Exoplanets, -moons, -comets

Super-Earth Formation In Systems With Cold Giants

By Keith Cowing

Status Report

astro-ph.EP

June 29, 2025

Filed under astro-ph.EP, exoplanet, gas giant, ice giant, super-Earth

Super-Earth Formation In Systems With Cold Giants — Planetary growth timescales in an irradiated disc, for different fragmentation velocities (v_frag = 1 m/s in the left panel, v_frag = 10 m/s in the right panel). The black dotted line represents the transition mass between the 3D and 2D Hill accretion regimes given by Eq. (28). In the high fragmentation velocity case, the transition line is segmented because of the pebble size transition from being drift limited in the Epstein regime to drift limited in the Stokes regime to fragmentation limited (see orange branch, right panel of Fig. 2), while in case of lower fragmentation velocity, the pebbles are always fragmentation limited (blue branch, Fig. 2). The light blue dotted line and the violet dotted line mark the initial embryo masses and the pebble isolation masses, respectively. Higher fragmentation velocities lead to faster growth rates, owed to an earlier transition between the 3D and the more efficient 2D Hill accretion regimes. Indeed, the larger pebbles (cfr. Fig. 2) are more settled towards the disc midplane (cfr. Eq. 15), which implies that the criterion in Eq. (27) is fulfilled earlier on than in the lower fragmentation velocity case. — astro-ph.EP

Around our Sun, terrestrial planets did not grow beyond Earth in mass, while super-Earths are found to orbit approximately every other solar-like star.

It remains unclear what divides these super-Earth systems from those that form terrestrial planets, and what role wide-orbit gas giants play in this process. Here, we show that the key uncertainty is the degree of viscous heating in the inner disc, which regulates the pebble accretion efficiency.

In this parameter study, we assume pebble sizes limited by fragmentation and radial drift. The initial seed planetesimals for embryo growth are taken from the top of the streaming instability mass distribution. We then evaluate the important role of the pebble scale height and the assumed pebble fragmentation velocity. In systems with maximally efficient viscous heating, where all the accretion heating is deposited in the disc midplane, pebble accretion in the terrestrial region is suppressed.

More realistic levels of viscous heating, at higher elevations, allow terrestrial embryo formation at Earth-like orbits. We also find that the role of the water iceline is minor, unless it is paired with extreme volatile loss and a change in the pebble fragmentation velocity.

Furthermore, we show that in systems with gas-giant formation, the role of mutual pebble filtering by outer pebble-accreting embryos is limited, unless some mechanism of delaying inner disc growth, such as viscous heating or the presence of an iceline, is simultaneously employed.

This latter point appears to be consistent with the fact that no strong suppression is seen in the occurrence rate of super-Earths in systems with known gas giants in wider orbits. We conclude that the diversity in inner-disc systems may largely be driven by complex, and as of yet poorly understood, disc accretion physics inside the water iceline.

Claudia Danti, Michiel Lambrechts, Sebastian Lorek
Comments: 22 pages, 9 figures, accepted for publication in Astronomy & Astrophysics
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2506.17091 [astro-ph.EP] (or arXiv:2506.17091v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2506.17091
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Submission history
From: Claudia Danti
[v1] Fri, 20 Jun 2025 15:54:44 UTC (2,006 KB)
https://arxiv.org/abs/2506.17091

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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