Vulcan Planets: Inside-Out Formation of the Innermost Super-Earths

Abstract

The compact multi-transiting systems discovered by Kepler challenge traditional planet formation theories. These fall into two broad classes: (1) formation further out followed by migration; (2) formation in situ from a disk of gas and planetesimals. In the former, an abundance of resonant chains is expected, which the Kepler data do not support. In the latter, required disk mass surface densities may be too high. A recently proposed mechanism hypothesizes that planets form in situ at the pressure trap associated with the dead-zone inner boundary (DZIB) where radially drifting "pebbles" accumulate. This scenario predicts planet masses ($M_p$) are set by the gap-opening process that then leads to DZIB retreat, followed by sequential, inside-out planet formation (IOPF). For typical disk accretion rates, IOPF predictions for $M_p$, $M_p$ versus orbital radius $r$, and planet-planet separations are consistent with observed systems. Here we investigate the IOPF prediction for how the masses, $M_{p,1}$, of the innermost ("Vulcan") planets vary with $r$. We show that for fiducial parameters, $M_{p,1}\simeq 5.0(r/0.1\mathrm{AU})M_{\oplus }$, independent of the disk's accretion rate at time of planet formation. Then, using Monte Carlo sampling of a population of these innermost planets, we test this predicted scaling against observed planet properties, allowing for intrinsic dispersions in planetary densities and Kepler's observational biases. These effects lead to a slightly shallower relation $M_{p,1}\propto r^{0.9\pm 0.2}$, which is consistent with $M_{p,1}\propto r^{0.7\pm 0.2}$ of the observed Vulcans. The normalization of the relation constrains the gap-opening process, favoring relatively low viscosities in the inner dead zone.

8 pages, 5 figures, accepted to ApJL

References in corpus (5)

• Dynamical Outcomes of Planet-Planet Scattering
• Resonant Repulsion of Kepler Planet Pairs
• On corotation torques, horseshoe drag and the possibility of sustained stalled or outward protoplanetary migration
• Hot super-Earths and giant planet cores from different migration histories
• Formation of close in Super-Earths \& Mini-Neptunes: Required Disk Masses \& Their Implications

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• The Compositional Diversity of Low-Mass Exoplanets
• Inner Planetary System Gap Complexity is a Predictor of Outer Giant Planets
• Edge-of-the-Multis: Evidence for a Transition in the Outer Architectures of Compact Multi-Planet Systems
• Influences of three-dimensional gas flow induced by protoplanets on pebble accretion --$\mathrm{I}$. shear regime
• Inside-Out Planet Formation. VII. Astrochemical Models of Protoplanetary Disks and Implications for Planetary Compositions
• Effects of Planetesimal Scattering: Explaining the Observed Offsets from Period Ratios 3:2 and 2:1
• Effects of Planetesimal Accretion on the Thermal and Structural Evolution of Sub-Neptunes
• Possible Outcomes of Coplanar High-eccentricity Migration: Hot Jupiters, Close-in Super-Earths, and Counter-orbiting Planets
• Inside-Out Planet Formation: VI. Oligarchic Coagulation of Planetesimals from a Pebble Ring?