Rapid growth of gas-giant cores by pebble accretion

By Michiel Lambrechts, Anders Johansen. 2012

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The observed lifetimes of gaseous protoplanetary discs place strong constraints on gas and ice giant formation in the core accretion scenario. The approximately 10-Earth-mass solid core responsible for the attraction of the gaseous envelope has to form before gas dissipation in the protoplanetary disc is completed within 1–10 million years. Building up the core by collisions between km-sized planetesimals fails to meet this timescale constraint, especially at wide stellar separations. Nonetheless, gas-giant planets are detected by direct imaging at wide orbital distances. In this paper, we numerically study the growth of cores by the accretion of cm-sized pebbles loosely coupled to the gas. We measure the accretion rate onto seed masses ranging from a large planetesimal to a fully grown 10-Earth-mass core and test different particle sizes. The numerical results are in good agreement with our analytic expressions, indicating the existence of two accretion regimes, one set by the azimuthal and radial particle drift for the lower seed masses and the other, for higher masses, by the velocity at the edge of the Hill sphere. In the former, the optimally accreted particle size increases with core mass, while in the latter the optimal size is centimeters, independent of core mass. We discuss the implications for rapid core growth of gas-giant and ice-giant cores. We conclude that pebble accretion can resolve the long-standing core accretion timescale conflict. This requires a near-unity dust-to-gas ratio in the midplane, particle growth to mm and cm and the formation of massive planetesimals or low radial pressure support. The core growth timescale is shortened by a factor 30–1000 at 5 AU and by a factor 100–10 000 at 50 AU, compared to the gravitationally focused accretion of, respectively, low-scale-height planetesimal fragments or standard km-sized planetesimals.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/0696d2f7c5219ce9170e21aa10a87ec8fc285e90

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Citations in the corpus

The great dichotomy of the Solar System: Small terrestrial embryos and massive giant planet cores (2015, background): “Growth of embryos and cores by pebble accretion In a series of recent papers (Lambrechts & Johansen, 2012, 2014; Ormel and Klahr, 2010; Johansen and Lacerda, 2010; Murray-Clay et al., 2011; Morbidelli and Nesvorny, 2012; Kretke and Levison, 2014b), a new model has been proposed in which…”
The structure of protoplanetary discs around evolving young stars (2014, background): “These embryos can then grow via collisions between each other and through pebble accretion (Lambrechts & Johansen 2012).”
Forming the cores of giant planets from the radial pebble flux in protoplanetary discs (2014, methodology): “Subsequently, pebbles are rapidly accreted by large planetesimals, provided that the planetesimal mass is above the transition mass, Mt ≈ √ 1 3 (ηvK)3 GΩK ≈ 0.0069 ( r 5 AU )3/2 ME, (27) which depends cubicly on the uncertain value of η(r) (Lambrechts & Johansen 2012).”
Planetesimal formation near the snowline: in or out? (2017, background): “After planetesimals of a certain size have formed, they can subsequently accrete pebbles and quickly grow to larger bodies (e.g., Ormel & Klahr 2010; Lambrechts & Johansen 2012, 2014; Bitsch et al. 2015; Visser & Ormel 2016).”
Fluffy dust forms icy planetesimals by static compression (2013, background): “Several possibilities have been proposed to overcome these barriers (Johansen et al. 2007; Pinilla et al. 2012; Windmark et al. 2012; Lambrechts & Johansen 2012; Ros & Johansen 2013; Garaud et al. 2013).”
RINGED SUBSTRUCTURE AND A GAP AT 1 au IN THE NEAREST PROTOPLANETARY DISK (2016, background): “The millimeter-sized particles needed to assemble larger planetesimals (Ormel & Klahr 2010; Lambrechts & Johansen 2012) experience aerodynamic drag with the gas that results in their rapid migration toward the host star (Weidenschilling 1977; Takeuchi & Lin 2002).”
Exoplanetary Atmospheres: Key Insights, Challenges, and Prospects (2019, background): “The important aspect of this formation mechanism (139) is that the solid accretion occurs predominantly during the formation of the core until the pebble isolation mass is reached, following which gas accretion dominates”
Pebble-isolation mass: Scaling law and implications for the formation of super-Earths and gas giants (2018, background): “In recent years, a new paradigm of solid accretion has emerged: pebble accretion (Johansen & Lacerda 2010; Ormel & Klahr 2010; Lambrechts & Johansen 2012; Morbidelli & Nesvorny 2012).”
IN SITU FORMATION AND DYNAMICAL EVOLUTION OF HOT JUPITER SYSTEMS (2015, background): “Rapid conversion of planetesimals into SuperEarths by accretion of pebbles appears to be fully consistent with the envisioned evolutionary sequence (Lambrechts and Johansen 2012, 2014; Levison et al. 2015).”
Close-in planetesimal formation by pile-up of drifting pebbles (2016, background): “Pebbles corresponding to Stokes number of 10−2−1 may be accreted very efficiently onto planetary embryos (Ormel & Klahr 2010; Lambrechts & Johansen 2012).”
DUST AND GAS IN THE DISK OF HL TAURI: SURFACE DENSITY, DUST SETTLING, AND DUST-TO-GAS RATIO (2015, background): “The increased gas/dust ratio and flat dust sub-disk can also lead to ”pebble accretion” (Lambrechts & Johansen 2012), significantly reducing the growth timescale of a planetary core massive enough to trigger runaway gas accretion.”
The nature of the TRAPPIST-1 exoplanets. (2018, background): “Next, the largest planetesimals grow rapidly by accreting inwards-drifting pebbles (Ormel & Klahr 2010; Lambrechts & Johansen 2012).”
Separating gas-giant and ice-giant planets by halting pebble accretion (2014, methodology): “Pebble accretion rates are sufficiently high to form the cores of giant planets in less than 1 million years, even in wide orbits (Lambrechts & Johansen 2012).”
Dust Evolution and the Formation of Planetesimals (2016, background): “With growth and drift providing a constant supply of inward drifting grains of the right sizes to be accreted, the growth time scale of cores, particularly in the outer regions of disks can be reduced significantly Lambrechts & Johansen (2012, 2014).”
Origin of water in the inner Solar System: Planetesimals scattered inward during Jupiter and Saturn's rapid gas accretion (2017, background): “core growth (e.g. Levison and Stewart, 2001). In contrast, the pebble accretion model proposes that giant planet cores grow preferentially by the accretion of small pebbles rather than planetesimals (Lambrechts and Johansen, 2012, 2014; Levison et al., 2015). While some planetesimals are needed to a”