The Emerging Paradigm of Pebble Accretion

Abstract

Pebble accretion is the mechanism in which small particles ("pebbles") accrete onto big bodies (planetesimals or planetary embryos) in gas-rich environments. In pebble accretion, accretion occurs by settling and depends only on the mass of the gravitating body, not its radius. I give the conditions under which pebble accretion operates and show that the collisional cross section can become much larger than in the gas-free, ballistic, limit. In particular, pebble accretion requires the pre-existence of a massive planetesimal seed. When pebbles experience strong orbital decay by drift motions or are stirred by turbulence, the accretion efficiency is low and a great number of pebbles are needed to form Earth-mass cores. Pebble accretion is in many ways a more natural and versatile process than the classical, planetesimal-driven paradigm, opening up avenues to understand planet formation in solar and exoplanetary systems.

The CARMENES search for exoplanets around M dwarfs

“…Their final masses can be best explained if the supply of pebbles was cut off during their formation, preventing their evolution into gas giants. One way to stop the supply of solid material from the outer disk is the emergence of a massive companion that stops the pebble flux by opening a gap in the protoplanetary disk (Ormel 2017). However, we do not see evidence for additional planets in any of the three systems, even though the available long-baseline data allow for a strong sensitivity for the detection of such companions in the cases of GJ 251 and Lalande 21185.…”

The Gemini Planet Imager Exoplanet Survey: Giant Planet and Brown Dwarf Demographics from 10 to 100 au

“…Brown dwarfs and perhaps also giant planets of the highest masses are thought to represent the low-mass outcome of the process by which stars form (i.e., gravitational instability; Forgan & Rice 2013; Kratter & Lodato 2016) or turbulent fragmentation (e.g., Hopkins 2013). Objects of lower mass (Jupiters and below) may have formed from the "bottom up" via core accretion (i.e., by coagulation of solids; e.g., Goldreich et al 2004;Ormel 2017) and subsequent accretion of gas (e.g., Harris 1978;Mizuno et al 1978;Mizuno 1980;Pollack et al 1996). Evidence for this dichotomy is seen for close-in companions observed by RV and transit surveys, where there exists a brown dwarf desert lacking companions near 30 M Jup (e.g., Grether & Lineweaver 2006;Triaud et al 2017).…”

A balanced budget view on forming giant planets by pebble accretion

“…The problem is tied up with the longstanding mystery of planetesimal formation (Chiang & Youdin 2010). "Pebble accretion" makes inroads on the problem of core assembly by exploiting the ability of seed cores to attract particles small enough to have their velocity dispersions damped by aerodynamic drag from the ambient gas disc (for a comprehensive review, see Ormel 2017). Without addressing the question of the origin of the seed core (seeds as low in mass as ∼10 −3 M⊕ have been assumed), the theory of pebble accretion points out that for particles whose aerodynamic stopping times are comparable to orbital times-"pebbles" with order-unity Stokes numbers-the accretion cross-section can approach its maximum value set by the Hill sphere of the seed core.…”