Forming Planetesimals in Solar and Extrasolar Nebulae

By E. Chiang, A. Youdin. 2009

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Planets are built from planetesimals: solids larger than a kilometer that grow by colliding in pairs. Planetesimals themselves are unlikely to form by twobody collisions alone; subkilometer objects have gravitational fields individually too weak, and electrostatic attraction is too feeble for growth beyond a few centimeters. We review the possibility that planetesimals form when self-gravity brings together vast ensembles of small particles. Even when self-gravity is weak, aerodynamic processes can accumulate solids relative to gas, paving the way for gravitational collapse. Particles pile up as they drift radially inward. Gas turbulence stirs particles but can also seed collapse by clumping them. Whereas the feedback of solids on gas triggers vertical-shear instabilities that obstruct self-gravity, this same feedback triggers streaming instabilities that strongly concentrate particles. Numerical simulations find that solids ∼10–100 cm in size gravitationally collapse in turbulent disks. We outline areas for progress, including the possibility that still smaller objects self-gravitate.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/d8b0ae5efec046f33bfe7fc1eeb3df0391ebaaa9

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Citations in the corpus

THE EXOPLANET CENSUS: A GENERAL METHOD APPLIED TO KEPLER (2011, background): “This trend has been a powerful guide to identifying the mechanisms responsible for planetesimal formation (Youdin & Shu 2002; Youdin & Goodman 2005; Johansen et al. 2009), see Chiang & Youdin (2010) for a review.”
Protoplanetary Disks and Their Evolution (2011, background): “…in long-lived vortices (e.g., Heng & Kenyon 2010), or via the gravitational instability of millimeter-sized chondrules in a gas deficient subdisk (e.g., Youdin & Shu 2002), overcoming this barrier remains one of the biggest challenges for planet formation theories (Chiang & Youdin 2010).”
The minimum-mass extrasolar nebula: in situ formation of close-in super-Earths (2012, methodology): “To estimate vrel, we recognize that gas is supported by a radial pressure gradient and orbits the star at the sub-Keplerian velocity (1 − η)Ωa, where η ∼ (hgas/a)2 ∼ 8 × 10−4(a/0.2 AU)1 (Adachi et al. 1976; Chiang & Youdin 2010).”
A Perspective from Extinct Radionuclides on a Young Stellar Object: The Sun and Its Accretion Disk (2011, neutral): “As discussed in Section 5, this simple picture has been questioned by two recent studies that proposed new models for the formation of planetesimals [see the review by Chiang & Youdin (2010)].”
Ice Condensation as a Planet Formation Mechanism (2013, background): “We useηvK = 0.05cs, a reasonable value at 3 AU in the MMSN (Cuzzi et al., 1993; Chiang & Youdin, 2010).”
Rapid growth of gas-giant cores by pebble accretion (2012, methodology): “…comparison of the MMSN model with observed protostellar accretion discs, (e.g. Bell et al. 1997) and studies of solar nebula metallicities (Lodders 2003) have prompted updated MMSN models, with a less steep pressure gradient, Δ = 0.036 (r/AU)2/7 (Chiang & Youdin 2010), as illustrated in Fig.”
How to form planetesimals from mm-sized chondrules and chondrule aggregates (2015, background): “Planetesimals are super-kilometer-sized bodies that are the seeds of terrestrial planets, as well as the cores of ice and gas giants (Safronov 1972; Chiang & Youdin 2010; Johansen et al. 2014).”
Separating gas-giant and ice-giant planets by halting pebble accretion (2014, background): “A more detailed discussion can be found in the reviews by Chiang & Youdin (2010) and Johansen et al. (2014).”
Adding particle collisions to the formation of asteroids and Kuiper belt objects via streaming instabilities (2011, background): “The formation of super-km-sized planetesimals is an important step towards terrestrial planets and the solid cores of gas and ice giants (e.g. Safronov 1969; Goldreich et al. 2004; Chiang & Youdin 2010).”
Astrophysics with Extraterrestrial Materials (2016, background): “The merits and challenges to each model have recently been summarized from a dynamic perspective in Chiang & Youdin (2010).”
Dynamics of Protoplanetary Disks (2010, background): “The most important is probably the effect that a dead zone would have on planetesimal formation, since many candidate mechanisms for building planetesimals are sensitive to the level of intrinsic disk turbulence (Chiang & Youdin 2010).”
Multiple Disk Gaps and Rings Generated by a Single Super-Earth (2017, background): “Once dust is concentrated into rings, coagulation can be facilitated (e.g., Youdin & Shu 2002; Johansen et al. 2007; Pinilla et al. 2016), and instabilities such as the streaming instability (Youdin & Goodman 2005; Chiang & Youdin 2010) can be triggered.”
The structure of protoplanetary discs around evolving young stars (2014, methodology): “Streaming ∆ v / c s r in AU ∂ ln P / ∂ ln r Transition Ṁ = 3.5× 10−8 MMSN Chiang & Youdin (2010) 0 0.02 0.04 0.06 0.08 0.12 2 5 10 201 Transition Ṁ = 3.5× 10−8 MMSN Chiang & Youdin (2010) -3.2 -3 -2.8 -2.6 -2.4 -2.2 Fig.”
Debris disks as signposts of terrestrial planet formation (2011, background): “In particular, planetesimal formation is poorly understood (for a review, see Chiang & Youdin 2010), and may be coupled to the level of intrinsic turbulence within the gaseous disk, which probably varies with radius (Gammie 1996; Armitage 2011).”
Chondrule size and related physical properties: a compilation and evaluation of current data across all meteorite groups (2014, background): “The size distributions of chondrules among the different chondrite groups could be a result of their mechanism of formation, a result of post-solidification nebular sorting, the result of a process on the parent body, or a combination of factors (e.g., Shu et al., 1996; Weidenschilling, 2000; Cuzzi ”