Forming the cores of giant planets from the radial pebble flux in protoplanetary discs (1408.6094v2)

Abstract: The formation of planetary cores must proceed rapidly in order for the giant planets to accrete their gaseous envelopes before the dissipation of the protoplanetary gas disc (<3 Myr). In orbits beyond 10 AU, direct accumulation of planetesimals by the cores is too slow. Fragments of planetesimals could be accreted faster, but planetesimals are likely too large for fragmentation to be efficient, and resonant trapping poses a further hurdle. Here we instead investigate the accretion of small pebbles (mm-cm sizes) that are the natural outcome of an equilibrium between the growth and radial drift of particles. We construct a simplified analytical model of dust coagulation and pebble drift in the outer disc, between 5 AU and 100 AU, which gives the temporal evolution of the solid surface density and the dominant particle size. These two key quantities determine how core growth proceeds at various orbital distances. We find that pebble surface densities are sufficiently high to achieve the inside-out formation of planetary cores within the disc lifetime. The overall efficiency by which dust gets converted to planets can be high, close to 50 % for planetary architectures similar to the Solar System. Growth by pebble accretion in the outer disc is sufficiently fast to overcome catastrophic Type I migration of the cores. These results require protoplanetary discs with large radial extent (~100 AU) and assume a low number of initial seed embryos. Our findings imply that protoplanetary discs with low disc masses, as expected around low-mass stars (<1 M_sun), or with sub-solar dust-to-gas ratios, do not easily form gas-giant planets (M > 100 M_E), but preferentially form Neptune-mass planets or smaller (< 10 M_E). This is consistent with exoplanet surveys which show that gas giants are relatively uncommon around stars of low mass or low metallicity.

• The paper demonstrates that pebble accretion efficiently builds massive planetary cores in wide protoplanetary discs.
• It employs an analytical model to predict the evolution of solid surface density and particle size for inside-out core formation.
• The study finds up to a 50% efficiency in converting dust into cores, aligning with observed exoplanet architectures.

Overview of Core Formation from Pebble Accretion

The paper by Lambrechts and Johansen offers a comprehensive analysis of the formation of planetary cores in protoplanetary discs through a mechanism that hinges on the radial flux of small pebbles, which range from millimeter to centimeter in size. This pea-sized material forms the building blocks for the giant planetary cores and may ultimately dictate the architecture of planetary systems similar to our own.

Core Growth and Pebble Accretion Model

A key proposition of the research is that pebble accretion could be the dominant method for core growth, especially in wide orbits where planetesimal accretion fails due to its sluggish nature. The model presented in the paper is analytical and simplifies the complexities of dust coagulation and pebble drift. It predicts the temporal evolution of the solid surface density and the prevalent particle size, which are critical to understanding core growth. The model assumes a balanced state between drift and growth in these outer regions of protoplanetary discs, specifically between 5 AU and 100 AU.

The research validates these assumptions by demonstrating high pebble surface densities, allowing planet cores to form inside-out during the disc's lifetime. The inside-out methodology is efficient, with the research estimating efficiencies of up to 50% for converting dust to planets in Solar System-like architectures.

Implications for Disc Structure and Core Formation

The results have significant implications on assumptions of disc structures and core formation processes. Firstly, the theory posits that for effective pebble accretion and subsequent core formation, the protoplanetary disc must have considerable radial extent (≥100 AU). Moreover, the paper suggests that such a process could occur with a few seed embryos present initially.

Interestingly, the analysis also suggests that in less massive discs, such as those around low-mass stars (<1 M₀), or in discs with sub-solar dust-to-gas ratios, the formation of gas giants is unlikely. Neptune-mass planets or smaller bodies are more likely to form under these conditions. This conclusion aligns with exoplanet observational data, which indicate fewer gas giants around stars of lower mass or metallicity.

Theoretical and Practical Implications

Theoretically, this work contributes significantly to understanding planetary formation by presenting pebble accretion as a viable model for forming cores quickly enough before the dissipation of the gas disc. This model overcomes previous challenges such as Type I migration by rapidly increasing core masses in a manner that averts catastrophic inward spiraling.

Practically, these results suggest a reevaluation of disc surveys. Observations of protoplanetary and debris discs should increasingly focus on radial extents and conditions conducive to pebble accretion rather than simply on total mass. The dissipative dynamics of gas and dust ultimately govern the lifespan and potential diversity of resulting planetary systems.

Future Implications and Developments

This research opens avenues for several future investigations. More granular approaches are warranted, particularly in simulating the distribution of pebbles across differing disc compositions and extents. Furthermore, integrating these simulations with observational data, especially regarding young stars like TW Hya, could refine predictions on system formation and compare theoretical outcomes with detected exoplanetary systems. Addressing these directions could pivot current assumptions and equip astronomers with better models for investigating exoplanet formation and distribution across the galaxy.

In conclusion, Lambrechts and Johansen's paper provides an insightful exploration of planet formation through pebble accretion, inviting further exploration into the nature and dynamics of pebble drift within protoplanetary discs. They present a compelling framework to explain the emergence of various planetary sizes, in equilibrium with observable disc parameters, while also suggesting a meticulous groundwork for future theoretical and observational research.