http://arxiv.org/abs/2003.03388
The orbital period ratios of neighbouring sub-Neptunes are distributed asymmetrically near first-order resonances. There are deficits of systems—“troughs” in the period ratio histogram—just short of commensurability, and excesses—“peaks”—just wide of it. We reproduce quantitatively the strongest peak-trough asymmetries, near the 3:2 and 2:1 resonances, using dissipative interactions between planets and their natal disks. Disk eccentricity damping captures bodies into resonance and clears the trough, and when combined with disk-driven convergent migration, draws planets initially wide of commensurability into the peak. The migration implied by the magnitude of the peak is modest; reductions in orbital period are $\sim$10\%, supporting the view that sub-Neptunes complete their formation more-or-less in situ. Once captured into resonance, sub-Neptunes of typical mass $\sim$$5$–$15 M_{\oplus}$ stay captured (contrary to an earlier claim), as they are immune to the overstability that afflicts lower mass planets. Driving the limited, short-scale migration is a gas disk whose surface density is fairly constant inside 1 AU and depleted relative to a solar-composition disk by 3–5 orders of magnitude. Such gas-poor but not gas-empty environments are quantitatively consistent with sub-Neptune core formation by giant impacts (and not, e.g., pebble accretion). While disk-planet interactions at the close of the planet formation era adequately explain the 3:2 and 2:1 asymmetries at periods $\gtrsim$ $5$–$15$ days, subsequent modification by stellar tides appears necessary at shorter periods, particularly for the 2:1.
N. Choksi and E. Chiang
Tue, 10 Mar 20
41/63
Comments: Submitted to MNRAS