arXiv subjects

David R. Rice

Publications and source records attributed to David R. Rice.

11 recordsLinked to original sources

arXiv2026

Modeling Carbon-Based Planets with MAGRATHEA

A planet's interior structure not only influences its habitability, but it also provides information about the planet's formation and history. In the quest to characterize the variety of planetary discoveries, the most common practice is to rely on our understanding of the Earth's differentiated interior structure, with an iron core and silicate mantle as a starting point. However, recent work by arXiv:2408.07761 [astro-ph.EP] shows that these assumptions may not be true for all planetary systems as the composition of the condensed material can change drastically towards a carbon mantle instead of a silicate one, once the C/O ratio of the system approaches and exceeds approximately 0.9. In an effort to more fully understand the range of possible planetary bodies, we added the ability to model carbon-based mantles to our open-source planetary interior solver, MAGRATHEA. The newly added carbon mantle includes three relevant allotropes of carbon: graphite, diamond, and BC8 (body centered cubic with eight atoms per cell).

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arXiv2026

Plan 9: Detecting Atmospheric Deterrence Against Interstellar Monsters

Exoplanet atmospheres are usually discussed as tracers of climate, chemistry, and habitability, but they may also preserve signatures of planetary defense. We consider three folklore-motivated deterrents against monsters: reduced organosulfur gases as anti-hematophage repellents, argentiferous reflective aerosols as anti-lycanthropic countermeasures, and haline aerosols as a counting problem for specters. We show that globally-mixed garlic-smelly levels of DMS/DMDS could produce observable mid-infrared transmission features, that silver hazes would show up as anomalous optical brightening, and that sea-salt lofting sustained by strong near-surface winds appears as muted spectra. None of these signatures is unique, which is precisely the observational challenge. A defended world may first appear merely sulfur-rich, bright, or hazy. Therefore, some atmospheres may encode not only biosignatures, but also evidence that the local biosphere has stopped being afraid of the dark.

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arXiv2025

Magrathea v2: A planetary interior modeling platform in C++

Magrathea is an open-source C++ code for modeling the internal structure of differentiated planets. The initial release, Huang et al. (2022), introduced the base solver, a modular framework for defining equations of state (EOSs) used within phase diagrams for each differentiated layer, and outlined a series of planned extensions. Many of those updates are now implemented. Magrathea v2 is a more versatile platform that supports a wider range of compositions, adds new tools for composition retrieval, and makes it easier for users to adapt the code to their own models.

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arXiv2025

Uncertainties in the Inference of Internal Structure: The Case of TRAPPIST-1 f

We use the TRAPPIST-1 system as a model observation of Earth-like planets. The densities of these planets being 1-10% less than the Earth suggest that the outer planets may host significant hydrospheres. We explore the uncertainty in water mass fraction from observed mass and radius. We investigate the interior structure of TRAPPIST-1 f using the open-source solver MAGRATHEA and varying assumptions in the interior model. We find that TRAPPIST-1 f likely has a water mass fraction of 16.2% $\pm$ 9.9% when considering all possible core mass fractions and requires 6.9% $\pm$ 2.0% water at an Earth-like mantle to core ratio. We quantify uncertainties from observational precision, model assumptions, and experimental and theoretical data on the bulk modulus of planet building materials. We show that observational uncertainties are smaller than model assumptions of mantle mineralogy and core composition but larger than hydrosphere, temperature, and equation of state assumptions/uncertainties. Our findings show that while precise mass and radius measurements are crucial, uncertainties in planetary models can often outweigh those from observations, emphasizing the importance of refining both theoretical models and experimental data to better understand exoplanet interiors.

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arXiv2024

The Distribution of Planet Radius in Kepler Multiplanet Systems Depends on Gap Complexity

The distribution of small planet radius ($<$4 R$_\oplus$) is an indicator of the underlying processes governing planet formation and evolution. We investigate the correlation between the radius distribution of exoplanets in \textit{Kepler} multiplanet systems and the system-level complexity in orbital period spacing. Utilizing a sample of 234 planetary systems with three or more candidate planets orbiting FGK main-sequence stars, we measure the gap complexity ($C$) to characterize the regularity of planetary spacing and compare it with other measures of period spacing and spacing uniformity. We find that systems with higher gap complexity exhibit a distinct radius distribution compared to systems with lower gap complexity. Specifically, we find that the radius valley, which separates super-Earths and sub-Neptunes, is more pronounced in systems with lower gap complexity ($C$$<$0.165). Planets in high complexity systems ($C$$>$0.35) exhibit a lower frequency of sub-Earths (2.5 times less) and sub-Neptunes (1.3 times less) and a higher frequency of super-Earths (1.4 times more) than planets in low complexity systems. This may suggest that planetary systems with more irregular spacings are more likely to undergo dynamic interactions that influence planet scattering, composition, and atmospheric retention. The gap complexity metric proves to be a valuable tool in linking the orbital configurations of planets to their physical characteristics.

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arXiv2024

The TESS-Keck Survey. XVIII. A sub-Neptune and spurious long-period signal in the TOI-1751 system

We present and confirm TOI-1751 b, a transiting sub-Neptune orbiting a slightly evolved, solar-type, metal-poor star ($T_{eff} = 5996 \pm 110$ K, $log(g) = 4.2 \pm 0.1$, V = 9.3 mag, [Fe/H] = $-0.40 \pm 0.06$ dex) every 37.47 d. We use TESS photometry to measure a planet radius of $2.77_{-0.07}^{+0.15}~\rm{R_\oplus}$. We also use both Keck/HIRES and APF/Levy radial velocities (RV) to derive a planet mass of $14.5_{-3.14}^{+3.15} ~\rm{M_\oplus}$, and thus a planet density of $3.6 \pm 0.9 \, {\rm g}\,{\rm cm}^{-3}$. There is also a long-period ($\sim400~\rm{d}$) signal that is observed in only the Keck/HIRES data. We conclude that this long-period signal is not planetary in nature, and is likely due to the window function of the Keck/HIRES observations. This highlights the role of complementary observations from multiple observatories to identify and exclude aliases in RV data. Finally, we investigate potential compositions of this planet, including rocky and water-rich solutions, as well as theoretical irradiated ocean models. TOI-1751 b is a warm sub-Neptune, with an equilibrium temperature of $\sim 820$ K. As TOI-1751 is a metal-poor star, TOI-1751 b may have formed in a water-enriched formation environment. We thus favor a volatile-rich interior composition for this planet.

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arXiv2023

Composition constraints of the TRAPPIST-1 planets from their formation

We study the formation of the TRAPPIST-1 (T1) planets starting shortly after Moon-sized bodies form just exterior to the ice line. Our model includes mass growth from pebble accretion and mergers, fragmentation, type-I migration, and eccentricity and inclination dampening from gas drag. We follow the composition evolution of the planets fed by a dust condensation code that tracks how various dust species condense out of the disc as it cools. We use the final planet compositions to calculate the resulting radii of the planets using a new planet interior structure code and explore various interior structure models. Our model reproduces the broader architecture of the T1 system and constrains the initial water mass fraction of the early embryos and the final relative abundances of the major refractory elements. We find that the inner two planets likely experienced giant impacts and fragments from collisions between planetary embryos often seed the small planets that subsequently grow through pebble accretion. Using our composition constraints we find solutions for a two-layer model, a planet comprised of only a core and mantle, that match observed bulk densities for the two inner planets b and c. This, along with the high number of giant impacts the inner planets experienced, is consistent with recent observations that these planets are likely dessicated. However, two-layer models seem unlikely for most of the remaining outer planets which suggests that these planets have a primordial hydrosphere. Our composition constraints also indicate that no planets are consistent with a core-free interior structure.

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arXiv2023

Stable lifetime of compact, evenly-spaced planetary systems with non-equal masses

Compact planetary systems with more than two planets can undergo orbital crossings from planet-planet perturbations. The time which the system remains stable without orbital crossings has an exponential dependence on the initial orbital separations in units of mutual Hill radii. However when a multi-planet system has period ratios near mean-motion resonances, its stability time differs from the time determined by planet separation. This difference can be up to an order of magnitude when systems are set up with chains of equal period ratios. We use numerical simulations to describe the stability time relationship in non-resonant systems with equal separations but non-equal masses which breaks the chains of equal period ratios. We find a deviation of 30 per cent in the masses of Earth-mass planets creates a large enough deviation in the period ratios where the average stability time of a given spacing can be predicted by the stability time relationship. The mass deviation where structure from equal period ratios is erased increases with planet mass but does not depend on planet multiplicity. With a large enough mass deviation, the distribution of stability time at a given spacing is much wider than in equal-mass systems where the distribution narrows due to period commensurabilities. We find the stability time distribution is heteroscedastic with spacing -- the deviation in stability time for a given spacing increases with said spacing.

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arXiv2022

MAGRATHEA: an open-source spherical symmetric planet interior structure code

MAGRATHEA is an open-source planet structure code that considers the case of fully differentiated spherically symmetric interiors. Given the mass of each layer and the surface temperature, the code iterates the boundary conditions of the hydrostatic equations using the method of shooting to a fitting point in order to find the planet radius. The first version of MAGRATHEA supports a maximum of four layers of iron, silicates, water, and ideal gas. With a few exceptions, the temperature profile can be chosen between isothermal, isentropic, and user-defined functions. The user has many options for the phase diagram and equation of state in each layer and we document how to add additional equations of state. We present MAGRATHEA's capabilities and discuss its applications. We encourage the community to participate in the development of MAGRATHEA at https://github.com/Huang-CL/Magrathea.

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arXiv2021

Implications of an improved water equation of state for water-rich planets

Water (H$_{2}$O), in all forms, is an important constituent in planetary bodies, controlling habitability and influencing geological activity. Under conditions found in the interior of many planets, as the pressure increases, the H-bonds in water gradually weaken and are replaced by ionic bonds. Recent experimental measurements of the water equation of state (EOS) showed both a new phase of H-bonded water ice, ice-VII$_t$, and a relatively low transition pressure just above 30 GPa to ionic bonded ice-X, which has a bulk modulus 2.5 times larger. The higher bulk modulus of ice-X produces larger planets for a given mass, thereby either reducing the atmospheric contribution to the volume of many exoplanets or limiting their water content. We investigate the impact of the new EOS measurements on the planetary mass-radius relation and interior structure for water-rich planets. We find that the change in the planet mass-radius relation caused by the systematic differences between previous and new experimental EOS measurements are comparable to the observational uncertainties in some planet sizes -- an issue that will become more important as observations continue to improve.

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arXiv2018

Survival of non-coplanar, closely-packed planetary systems after a close encounter

Planetary systems with more than two bodies will experience orbital crossings at a time related to the initial orbital separations of the planets. After a crossing, the system enters a period of chaotic evolution ending in the reshaping of the system's architecture via planetary collisions or ejections. We carry out N-body integrations on a large number of systems with equally-spaced planets (in units of the Hill radius) to determine the distribution of instability times for a given planet separation. We investigate both the time to the initiation of instability through a close encounter and the time to a planet-planet collision. We find that a significant portion of systems with non-zero mutual inclinations survive after a close encounter and do not promptly experience a planet-planet collision. Systems with significant inclinations can continue to evolve for over 1,000 times longer than the encounter time. The fraction of long lived systems is dependent on the absolute system scale and the initial inclination of the planets. These results have implications to the assumed stability of observed planetary systems.

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