitable zone. However, even within the habitable zone planets can still develop uninhabitable climate states. Sustaining a temperate climate over geologic ($\sim$Gyr) timescales requires a planet contain sufficient internal energy to power a planetary-scale carbon cycle. A major component of a rocky planet's energy budget is the heat produced by the decay of radioactive elements, especially $^{40}$K, $^{232}$Th, $^{235}$U and $^{238}$U. As the planet ages and these elements decay, this radiogenic energy source dwindles. Here we estimate the probability distribution of the amount of these heat producing elements (HPEs) that enter into rocky exoplanets through Galactic history, by combining the system-to-system variation seen in stellar abundance data with the results from Galactic chemical evolution models. Using these distributions, we perform Monte-Carlo thermal evolution models that maximize the mantle cooling rate. This allows us to create a pessimistic estimate of lifetime a rocky, stagnant-lid exoplanet can support a global carbon cycle and temperate climate as a function of its mass and when it in Galactic history. We apply this framework to a sample of 17 likely rocky exoplanets with measured ages, 7 of which we predict are likely to be actively degassing today despite our pessimistic assumptions. For the remaining planets, including those orbiting TRAPPIST-1, we cannot confidently assume they currently contain sufficient internal heat to support mantle degassing at a rate sufficient to sustain a global carbon cycle or temperate climate without additional tidal heating or undergoing plate tectonics.

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arXiv2021

Orbital Dynamics and the Evolution of Planetary Habitability in the AU Mic System

The diversity of planetary systems that have been discovered are revealing the plethora of possible architectures, providing insights into planet formation and evolution. They also increase our understanding of system parameters that may affect planetary habitability, and how such conditions are influenced by initial conditions. The AU~Mic system is unique among known planetary systems in that it is a nearby, young, multi-planet transiting system. Such a young and well characterized system provides an opportunity to study orbital dynamical and habitability studies for planets in the very early stages of their evolution. Here, we calculate the evolution of the Habitable Zone of the system through time, including the pre-main sequence phase that the system currently resides in. We discuss the planetary atmospheric processes occurring for an Earth-mass planet during this transitionary period, and provide calculations of the climate state convergence age for both volatile rich and poor initial conditions. We present results of an orbital dynamical analysis of the AU~Mic system that demonstrate the rapid eccentricity evolution of the known planets, and show that terrestrial planets within the Habitable Zone of the system can retain long-term stability. Finally, we discuss follow-up observation prospects, detectability of possible Habitable Zone planets, and how the AU Mic system may be used as a template for studies of planetary habitability evolution.

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arXiv2020

A Volatile-Poor Formation of LHS 3844b based on its Lack of Significant Atmosphere

Exoplanet discoveries have reached into the realm of terrestrial planets that are becoming the subject of atmospheric studies. One such discovery is LHS 3844b, a 1.3 Earth radius planet in a 0.46 day orbit around an M4.5-5 dwarf star. Follow-up observations indicate that the planet is largely devoid of substantial atmosphere. This lack of significant atmosphere places astrophysical and geophysical constraints on LHS 3844b, primarily the degree of volatile outgassing and the rate of atmosphere erosion. We estimate the age of the host star as $7.8\pm1.6$ Gyrs and find evidence of an active past comparable to Proxima Centauri. We use geodynamical models of volcanic outgassing and atmospheric erosion to show that the apparent lack of atmosphere is consistent with a volatile-poor mantle for LHS 3844b. We show the core is unlikely to host enough C to produce a sufficiently volatile-poor mantle, unless the bulk planet is volatile-poor relative to Earth. While we cannot rule out a giant impact stripping LHS 3844b's atmosphere, we show this mechanism would require significant mantle stripping, potentially leaving LHS 3844b as an Fe-rich "super-Mercury". Atmospheric erosion by smaller impacts is possible, but only if the planet has already begun degassing and is bombarded by $10^3$ impactors of radius 500-1000 km traveling at escape velocity. We discuss formation and migration scenarios that could account for a volatile poor origin, including the potential for an unobserved massive companion planet. A relatively volatile-poor composition of LHS 3844b suggests that the planet formed interior to the system snow-line.

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arXiv2020

The GALAH Survey: Using Galactic Archaeology to Refine our Knowledge of TESS Target Stars

An unprecedented number of exoplanets are being discovered by the Transiting Exoplanet Survey Satellite (TESS). Determining the orbital parameters of these exoplanets, and especially their mass and radius, will depend heavily upon the measured physical characteristics of their host stars. We have cross-matched spectroscopic, photometric, and astrometric data from GALAH Data Release 2, the TESS Input Catalog and Gaia Data Release 2, to create a curated, self-consistent catalog of physical and chemical properties for 47,285 stars. Using these data we have derived isochrone masses and radii that are precise to within 5\%. We have revised the parameters of three confirmed, and twelve candidate, TESS planetary systems. These results cast doubt on whether CTOI-20125677 is indeed a planetary system since the revised planetary radii are now comparable to stellar sizes. Our GALAH-TESS catalog contains abundances for up to 23 elements. We have specifically analysed the molar ratios for C/O, Mg/Si, Fe/Si and Fe/Mg, to assist in determining the composition and structure of planets with $R_p < 4R_\oplus$. From these ratios, 36% fall within 2 sigma of the Sun/Earth values, suggesting that these stars may host rocky exoplanets with geological compositions similar to planets found within our own Solar system.

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arXiv2020

Exogeoscience and Its Role in Characterizing Exoplanet Habitability and the Detectability of Life

The search for exoplanetary life must encompass the complex geological processes reflected in an exoplanet's atmosphere, or we risk reporting false positive and false negative detections. To do this, we must nurture the nascent discipline of "exogeoscience" to fully integrate astronomers, astrophysicists, geoscientists, oceanographers, atmospheric chemists and biologists. Increased funding for interdisciplinary research programs, supporting existing and future multidisciplinary research nodes, and developing research incubators is key to transforming true exogeoscience from an aspiration to a reality.

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arXiv2020

A Geologically Robust Procedure For Observing Rocky Exoplanets to Ensure that Detection of Atmospheric Oxygen is an Earth-Like Biosignature

In the next decades, the astrobiological community will debate whether the first observations of oxygen in an exoplanet$'$s atmosphere signifies life, so it is critical to establish procedures now for collection and interpretation of such data. We present a step-by-step observational strategy for using oxygen as a robust biosignature, to prioritize exoplanet targets and design future observations. It is premised on avoiding planets lacking subaerial weathering of continents, which would imply geochemical cycles drastically different from Earth$'$s, precluding use of oxygen as a biosignature. The strategy starts with the most readily obtained data: semi-major axis and stellar luminosity to ensure residence in the habitable zone; stellar XUV flux, to ensure an exoplanet can retain a secondary (outgassed) atmosphere. Next, high-precision mass and radius information should be combined with high-precision stellar abundance data, to constrain the exoplanet$'$s water content; those incompatible with less than 0.1 wt % H$_{2}$O can be deprioritized. Then, reflectance photometry or low-resolution transmission spectroscopy should confirm an optically thin atmosphere. Subsequent long-duration, high-resolution transmission spectroscopy should search for oxygen and ensure that water vapor and CO$_{2}$ are present only at low (10$^{2}$-10$^{4}$ ppm levels). Assuming oxygen is found, attribution to life requires the difficult acquisition of a detailed, multispectral light curve of the exoplanet to ensure both surface land and water. Exoplanets failing some of these steps might be habitable, even have observable biogenic oxygen, but should be deprioritized because oxygen could not be attributed unambiguously to life. We show how this is the case for the Solar System, the 55 Cnc System, and the TRAPPIST-1 System, in which only the Earth and TRAPPIST-1e successfully pass through our procedure.

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arXiv2020

Detectability of Life Using Oxygen on Pelagic Planets and Water Worlds

The search for life on exoplanets is one of the grand scientific challenges of our time. The strategy to date has been to find (e.g., through transit surveys like Kepler) Earth-like exoplanets in their stars habitable zone, then use transmission spectroscopy to measure biosignature gases, especially oxygen, in the planets atmospheres (e.g., using JWST, the James Webb Space Telescope). Already there are more such planets than can be observed by JWST, and missions like the Transiting Exoplanet Survey Satellite and others will find more. A better understanding of the geochemical cycles relevant to biosignature gases is needed, to prioritize targets for costly follow-up observations and to help design future missions. We define a Detectability Index to quantify the likelihood that a biosignature gas could be assigned a biological vs. non-biological origin. We apply this index to the case of oxygen gas, O2, on Earth-like planets with varying water contents. We demonstrate that on Earth-like exoplanets with 0.2 weight percent (wt%) water (i.e., no exposed continents) a reduced flux of bioessential phosphorus limits the export of photosynthetically produced atmospheric O2 to levels indistinguishable from geophysical production by photolysis of water plus hydrogen escape. Higher water contents >1wt% that lead to high-pressure ice mantles further slow phosphorus cycling. Paradoxically, the maximum water content allowing use of O2 as a biosignature, 0.2wt%, is consistent with no water based on mass and radius. Thus, the utility of an O2 biosignature likely requires the direct detection of both water and land on a planet.

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arXiv2019

The Pressure and Temperature Limits of Likely Rocky Exoplanets

The interior composition of exoplanets is not observable, limiting our direct knowledge of their structure, composition, and dynamics. Recently described observational trends suggest that rocky exoplanets, that is, planets without significant volatile envelopes, are likely limited to $<$1.5 Earth radii. We show that given this likely upper limit in the radii of purely-rocky super-Earth exoplanets, the maximum expected core-mantle boundary pressure and adiabatic temperature is relatively moderate, 630 GPa and 5000 K, while the maximum central core pressure varies between 1.5 and 2.5 TPa. We further find that for planets with radii less than 1.5 Earth radii, core-mantle boundary pressure and adiabatic temperature are mostly a function of planet radius and insensitive to planet structure. The pressures and temperatures of rocky exoplanet interiors, then, are less than those explored in recent shock-compression experiments, ab-initio calculations, and planetary dynamical studies. We further show that the extrapolation of relevant equations of state does not introduce significant uncertainties in the structural models of these planets. Mass-radius models are more sensitive to bulk composition than any uncertainty in the equation of state, even when extrapolated to TPa pressures.

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arXiv2018

Updated Compositional Models of the TRAPPIST-1 Planets

After publication of our initial mass-radius-composition models for the TRAPPIST-1 system in Unterborn et al. (2018), the planet masses were updated in Grimm et al. (2018). We had originally adopted the data set of Wang et al., 2017 who reported different densities than the updated values. The differences in observed density change the inferred volatile content of the planets. Grimm et al. (2018) report TRAPPIST-1 b, d, f, g, and h as being consistent with <5 wt% water and TRAPPIST-1 c and e has having largely rocky interiors. Here, we present updated results recalculating water fractions and potential alternative compositions using the Grimm et al., 2018 masses. Overall, we can only reproduce the results of Grimm et al., 2018 of planets b, d and g having small water contents if the cores of these planets are small (<23 wt%). We show that, if the cores for these planets are roughly Earth-sized (33 wt%), significant water fractions up to 40 wt% are possible. We show planets c, e, f, and h can have volatile envelopes between 0-35 wt% that are also consistent with being totally oxidized and lacking an Fe-core entirely. We note here that a pure MgSiO$_3$ planet (Fe/Mg = 0) is not the true lowest density end-member mass-radius curve for determining the probability of a planet containing volatiles. All planets that are rocky likely contain some Fe, either within the core or oxidized in the mantle. We argue the true low density end-member for oxidizing systems is instead a planet with the lowest reasonable Fe/Mg and completely core-less. Using this logic, we assert that planets b, d and g likely must have significant volatile layers because the end-member planet models produce masses too high even when uncertainties in both mass and radius are taken into account.

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arXiv2018

Stellar and Planetary Characterization of the Ross 128 Exoplanetary System from APOGEE Spectra

The first detailed chemical abundance analysis of the M dwarf (M4.0) exoplanet-hosting star Ross 128 is presented here, based upon near-infrared (1.5--1.7 \micron) high-resolution ($R$$\sim$22,500) spectra from the SDSS-APOGEE survey. We determined precise atmospheric parameters $T_{\rm eff}$=3231$\pm$100K, log$g$=4.96$\pm$0.11 dex and chemical abundances of eight elements (C, O, Mg, Al, K, Ca, Ti, and Fe), finding Ross 128 to have near solar metallicity ([Fe/H] = +0.03$\pm$0.09 dex). The derived results were obtained via spectral synthesis (1-D LTE) adopting both MARCS and PHOENIX model atmospheres; stellar parameters and chemical abundances derived from the different adopted models do not show significant offsets. Mass-radius modeling of Ross 128b indicate that it lies below the pure rock composition curve, suggesting that it contains a mixture of rock and iron, with the relative amounts of each set by the ratio of Fe/Mg. If Ross 128b formed with a sub-solar Si/Mg ratio, and assuming the planet's composition matches that of the host-star, it likely has a larger core size relative to the Earth. The derived planetary parameters -- insolation flux (S$_{\rm Earth}$=1.79$\pm$0.26) and equilibrium temperature ($T_{\rm eq}$=294$\pm$10K) -- support previous findings that Ross 128b is a temperate exoplanet in the inner edge of the habitable zone.

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arXiv2018

Inward Migration of the TRAPPIST-1 Planets as Inferred From Their Water-Rich Compositions

Multiple planet systems provide an ideal laboratory for probing exoplanet composition, formation history and potential habitability. For the TRAPPIST-1 planets, the planetary radii are well established from transits (Gillon et al., 2016, Gillon et al., 2017), with reasonable mass estimates coming from transit timing variations (Gillon et al., 2017, Wang et al., 2017) and dynamical modeling (Quarles et al., 2017). The low bulk densities of the TRAPPIST-1 planets demand significant volatile content. Here we show using mass-radius-composition models, that TRAPPIST-1f and g likely contain substantial ($\geq50$ wt\%) water/ice, with b and c being significantly drier ($\leq15$ wt\%). We propose this gradient of water mass fractions implies planets f and g formed outside the primordial snow line whereas b and c formed inside. We find that compared to planets in our solar system that also formed within the snow line, TRAPPIST-1b and c contain hundreds more oceans worth of water. We demonstrate the extent and timescale of migration in the TRAPPIST-1 system depends on how rapidly the planets formed and the relative location of the primordial snow line. This work provides a framework for understanding the differences between the protoplanetary disks of our solar system versus M dwarfs. Our results provide key insights into the volatile budgets, timescales of planet formation, and migration history of likely the most common planetary host in the Galaxy.

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arXiv2018

A procedure for observing rocky exoplanets to maximize the likelihood that atmospheric oxygen will be a biosignature

Here we advocate an observational strategy to help prioritize exoplanet observations. It starts with more easily obtained observational data, and ranks exoplanets for more difficult follow-up observations based on the likelihood of avoiding planets for which oxygen is a false positives or even an inconclusive signature of life. We find that for oxygen to be a robust biosignature, both land and surface water must be present. Landless exoplanets have much slower biogeochemical cycles, so while oxygenic photosynthesizing life could exist on such planets, it could not produce oxygen at a rate competitive with abiotic rates such as photolysis. These habitable planets, whose life would not be detectable, should be avoided.

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