Pebble accretion
Pebble accretion is the
Description
Pebbles ranging in size from centimeters up to a meter in size are accreted at an enhanced rate in a
The remainder would follow hyperbolic paths, accelerating toward the planetesimal on their approach and decelerating as they recede. However, the drag the pebbles experience grows as their velocities increase, slowing some enough that they become gravitationally bound to the planetesimal.[5] These pebbles continue to lose energy as they orbit the planetesimal causing them to spiral toward and be accreted by the planetesimal.[6][7]
Small planetesimals accrete pebbles that are drifting past them at the relative velocity of the gas. Those pebbles with stopping times similar to the planetesimal's Bondi time are accreted from within its Bondi radius. In this context the Bondi radius is defined as the distance at which an object approaching a planetesimal at the relative velocity of the gas is deflected by one radian; the stopping time is the exponential timescale for the deceleration of an object due to gas drag, and the Bondi time is the time required for an object to cross the Bondi radius. Since the Bondi radius and Bondi time increase with the size of the planetesimal, and the stopping time increases with the size of the pebble, the optimal pebble size increases with size of planetesimal.
Smaller objects, with ratios of stopping times to Bondi times less than 0.1, are pulled from the flow past the planetesimal and accreted from a smaller radius which declines with the square root of this ratio. Larger, weakly coupled pebbles are also accreted less efficiently due to three body effects with the radius accreted from declining rapidly between ratios of 10 and 100. The Bondi radius is proportional to the mass of the planetesimal so the relative growth rate is proportional to mass squared resulting in runaway growth.[9] The aerodynamic deflection of the gas around the planetesimal reduces the efficiency of pebble accretion resulting in a maximum growth timescale at 100 km.[10]
Larger planetesimals, above a transition mass of roughly Ceres mass in the inner solar system and Pluto mass in the outer solar system,[11] accrete pebbles with Stokes numbers near one from their Hill radii. The "Stokes number" in this context is the product of stopping time and the Keplerian frequency. As with small planetesimals the radius from which pebbles accrete declines for smaller and larger pebble sizes. The optimal pebble size for large planetesimals measures in cm's due to a combination of the accretion radius and the radial drift rates of the pebbles. As objects grow their accretion changes from 3-D, with accretion from part of the thickness of the pebble disk, to 2D with accretion from full thickness of pebble disk. The relative growth rate in 2-D accretion is proportional to mass leading to oligarchical growth and the formation of similar sized bodies.[9] Pebble accretion can result in doubling of mass of an Earth-massed core in as little as 5500 years,[11] reducing the timescales for growth of the cores of giant planets by 2 or 3 orders of magnitude relative to planetesimal accretion.[9] The gravitational influence of these massive bodies can create a partial gap in the gas disk altering the pressure gradient.[11] The velocity of gas then becomes super-keplerian outside the gap stopping the inward drift of pebbles and ending pebble accretion.[3]
Outer Solar System
If the formation of pebbles is slow, pebble accretion leads to the formation of a few
However, if the formation or the delivery of pebbles is slow growth timescales becomes longer than the time required for gravitationally stirring. The largest planetesimals then excite the
Inner Solar System
The terrestrial planets may be much smaller than the giant planets due to the sublimation of water ice as pebbles crossed the
In the terrestrial zone pebble accretion plays a smaller role.[23] Here growth is due to a mix of pebble and planetesimal accretion until an oligarchical configuration of isolated lunar-massed embryos forms. Continued growth due to the accretion of inward drifting chondrules increases the mass of these embryos until their orbits are destabilized, leading to giant impacts between the embryos and the formation of Mars-sized embryos.[23][24] The cutoff of the inward drift of icy pebbles by the formation of Jupiter before the ice line moved into the terrestrial region would limit the water fraction of the planets formed from these embryos.[25]
The small mass of Mars and the low mass asteroid belt may be the result of pebble accretion becoming less efficient as the density of gas in the protoplanetary disk decreases. The protoplanetary disk from which the Solar System formed is thought to have had a
References
- ^ a b Lewin, Sarah (19 August 2015). "To Build a Gas Giant Planet, Just Add Pebbles". Space.com. Retrieved 22 November 2015.
- ^ S2CID 119278457.
- ^ S2CID 55923519.
- S2CID 118553344.
- S2CID 86864111.
- ^ a b "Scientists predict that rocky planets formed from "pebbles"". Southwest Research Institute. Archived from the original on 23 November 2015. Retrieved 22 November 2015.
- ^ Küffmeier, Michael (9 September 2015). "Chondrules are old and everywhere – are solar system's solid bodies built by them?". Astrobites. Retrieved 20 November 2016.
- .
- ^ S2CID 53961588.
- S2CID 118672882.
- ^ S2CID 119259438.
- ^ "Scientists think 'planetary pebbles' were the building blocks for the largest planets". Phys.org. Retrieved 22 November 2015.
- S2CID 18964068.
- S2CID 118572605.
- ^ a b Hand, Eric. "How Jupiter and Saturn were born from pebbles". Science. Retrieved 22 November 2015.
- S2CID 4458098.
- S2CID 182381427. Retrieved 22 November 2015.
- .
- ^ Lichtenberg, Tim (18 August 2015). "Giant planets from far out there". astrobites. Retrieved 20 November 2016.
- S2CID 118878865.
- .
- ^ S2CID 119298280.
- ^ S2CID 119187749.
- ^ PMID 26601169.
- S2CID 54642403.
- ^ PMID 26512109.