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| Image: The Kuiper Belt. Credit: NASA. |
The Kuiper belt is a circumstellar disc in the Solar System beyond the planets, extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt (the circumstellar disc located roughly between the orbits of the planets Mars and Jupiter), but it is far larger-20 times as wide and 20 to 200 times as massive.
Like the asteroid belt, it consists mainly of small bodies, or remnants from the Solar System's formation. Although many asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles (termed "ices"), such as methane, ammonia and water. The Kuiper belt is home to three officially recognized dwarf planets: Pluto, Haumea, and Makemake. Some of the Solar System's moons, such as Neptune's Triton and Saturn's Phoebe, are also thought to have originated in the region.
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| Image: Phoebe - Comet Moon of Saturn - may have originated in the outer solar system. Phoebe's irregular surface, retrograde orbit, unusually dark surface, assortment of large and small craters, and low average density appear consistent with the hypothesis that Phoebe was part of the Kuiper Belt of icy comets beyond Neptune before it was captured by Saturn. Visible in this image of Phoebe are craters, streaks, and layered deposits of light and dark material. The image was taken from around 19,000 miles out from this 137-mile wide moon. Credit: NASA/ESA/JPL/SSI |
Large Kuiper Belt Objects are conventionally thought to have formed out of a massive planetesimal belt that is a few thousand times its current mass. Such a picture, however, is incompatible with multiple lines of evidence.
The Cold Classical Kuiper Belt, in particular, is an interesting population of low-inclination and low eccentricity objects lying between 42 and 47 AU with a total mass of ~ 0.1Mearth and radius up to ~ 200 km. While the other Kuiper belt populations appear to have been injected into the region via interactions with planets the evidences strongly favour an in situ formation of the Cold Classical Kuiper Belt.
In a recent paper (Shannon, Wu & Lithwick 2016, ApJ) the authors present a new model for the formation of Cold Classical Kuiper belt objects, out of a solid belt only a few times its current mass. This is made possible by depositing most of the primordial mass in grains of size centimetre or smaller. These grains collide frequently and maintain a dynamically cold belt out of which large bodies grow efficiently.
Such a light belt may represent the true outer edge of the Solar system, and it may have effectively halted the outward migration of Neptune.
Finally, more recognition of the in in situ formation of cold classical KBOs; however, I think their typically similar-sized binary nature requires gravitational collapse (Nesvorny et al., 2010) rather than some form of accretion. And we shouldn't try to shoe horn models into Grand Tack. Instead, let the evidence dictate the theory.
ReplyDeleteOccam's razor simplicity is a slippery subject when greater complexity in one area results in greater simplicity in another. Is Grand Tack which requires one reservoir with fine tuning a simpler model than an alternative ideology that requires fewer variables (more stable) but requires 3 material reservoirs and several alternative planet/planetesimal formation mechanisms? Clearly academia comes down on the side of fine tuning a creaking model, rather than exploring more-predictive alternative ideologies. Here's where it would help to have scientific philosophers who could weigh in on the relative complexity of competing models and ideologies of mundane science of planet and planetesimal formation mechanisms, rather than the almost exclusive focus on sexy theories of everything (ToEs).
I'm very much in favor of 'self-centering' ideology that reduces variables, even if it requires 3 new planet formation mechanisms. I suggest that simplicity should count variables--period, end of discussion--not that counting variables is easy or even necessarily possible.
I suggest 3 reservoirs:
1) A protoplanetary disk, condensing SDOs and comets
2) A 4,567 Ma 'primary debris disk', condensing asteroids, chondrites and in situ hot classical KBOs
3) A 542 Ma 'secondary debris disk', condensing in situ cold classical KBOs, including geologically-young Pluto
I suggest our former quadruple star system was originally composed of binary-Sun and binary-Companion in a wide-binary separation. Secular perturbation caused the two close-binary pairs to in-spiral, transferring their orbital energy and angular momentum to the wide-binary system, causing Sun-Companion to spiral out for 4 billion years. Binary-Sun in-spiraled to merge at 4,567 Ma, creating the short-lived r-process radionuclides, such as 26Al and 60Fe, and the stable isotopes 12C and 16O. The in-spiral of binary-Companion lasted 4 billion years, with the solar system barycenter (SSB) spiraling out through the Kuiper belt from 4.1–3.8 Ga, causing the late heavy bombardment as it passed through the cubewano population, turning an originally cold population (low inclination, low eccentricity) into a hot population (high inclination, high eccentricity) by SSB perturbation.
Then the asymmetrical 542 Ma in-spiral merger of a former binary-(brown dwarf)-Companion gave the Companion escape velocity from the Sun and created a 'secondary debris disk' from which cold classical KBOs condensed by gravitational instability against Neptune's 2:3 resonance.
http://arxiv.org/abs/1510.01323
Forming the Cold Classical Kuiper Belt in a light Disk
Nesvorny et al., 2010, Formation of Kuiper Belt Binaries by Gravitational Collapse
I understand that the formation mechanism of the Kuiper belt is quite complex (models take into account gravitational collapse of the original cloud, planets resonances, collisions, accretion etc.) and still unclear. The new paper (Shannon et al. 2016) show only that the grains' growth by collision could be an efficient mechanism for the Cold Kuiper Belt but clearly it does not mean it is the right mechanism.
DeleteA better resolution in the dynamical simulation codes will probably help to clarify the issue. Also the new observations of extrasolar Kuiper belts could serve to settle the question.
It's the first time I've heard about the double binary star system. I know nothing about it.