Origins, Worlds, and Life A Decadal Strategy for Planetary Science and Astrobiology 2023-2032 (2023) / Chapter Skim
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5 Question 2: Accretion in the Outer Solar System
5 Question 2: Accretion in the Outer Solar System
Pages 127-156
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From page 127... ...
Uranus and Neptune represent a unique class of planets that clearly differ from terrestrial and gas giant planets, and their formation challenges planet formation models because it is unclear whether they are simply failed gas giants or if they formed in a different way. That planets with similar sizes/masses appear abundant in the galaxy suggests that the formation of such intermediate-mass gas planets is common, emphasizing the need to understand the formation of Uranus and Neptune (see Questions 7 and 12; Chapters 10 and 15, respectively)
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From page 128... ...
However, their exact formation timescale remains an open question. The formation timescale of the gas giant planets is thought to be on the order of 106 to 107 years.
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From page 129... ...
In general, core accretion requires a heavy-element core, but the presence of a core in the disk instability model cannot be ruled out. Overall, better understanding of giant planet origin requires improved information on their bulk compositions and internal structures, which can be inferred from structure models that use accurate measurements of their gravitational and magnetic fields, and atmospheric compositions (see Question 7)
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From page 130... ...
Details on the evolution of the internal structures of giant planets are given in Chapter 10 and references therein. Q2.1d What Were the Roles of Early Giant Impacts and Magnetic Fields in Shaping the Properties of the Outer Planets?
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From page 131... ...
. Although Uranus and Neptune accreted vastly smaller gas components than Jupiter and Saturn, and per above appear to have had their final spin states affected by late giant impacts, the current rotation periods of all four giant planets are broadly similar (about 10–17 hours)
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From page 132... ...
. Q2.2a How Was the Overall Bulk Fraction of Heavy Elements in the Giant Planets Established?
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From page 133... ...
Because of measurements from the Galileo probe and Juno, Jupiter's atmospheric composition is far better understood than that of any of the other giant planets. SOURCE: Atreya et al.
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From page 134... ...
Composition varied spatially and temporally within the protosolar disk, with effects preserved in the presentday composition of the giant planets. The planets formed at different heliocentric distances and thus sampled spatial variation within the disk, and time variation of disk composition affected the planets differently owing to variation in individual gas accretion timescales.
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From page 135... ...
. While not giant planets, trans-neptunian dwarf planets, when telescopic observations are sufficient, are also seen to possess one or more satellites.
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From page 136... ...
Maruta, and M.N. Machida, 2014, "Accretion of Solid Materials onto Circumplanetary Disks from Protoplanetary Disks," The Astrophysical Journal 784(2)
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From page 137... ...
Q2.3b What Were the Roles of Giant Impact and Capture in the Outer Solar System for the Origin of Primordial Satellites and Planetary Rings? It is unclear whether Uranus and Neptune formed circumplanetary disks in a manner like that posited for Jupiter and Saturn (Peale and Canup 2015)
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From page 138... ...
, dynamical transport of heliocentric planetesimals and dwarf planets throughout the giant planet region would have occurred but would have been especially important for Neptune as it migrated outward through the primordial Kuiper belt population. Capture into the satellite region of a giant planet could have occurred by direct collision with a preexisting regular satellite or by tidal stripping of a binary (e.g., Agnor and Hamilton 2006; Nogueira et al.
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From page 139... ...
. It is possible that such cavities could be detectable via polarimetric measurements of accreting extra-solar giant planets (sensitive to magnetic fields)
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From page 140... ...
Q2.4 HOW DID THE GIANT PLANETS GRAVITATIONALLY INTERACT WITH EACH OTHER, THE PROTOSOLAR DISK, AND SMALLER BODIES IN THE OUTER SOLAR SYSTEM? It is now thought that the giant planets did not form where they currently reside, but instead migrated inward and/or outward because of disk torques during the protosolar nebular phase or by later gravitational interactions with remnant planetesimals.
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From page 141... ...
Early versions of such models postulated that the instability and migration could have occurred hundreds of millions of years after the formation of the solar system, but more recent dynamical work favors an earlier instability; as the nebula clears, the giant planets emerge in a spacing too close to remain stable without the eccentricity and inclination damping effects of nebular gas. Regardless, the dispersal of the primordial Kuiper belt led to the heavy bombardment of the solar system worlds that existed at that time by ice-rich planetesimals (see Question 4, Chapter 7)
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From page 142... ...
. This migration dynamically affects the terrestrial planets, asteroid belt, and primordial Kuiper belt (e.g., Nesvorný 2018)
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From page 143... ...
Morbidelli, 2012, "Statistical Study of the Early Solar System's Instability with Four, Five, and Six Giant Planets," The Astrophysical Journal 144(4) :117, https://doi.
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From page 144... ...
But their ultimate origin is thought to be the primordial Kuiper belt, the same region that birthed Pluto and the other larger members of the TNO population. Debate centers on whether these "proto-comets" formed as primordial small bodies (e.g., Davidsson et al.
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From page 145... ...
that quantity allows the depletion of the primordial Kuiper belt by Neptune's migration to explain various small body populations captured during the giant planet instability (Nesvorný and Vokrouhlický 2016; Vokrouhlický et al. 2016, 2019; Morbidelli and Nesvorný 2020)
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From page 146... ...
Q2.5e During Accretion, (How) Did the Interiors of Outer Solar System Moons and Dwarf Planets Transition from Homogeneous to Layered?
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From page 147... ...
FIGURE 5-8 Possible early internal structures of outer solar system moons and dwarf planets. The degree of separation between rock and organic material is unknown, as is the organic fraction.
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From page 148... ...
Q2.6 HOW DID THE ORBITAL STRUCTURE OF THE TRANS-NEPTUNIAN BELT, THE OORT CLOUD, AND THE SCATTERED DISK ORIGINATE, AND HOW DID GRAVITATIONAL INTERACTIONS IN THE EARLY OUTER SOLAR SYSTEM LEAD TO SCATTERING AND EJECTION? Small bodies across the solar system can constrain the migration of the giant planets and planet formation.
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From page 149... ...
These bodies then had their perihelia increased by a combination of gravitational perturbations from galactic tides and close passing stars. Approximately ~5 percent of the primordial Kuiper belt still exists within the Oort cloud (see Vokrouhlický et al.
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From page 150... ...
these encounters are happening concurrently with Neptune's migration through the primordial Kuiper belt, which sends approximately 20 Earth masses of TNOs into the giant planet zone. Some TNOs could have been located at the right place and time to be captured within stable reservoirs across the solar system by three-body reactions during this time (see also Question 3)
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From page 151... ...
. One possible explanation is that the transition to very red objects occurs beyond 30 AU in the primordial Kuiper belt, either from sublimation-driven surface depletion in some organic molecules or from collisional evolution (Nesvorný et al.
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From page 152... ...
2018. "On the Terminal Rotation Rates of Giant Planets." Astronomical Journal 155:178.
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From page 153... ...
2017. "Ring Formation Around Giant Planets by Tidal Disruption of a Single Passing Large Kuiper Belt Object." Icarus 282:195–213.
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From page 154... ...
2012. "Statistical Study of the Early Solar System's Instability with Four, Five, and Six Giant Planets." Astronomical Journal 14:117.
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From page 155... ...
1996. "Despin Mechanism for Protogiant Planets and Ionization State of Protogiant Planetary Disks." Icarus 123:404–421.
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From page 156... ...
Q3 PLATE: An enhanced-color image mosaic of Mercury acquired by the MESSENGER spacecraft in 2013. The colors reveal different surface compositions.
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