Exoplanetology: Exoplanets & Exomoons

Can Metal-rich Worlds Form By Giant Impacts?

By Keith Cowing

Status Report

astro-ph.EP

August 30, 2024

Filed under astro-ph.EP, exoplanet, Impact event, metal-rich star, rock exoplanet, super-Earth

Can Metal-rich Worlds Form By Giant Impacts? — Are the observed high-density exoplanets metal-rich remnants of giant impacts? a, Super-Earths are predicted to complete their formation through giant impacts whose energies are too low to strip their rocky mantles and form metal-rich planets (Scora et al., 2020, 2022; Poon et al., 2020; Esteves et al., 2022; Goldberg and Batygin, 2022). b, After their formation ceases, super-Earths may remain on closely spaced orbits typical of observed compact systems (Weiss et al., 2022) and can experience instabilities over timescales comparable to the main-sequence lifetime of the host stars (Volk and Gladman, 2015; Pu and Wu, 2015; Tamayo et al., 2020; Volk and Malhotra, 2020). c, If a compact system becomes unstable, super-Earths may experience a second stage of giant impacts that are more energetic than those that occurred during their formation (Volk and Gladman, 2015). These late giant impacts may erode the silicate mantles of the super-Earths to form metal-rich, high-density planets. d, The final, post-instability planetary system may have a metal-rich world with density akin to the measured densities of observed high-density exoplanets (Table 1). Colors indicate different planetary compositions: orange is metal-rich, blue indicates planets with a terrestrial composition (that is, differentiated in an iron core and a rocky mantle).– astro-ph.EP

Planets and stars are expected to be compositionally linked because they accrete from the same material reservoir. However, astronomical observations revealed the existence of exoplanets whose bulk density is far higher than what is expected from host-stars’ composition.

A commonly-invoked theory is that these high-density exoplanets are the metallic cores of super-Earth-sized planets whose rocky mantles were stripped by giant impacts. Here, by combining orbital dynamics and impact physics, we show that mantle-stripping giant impacts between super-Earths are unlikely to occur at rates sufficient to explain the observed size and currently estimated abundance of the high-density exoplanets.

We explain this as the interplay of two main factors: the parent super-Earths being in most cases smaller than 2 Earth radii; and the efficiency of mantle stripping decreasing with increasing planetary size. We conclude that most of the observed high-density exoplanets are unlikely to be metal-rich giant-impact remnants.

Flowchart of the statistical model of orbital instability. For details on each step, see Section 2.2.2. The symbols MT and MP indicate the masses of the target and projectile in the collisions, respectively. The symbols ZT and ZP indicate the core-mass fractions of the target and projectile of the collisions, respectively. The distributions from which the relative impact velocity, v∞ (in units of Keplerian velocity, vk) and impact angle, θ, are sampled are shown in Figure A.2c,d. — astro-ph.EP

Saverio Cambioni, Benjamin P. Weiss, Erik Asphaug, Kathryn Volk, Alexandre Emsenhuber, John B. Biersteker, Zifan Lin, Robert Melikyan

Comments: 48 pages, 15 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2408.15340 [astro-ph.EP] (or arXiv:2408.15340v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2408.15340
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Submission history
From: Saverio Cambioni
[v1] Tue, 27 Aug 2024 18:04:12 UTC (4,015 KB)
https://arxiv.org/abs/2408.15340

Astrobiology

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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