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Earth’s core accounts for ~33% of its total mass, consistently with what expected from the Sun's abundance of rock-forming elements. Mars’ has a smaller core-mass fraction than Earth’s according to data from NASA Insight. Mercury’s, some super-Earths like K2-229b, and perhaps some asteroids like 16 Psyche have an anomalously large metal content. How does this planetary diversity arise?
Is asteroid (16) Psyche a metal-rich giant-impact remnant?
Cambioni et al. (2026) JGR: Planets
Asteroid (16) Psyche is the largest likely metal-rich asteroid in the Solar System and the target of the NASA Psyche mission. The mission aims to determine whether the asteroid is the core of a differentiated planetesimal that lost its mantle via a giant impact. To prepare for spacecraft observations of the asteroid, we combined impact and geodynamic models to investigate the consequences of such a formation scenario on Psyche's magnetization, composition, and internal structure. We found that, after the impact, churning of molten metal in Psyche's core could have generated a magnetic field that was recorded by Psyche's exterior while cooling, analogous to how igneous rocks become magnetized in Earth's magnetic field. Magnetization of Psyche by its own core's dynamo magnetic field and not by other magnetic fields generated in the Sun's protoplanetary disk is favored if Psyche formed from sulfur-rich (≳ 10 wt.%) materials and has a bulk metal content ≳ 50 wt.%. Bulk sulfur contents ≳ 10 wt.% correspond to a longer lifetime of the dynamo field, allowing more time for ferromagnetic grains to cool down and become magnetized. Formation of Psyche-sized bodies with ≳ 50 wt.% metal content requires higher-energy impacts that more favorably occur after the protoplanetary disk dissipates.
We concluded that, if the NASA Psyche spacecraft measures a strong magnetization, a high sulfur content, and a high metal content, then asteroid Psyche most likely formed as a mantle-stripped planetary core.
VideoS2.mp4Smoothed particle hydrodynamics (SPH) simulation run with the code SPHLATCH of a giant impact between two differentiated planetesimal. The projectile's core loses its silicate mantle but its core survives as a distinct body with mass and density analogous to that of asteroid (16) Psyche.
High-density exoplanets are unlikely to be metal-rich giant impact remnants
Cambioni et al. (2025) Astronomy & Astrophysics
It has been proposed that planet Mercury has a high density because it is the metallic core of a terrestrial planet that lost its mantle in a giant impact. But what about Mercury's larger cousins outside the solar system --- the so-called "super-Mercury" exoplanets? Are these planets the metallic cores of super-Earths? We tested this hypothesis by combining our machine-learning models of giant impacts with planet formation models and exoplanetary data to explore >100,000 different scenarios. We found that despite giant impacts can strip the mantle of super-Earths and form metal-rich worlds, these events are too rare to explain why ~9% of terrestrial planets in the galaxy are high-density worlds. Moreover, giant impacts between super-Earths tend to form metal-rich worlds that are too small to match the observations. Taken together, these findings suggest that high-density exoplanets are unlikely to be metal-rich giant impact remnants.
The two remaining hypothesis is that high-density exoplanets are the naked fossil-compressed cores of giant planets or are primordial metal rich worlds that formed in metal-rich regions of the nebula, with the first former hypothesis being disfavoured based on our recent study (Lin, Cambioni, Seager 2025, ApJL).
Mass (M) as a function of radius (R) of observed and simulated high-density worlds, along with their corresponding probability density functions (PDF). The modelled versus observed high-density worlds have distinct mass-radius distributions, which indicates that the latter are unlikely to form by giant impacts.
The role of giant impacts in planetary formation
Gabriel & Cambioni 2023, Annual Review of Earth and Planetary Science
Planets are thougth to conclude their growth through a series of energetic, global impacts whose outcomes strongly depend on the impact conditions. How do these giant impacts affect planetary composition and evolution? Will the projectile survive as a distinct object or being buried forever in the target planet? Will a moon form from the debris disk? In this review, we discuss how continued improvement in computer models and theory have helped addressing these questions and improved our ability to deduce probable collision conditions from observations of collision remnants. Despite these advancements, many open questions remain, as even the type of giant impact that formed Earth's Moon remains debated. Among many findings, we encourage the use of probability theory to evaluate distinct formation hypothesis among different sequences of giant impacts that can produce similar planets/
Videos of collision simulations
(Supplementary materials in Gabriel & Cambioni 2023, AREPS)
Statistical diversity (hannon's information entropy) of metal contents for bodies in a population that have survived the same number of hit-and-run collisions) as a function of total number of giant impacts. For each collision, the outcome (accretion or hit-and-run) is decided via coin flip (50-50% probability); metal content increases only in case of hit-and-run. This statistical exercise shows that the more collisionally evolved a population is, the higher its expected compositional diversity.
The effect of inefficient accretion on planetary differentiation
Cambioni et al. 2021, PSJ
In this work, my colleagues and I studied the effect of collisional fragmentation of planets following giant impacts on their core formation following magma-ocean formation using the machine-learning models of giant impacts developed in previous studies (Cambioni et al. (2019), ApJ; Emsenhuber et al. 2020, ApJ, described below).
We find that the model of perfect merging and the machine-learning treatment provide similar predictions for the mass and core mass fraction of planets more massive than 0.1 Earth’s masses (~1 Mars’ mass). At smaller scales, however, the inefficient accretion model predicts a higher degree of planetary diversity in terms of core mass fraction due to a predominance of hit-and-run collisions. This suggests that giant impacts played a key role in shaping the diversity of the core-mass fractions of the terrestrial planets in the solar system.
Core mass fraction (mass of the core / mass of the planet) as a function of planet mass in units of Earth's masses (M🜨) from the N-body simulations of terrestrial planet formation. The spread in core mass fraction of the final planets qualitatively resembles that observed for the terrestrial planets in the solar system.
The effect of inefficient accretion on planetary formation
Emsenhuber et al. 2020, ApJ
Cambioni et al. (2019), ApJ
Despite the importance of giant impacts in shaping the mass, composition, and orbits of planets, their modeling in planet formation and evolution studies has been traditionally simplified assuming inelastic collisions. This oversimplification was due to the high computational cost of running “on-the-fly” impact simulations. I am leading the development of a new method to solve this bottleneck by training machine-learning algorithms to mimic the outcome of expensive, high-resolution giant impact simulations.
We used the resulting machine-learning code (https://github.com/aemsenhuber/collresolve) to explore the effect of collisional fragmentation on the masses and composition terrestrial planet in N-body simulations. The solar system architectures we obtain feature a much wider range of terrestrial planet masses and enhanced compositional diversity than what obtained using perfect merging of the colliding bodies, highlighting the importance of realistically modelling collisions in planet formation studies.
Final planets in N-body simulations of planet formation with the same initial conditions, but different models for giant impacts (perfect merging, top 4 systems, vs. machine learning, bottom 4 systems). The planets are color-coded in terms of the initial heliocentric distance of the early solar system materials that contributed to their growth.
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