Exoplanetology: Exoplanets & Exomoons

Bulk and Atmospheric Metallicities as Direct Probes of Sequentially Varying Accretion Mechanisms of Gas and Solids Onto Planets

By Keith Cowing
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astro-ph.EP
September 17, 2024
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Bulk and Atmospheric Metallicities as Direct Probes of Sequentially Varying Accretion Mechanisms of Gas and Solids Onto Planets
Four stages of planet formation via core accretion and its schematic diagram. Top: Sequential accretion of gas and solid divides the mass growth of planets into four stages. In Stage I, planetary cores form by solid accretion. As time goes on, solid accretion rates decrease, and gas accretion becomes dominated. One conservative estimate of a characteristic solid accretion rate is 10M⊕/1 Myr ∼ 10−5M⊕ yr−1 as denoted by the black dash-dotted line. Once the solid accretion rate becomes lower the value, gas accretion comes into play. In Stage II, accretion of both gas and solids occurs. The gas accretion rate is regulated by cooling of accreted gas (i.e., M˙ XY,KH denoted by the blue solid line). In Stage III, gas accretion is controlled by disk evolution. The corresponding rate becomes very high (i.e., M˙ XY,hydro denoted by the green dotted line), and it is the main channel of forming sub-giants. In Stage IV, gas accretion is reduced due to planet-disk interaction and the resulting gap formation in gas disks (i.e., M˙ XY,gap denoted by the yellow dashed line). This is the final stage of giant plane formation. Mathematical expressions of accretion rates are summarized in Appendix A. Bottom: Sequential accretion of gas and solid can be idealized by the onion-like model. As we show below, bulk and atmospheric metallicities can be used to trace when gap formation in both planetesimal and gas disks occurs, which corresponds to Stages III and IV, respectively. The purple and light salmon trapezoids represent gas and planetesimals disks, respectively. The brown circles denotes solids and the circular rings represent envelope gas. — astro-ph.EP

Core accretion is the standard scenario of planet formation, wherein planets are formed by sequential accretion of gas and solids, and is widely used to interpret exoplanet observations. However, no direct probes of the scenario have been discussed yet.

Here, we introduce an onion-like model as one idealization of sequential accretion and propose that bulk and atmospheric metallicities of exoplanets can be used as direct probes of the process. Our analytical calculations, coupled with observational data, demonstrate that the trend of observed exoplanets supports the sequential accretion hypothesis.

In particular, accretion of planetesimals that are ≳ 100 km in size is most favored to consistently explain the observed trends. The importance of opening gaps in both planetesimal and gas disks following planetary growth is also identified.

New classification is proposed, wherein most observed planets are classified into two interior statuses: globally mixed and locally (well-)mixed. Explicit identification of the locally (well-)mixed status enables reliable verification of sequential accretion. During the JWST era, the quality and volume of observational data will increase drastically and improve exoplanet characterization.

This work provides one key reference of how both the bulk and atmospheric metallicities can be used to constrain gas and solid accretion mechanisms of planets.

Yasuhiro Hasegawa, Mark R. Swain

Comments: 14pages, 4 figures, 1 table; accepted for publication in ApJ Letters
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2409.06670 [astro-ph.EP] (or arXiv:2409.06670v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2409.06670
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Submission history
From: Yasuhiro Hasegawa
[v1] Tue, 10 Sep 2024 17:38:09 UTC (405 KB)
https://arxiv.org/abs/2409.06670
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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