The TREC (Tracing Rocky Exoplanet Compositions) project focuses on understanding the chemical compositions of rocky exoplanets, particularly their surfaces, and the impact of these variations on climate, habitability, and the potential for life detection. Our team is dedicated to addressing the fundamental questions: What is the range of chemical compositions on rocky exoplanets and which chemical variations are most crucial for climate, habitability, and life detection?
To tackle these questions, TREC is implementing a comprehensive research program. This initiative aims to determine the initial chemical compositions of stellar systems and trace the changes that occur during the formation and evolution of planets. From the protoplanetary disk to mantle-surface interactions over geological time, our work will synthesize these findings to establish statistical distributions of surface abundances of key elements that influence climate, habitability, and life detection.
We will enhance existing catalogs by measuring key elemental abundances in Sun-like stars, focusing on rarely measured elements such as phosphorus (P), thorium (Th), and uranium (U). We aim to refine the stellar ages of these stars with a precision of less than 1 Gyr and test whether variations among solar twins are due to planetary material accretion.
We will investigate the changes in planetary materials resulting from radial pebble fluxes and condensation fronts within protoplanetary disks, testing these models using meteoritic data.
Our work will explore how elements are sequestered into planetary cores, removing them from surface interaction. We will compile metal-silicate partition coefficients, model mantle mineralogy post-magma ocean crystallization, and assess the impact of giant collisions.
We will study how mantle compositions evolve, focusing on how melts interact with planetary surfaces, outgassing volatiles, and forming crusts. This task involves advanced thermodynamic and geodynamic modeling to simulate heat flux decline over time.
We will synthesize our findings to map the likely range of rocky exoplanet mantle and surface compositions as functions of age, mass, and stellar abundances. Through workshops and community consensus-building, we aim to identify which elemental abundance variations are most critical for climate, habitability, and life detection on exoplanets.
Our team is uniquely positioned to answer these questions due to our broad expertise across Earth and planetary sciences and astrophysics. We have a proven track record of facilitating interdisciplinary science and are committed to innovation in our approach. Some of our key innovations include:
Through our work, TREC aims to push the boundaries of our understanding of rocky exoplanets and their potential to host life, contributing to the broader goals of NASA's astrobiology missions and the scientific community at large.
The discovery of Earth-like exoplanets has become increasingly frequent, yet the field of exoplanetary science remains in its formative stages, similar to planetary science several decades ago. In the 1960s, Venus was believed to potentially support liquid water, based solely on its mass and radius. However, subsequent robotic missions revealed its stark chemical differences from Earth, fundamentally altering our understanding of its climate and habitability.
Recent advancements, such as the JWST's detection of atmospheric CO2 on WASP-39b and anticipated findings in the Trappist-1 system, are shifting the focus away from mass, radius, and temperature as primary indicators of Earth-like conditions. Instead, the composition and geochemical cycles of these planets are gaining recognition for their critical role in determining habitability.
Our team, part of the Nexus for Exoplanetary System Science (NExSS) from 2015 to 2021, was dedicated to bridging geology and planetary science with astronomical observations of exoplanets. A key achievement of our research was highlighting the significance of a planet’s surface water content in using oxygen as a biosignature. Detecting O2 or O3 in a rocky exoplanet’s atmosphere is a major goal in exoplanet science, given that Earth's 21% O2 atmosphere is a direct result of global photosynthesis.
However, our findings suggest that the presence of oxygen alone is not a definitive biosignature. Instead, it is the rate of O2 production that holds significance. On Earth, this rate far exceeds abiotic processes, making it a reliable indicator of life. However, on planets with vastly different conditions, such as those with more surface water, the geochemical cycling of essential elements like phosphorus could limit oxygen production to levels that are indistinguishable from abiotic sources.
Our research has provided valuable insights into target selection strategies and the interpretation of future data. While life requires water, our findings suggest that rocky exoplanets with excessive surface water should be deprioritized when searching for biosignatures, even if oxygen is detected.
The chemical composition of rocky exoplanets, particularly their surfaces, plays a crucial role in their climate, habitability, and the potential detection of life. The ASU-NExSS team has explored how variations in planetary composition, especially in elements like magnesium, silicon, and phosphorus, can lead to dramatically different outcomes for planets that might otherwise seem similar.
For instance, differences in mantle composition, influenced by factors such as the Mg/Si ratio, can affect surface recycling, volcanic activity, and overall planetary behavior. Additionally, processes like planetary differentiation, giant impacts, and mantle degassing can further alter the surface composition of these planets, influencing the availability of bio essential elements.
Understanding these variations is essential for assessing geophysical cycles and detecting potential biosignatures. However, this task is complex, as it requires detailed knowledge of the abundances of key elements on exoplanet surfaces and their interiors. For example, phosphorus, a limiting nutrient in primary production, varies significantly across planets and can influence the net production of oxygen, a potential biosignature.
Our research emphasizes the need for a comprehensive chemical inventory of exoplanet surfaces and interiors as a foundational step toward understanding their geophysical cycles and potential habitability. This approach will enable more accurate assessments of climate, habitability, and the potential for life on rocky exoplanets.
Our first objective focuses on quantifying how key elements partition between stars, disks, planets, cores, mantles, and surfaces over time. Understanding these distributions is essential for predicting the likely ranges of compositions on rocky exoplanets, which directly influences their climate, habitability, and the potential detection of life.
To achieve this objective, we will undertake the following tasks:
Our second objective is to identify which elemental variations significantly impact exoplanetary properties such as climate, habitability, and biosignatures. We aim to understand the physical processes responsible for these variations, which will guide future research and tie these properties to other observable characteristics of stellar systems. For example, while the abundance of zinc (Zn) may vary widely, its influence on climate and habitability is likely minimal. Conversely, even a modest increase in surface water (H₂O) can have profound effects, such as submerging continents, suppressing weathering rates, and altering climate and oxygen-based biosignatures.
Our research extends beyond theoretical modeling and data collection. We aim to facilitate transdisciplinary collaboration and advance the field of exoplanetary science through the following actions:
Our integrated and interdisciplinary research program bridges astronomy, Earth science, and planetary science. By tracing rocky exoplanet compositions through star and planet formation, disk processes, and planetary interior processes, we aim to inform the contextualization of JWST exoplanet atmospheric transmission spectra and the design of future space telescopes like HabEx and LUVOIR.
Our research aligns with the goals of the NExSS Research Coordination Network, specifically addressing the emphasis on habitability and the detection of life on exoplanets. It also supports two of the six major research goals of the Astrobiology Program as outlined in the 2015 Astrobiology Strategy Document: identifying and characterizing environments for habitability and biosignatures, and constructing habitable worlds.
Planets and their stars originate together within protoplanetary disks, originating from molecular clouds whose chemical makeup is shaped by Galactic Chemical Evolution (GCE). Understanding these initial compositions is vital, as they form the building blocks of planetary systems. To gain deeper insights into GCE and planetary evolution, we will expand our catalog of elemental abundances for Sun-like stars and improve the accuracy of stellar age estimates. Additionally, our research will examine whether the compositional differences observed among solar twins are attributable to the absence of planetary material during formation or the impact of magnetic forces during the star's early development.
We will determine the bulk abundances of Sun-like stars, focusing on key elements such as Fe, Mg, Si, and others, using both optical and near-infrared (NIR) spectroscopic data. Our sample includes 229 Sun-like stars, 36 of which are confirmed exoplanet hosts. By combining new observations with archival data, we will enhance the Hypatia Catalog, the most comprehensive database of stellar abundances near the Sun. This work will involve comparing observed abundances with GCE model predictions.
To accurately compare elemental abundances with GCE model predictions, we need precise stellar ages (~±1 Gyr). We will refine the ages of our sample stars by improving isochronal age estimates using updated stellar parameters. This will involve comparing stars against Tycho stellar evolution models and incorporating various methods like gyrochronology and chromospheric activity. Our goal is to achieve age uncertainties comparable to those obtained using gyrochronology, even for late G and early K stars.
We will test whether chemical fractionations occur during star formation, which could explain observed compositional differences among solar twins. Using models that incorporate magnetic field effects and dust grain retention, we will predict trends in elemental depletions and compare them with observational data. This work will help us understand the impact of magnetic fields on the chemical evolution of stellar systems.
Planets that form at different distances (r) from a star are expected to originate from the solids that condensed from a protoplanetary disk, reflecting the same composition as the disk and star (as outlined in Task 1a). However, two processes—condensation fronts and the pebble nature of solids—alter the compositions of planetary materials. Just outside the maximum r at which H₂O exists as vapor, the 'cold-finger' effect may cause the ice/rock ratio to exceed the system-wide average by an order of magnitude. Additionally, there is growing recognition (e.g., Ormel, 2017) that solids in disks exist as 'pebbles' that are slightly decoupled aerodynamically from the gas, allowing rapid planet growth by 'pebble accretion' and leading to large inward radial drifts of volatile-rich solids, altering compositions in planet-forming regions. Pebbles may undergo devolatilization during planetary accretion. In Task 2, we quantify how these effects shift the distributions of chemical compositions of planets relative to their host stars as they form from disks.
Volatiles like water are redistributed within evolving protoplanetary disks due to condensation fronts, or 'snow lines' (Lunine & Stevenson, 1988; Ciesla & Cuzzi, 2006). As ice-bearing particles drift inward to warmer regions where temperatures exceed the sublimation point of water (~170 K), the ice vaporizes. This water vapor can then diffuse outward to cooler regions, where it re-condenses, leading to ice buildup. These processes result in significant variations in the O/Si (H2O ice/dust) ratio, enhancing it in a narrow zone just beyond the condensation front and depleting it inside (Cuzzi & Zahnle, 2004). Different elements condense at different temperatures, leading to chemical fractionation that is observable in meteoritic and planetary data.
The redistribution of H2O likely altered the oxygen fugacity (fO2) of planetary materials in the Sun’s protoplanetary disk. Enstatite chondrites, formed inside the H2O snow line, have an oxygen fugacity about five log units below the iron-wüstite buffer (ΔIW ≈ -5), while carbonaceous chondrites, formed outside the snow line, are more oxidized (ΔIW ≈ 0) (Lin 2022). Recent research also highlights the role of silicate vaporization and forsterite recondensation near 'rock lines' in varying Mg/Si ratios in the solar nebula (Aguichine et al., 2020). Similar processes are thought to have influenced the formation of Ca-rich, Al-rich inclusions (CAIs) and the creation of regions with enhanced sulfur fugacity at 'sulfur snow lines' (Lehner et al., 2013).
Our research, led by Co-I Desch, will explore the extent of elemental fractionation at snow lines in protoplanetary disks through simulations. We will model condensation fronts for CAI-forming minerals, forsterite, major silicates, and sulfur-bearing species using detailed disk models from the literature, such as Desch et al. (2018). By incorporating limited chemical networks into existing codes, we will simulate the vaporization and condensation of these species, creating lookup tables to partition key elements between vapor and solids. This research will test hypotheses against Solar System meteoritic and planetary data, with findings published and all codes made available on GitHub by the project's conclusion.
Recent studies, like those by Ormel (2017), have highlighted the significance of pebble accretion in planet formation. Pebbles, defined by their Stokes numbers (St = ΩK tstop) and varying in size, are partially decoupled from the gas in protoplanetary disks, making them prone to accretion by planetary embryos. These pebbles, especially those rich in volatiles like water and carbon, drift inward from the outer disk and contribute significantly to the material composition of forming terrestrial planets. For instance, it’s estimated that approximately 300 Earth masses (ME) of carbonaceous pebbles could have passed through the inner disk, delivering excess carbon and water to regions where planets like Earth formed.
This influx of volatile-rich pebbles would have significant chemical consequences. For example, the delivery of excess carbon by these pebbles could lead to a dramatic increase in the carbon-to-oxygen (C/O) ratio within the disk, particularly inside the 'soot line,' where carbon combusts into carbon monoxide (CO) at temperatures around 500 K. This process would also reduce the oxygen fugacity (fO2) in the disk, potentially explaining observed variations in meteoritic compositions. Moreover, the introduction of carbonaceous pebbles could result in large-scale chemical fractionation, influencing the formation of calcium-aluminum-rich inclusions (CAIs) and contributing to the distinctive isotopic anomalies observed in different meteorite classes.
To better understand these processes, our team, including Co-I Desch and collaborators, will use advanced disk models to simulate the impact of carbonaceous pebble fluxes on the inner disk's composition. We aim to refine our understanding of how these fluxes altered the C/O ratio, oxygen fugacity, and the distribution of isotopic anomalies like ε54Cr and ε50Ti. By integrating observational data with these models, we hope to constrain the role of pebble accretion in shaping the chemical environment of the early solar system and predict how similar processes might occur in other protoplanetary disks.
Task 3 investigates the processes that lead to the differentiation of planetary bodies into cores and mantles, focusing on the geochemical and geophysical mechanisms driving this evolution. This task will produce extensive datasets and models, exploring various aspects of core formation, mantle dynamics, and the chemical partitioning between these layers. The data will include both theoretical models and experimental results, all of which will be archived in public repositories for broad accessibility.
We will develop models that explore the partitioning of elements between the core and mantle during planetary differentiation. These models will include Density Functional Theory (DFT) calculations to generate a comprehensive partition coefficient database. The data generated will be archived in text files with standards compatible with PDS and AstroMat, ensuring they are accessible and reusable for future research.
This subtask focuses on modeling the composition and dynamics of planetary mantles, particularly in relation to stagnant-lid tectonics and mantle convection. We will use a variety of geodynamic models, including SPH (Smoothed Particle Hydrodynamics) calculations, to simulate these processes. The output will include large datasets in text, CSV, and JSON formats, which will be archived according to the GNU standard and deposited in repositories like GitHub and Zenodo.
3c will simulate the impacts that contribute to the evolution of planetary mantles and cores, particularly focusing on giant impacts during the late stages of planet formation. The simulations will yield data on shockwave propagation, melting, and material ejection. These results will be stored in ASCII and text formats and made available through appropriate repositories, ensuring that they can be reproduced and verified by other researchers.
We will model the differentiation processes in the solar system’s terrestrial planets, providing comparative analyses with exoplanetary bodies. The data will include inputs and outputs from models of core formation, mantle dynamics, and surface evolution. The files will be archived in text and CSV formats, with metadata that aligns with PDS standards, and will be made available for public access via online repositories like AstroMat.
Tasks 1-3 outline the potential compositions of exoplanet mantles, referred to as ‘bulk silicate exoplanet’ (BSEP) compositions, based on stellar system origins and the compositional changes that occur during disk processes, planet formation, and planetary differentiation. The next phase involves quantifying how elements fractionate as materials reach the surface, focusing on processes like outgassing, melt production, and silicate crust formation, with a particular emphasis on key elements, including phosphorus (P). This will involve acquiring new experimental data for compositions distinct from Earth, particularly in terms of melting curves and the solubility of P in partial BSEP melts. Additionally, we will update the exoMELTS/ENKI code with this new data and model the thermal and geodynamic evolution of stagnant-lid rocky exoplanets to predict the rates at which key elements are emplaced on planetary surfaces.
This subtask seeks to understand the species brought to planetary surfaces during the melting of upper mantles with probable BSEP compositions. Elements are transferred to planet surfaces from upper mantles through their presence in silicate melts, which can be extruded onto the surface, leading to outgassing or the formation of silicate crusts. Accurate quantification of these effects depends on experimental data on melting curves, residual solids, and the solubility of key elements, especially P, in these melts across a range of conditions relevant to rocky exoplanets. This requires new experimental data for distinctly non-Earthlike compositions. Understanding the solubility of P, in particular, is crucial, as it remains poorly understood for compositions different from Earth, aside from a few studies focused on Solar System planets.
BSEP compositions may differ significantly from Earth’s Bulk Silicate Earth (BSE) in cation ratios such as Fe/Si and Ca/Si, which could greatly influence melt behavior and the solubility of elements like P, with significant implications for the use of oxygen as a biosignature. Co-I Brugman will lead efforts to acquire experimental data to empirically determine melting curves, residual solids, and P solubility across the range of likely BSEP compositions. Approximately 30 piston cylinder experiments per year will be conducted at the EPIC lab at ASU to determine likely mantle mineralogies, melting curves, magma and crust compositions, and P solubility.
These experiments will mimic conditions in the upper mantles of Mars- to super-Earth-sized rocky silicate bodies and will begin with the BSEP composition ‘HEX2,’ which has already been studied in previous experiments. By adding P to this composition, we will assess its impact on the melting curve and investigate P solubility in a BSEP melt. Co-Is Brugman, Till, and a postdoctoral researcher will design and conduct the experiments with the help of undergraduate researchers, exploring the appropriate range of silicate compositions and P-T-fO2 conditions informed by the results of Tasks 3 and 4b. This work will result in a greater petrological understanding of BSEP melting behavior, melt compositions, and elemental solubilities, which are essential for predicting the inventories of key elements on exoplanet surfaces.
This subtask examines how interior processes alter the distribution of elements between rocky planets' mantles and surfaces over time. After a planet's initial element distribution is determined by formation, differentiation, magma ocean crystallization, and giant impacts, the final stage involves the redistribution of elements between surface and interior through processes like volcanism, crust formation, and outgassing. These processes affect the abundance of key elements on a planet's surface and drive geochemical cycles. A major uncertainty in these models is the tectonic mode of rocky planet evolution—whether they exhibit plate tectonics like Earth, stagnant lids like Mars, or something in between. Co-I Foley will lead the development of thermal evolution models for rocky exoplanets, using codes that incorporate formation of basaltic crust, partitioning of key elements, and outgassing of CO2. This work will provide comprehensive insights into element redistribution through geological evolution on rocky exoplanets, applying thermodynamic tools from the ENKI project to predict crustal compositions and outgassed atmospheres. Dynamic mantle convection models will also be used to refine scaling laws and better understand the formation of crusts on exoplanets with non-Earthlike compositions.
In Task 5, we integrate the findings from the previous tasks to explore the range of possible compositions of rocky exoplanet surfaces and mantles. Our objective is to identify compositions that could have significant impacts on climate, habitability, or biosignatures. We aim to synthesize data from Tasks 1 through 4, which cover the compositional shifts from star formation to planet differentiation, and eventually to surface material formation through processes like melting and outgassing.
A key focus of our research is to identify the starting compositions of stars, track the changes as these compositions evolve through planetary formation, and ultimately develop a range of likely exoplanet compositions. Another focus is to determine which variations in composition are most significant for properties like climate, habitability, and the potential for life. For example, an Earth-like planet with twice the Earth's surface water might maintain similar geochemical cycles, but one with five times the water could submerge continents and hinder weathering processes. Small variations in elements like Ca/Si could result in significantly different planetary crusts. Additionally, a planet with a highly reduced mantle might sequester all phosphorus in its core, impacting the planet's potential for life.
We recognize that our team can make educated guesses about meaningful abundance ranges, but involving the broader scientific community will yield more robust assessments. We plan to leverage expertise from various teams within NASA's Research Coordination Networks, including NExSS, to develop a consensus on these questions. We aim to foster collaboration across disciplines, particularly between astrophysicists and Earth and planetary scientists, to address these complex, transdisciplinary challenges.
To facilitate this, we propose hosting two workshops. The first workshop, to be held in Year 2, will focus on determining how compositionally different a planet can be from Earth while maintaining Earth-like geochemical cycles. The second workshop, in Year 5, will synthesize our findings and reassess the consensus on significant compositional variations. These workshops will also serve as a platform to develop and disseminate best practices for transdisciplinary exoplanet science, leveraging online technologies and fostering knowledge exchange across disciplines.
Our commitment to promoting interdisciplinary research extends beyond our team to the entire NExSS community. We will maintain intra-team communication through weekly Coffee Hours and frequent team-wide meetings. We will also support students and postdocs in coordinating a Science Communications Working Group (SCWG) that will contribute content to the NExSS-level SCWG, build and maintain our website, and produce science communication materials.
To enhance interdisciplinary research, we will mentor undergraduate students, pairing small teams with research mentors to actively engage in our projects. We will assess their research experiences using a mixed-methods approach, including pre- and post-surveys and exit interviews, to evaluate their progress and impact on their career trajectories.
Our workshops will also be assessed using similar methods, with surveys and interviews helping us gauge the effectiveness of these events in promoting interdisciplinary collaboration and overcoming cultural barriers between scientific disciplines. Through these efforts, we aim to advance the field of exoplanet science and foster a collaborative, interdisciplinary research environment.