De onde vem a população de baixas inclinações do cinturão de Kuiper?

(Review: Gomes 2009 CMDA 104, 39-51)

A região do cinturão de Kuiper (40-50 AU) precisava ser 'vazia' para que Netuno não migrasse até 50 AU (Gomes et al. 2004).

Existe um mecanismo de transporte da população de altas inclinações de um disco truncado em 30 AU até o cinturão de Kuiper atual (Gomes 2003).

Não haveria também um mecanismo de transporte da população de baixas inclinações?

Um mecanismo pré modelo de NiceLevison & Morbidelli (2003) Nature 426 419

Diferentemente do modelo de produção da população de altas inclinações, este mecanismo não funciona para o modelo de Nice, já que exige uma migração suave por um trecho longo.

Transporte da população de baixas inclinações pelo modelo de Nice

Levison et al. 2008 Icarus 196, 258

Variação das excentricidades de objetos espalhados por Netuno devido à sua alta excentricidade

Exemplo semelhante vindo de uma simulação do modelo de Nice

Simulação 'mista': Netuno excêntrico e objetos espalhados vindos de uma integração original; circularização de Netuno artificial com partículas colocadas com massa zero.

O mesmo para outro conjunto de condições iniciais

Principal problema com este mecanismo: excentricidades ainda muito altas: mediana entre 0.1 e 0.13 dependendo das condições iniciais; mediana das excentricidades para as órbitas reais: 0.07.

Controvérsia sobre origem da população de baixas inclinações

Gomes 2009 CMDA 104, 39-51

L1. The initial Kuiper conjecture: a ‘real’ Kuiper belt should display low inclination/ low eccentricity orbits as the cold population does.

L2. The fact that it is really a different population from the hot one (by their different sizes / colors). A totally different origin for cold and hot population is naturally consistent with a local origin for the local population.

Controvérsia T1. As Gomes et al. (2004) showed, it is difficult to keep both an original

massive enough Kuiper belt at its present location and keep Neptune at 30 AU at the time of planetary migration.

T2. The outer border of the Kuiper belt suggests the transporting scenario. For the local formation scenario the border must be original, thus it could be virtually anywhere. For the transporting scenario it must be near the 1:2 resonance with Neptune (Levison et al. 2008a). So the fact that the outer border of the KB is located where the transporting scenario (which is consistent with other important features of the KB) predicts and where it would just by chance be located by the local formation scenario surely gives extra support to the transporting scenario.

T3. The fact that both the cold and hot populations have about the same mass argues against a local origin for the cold population. If the hot population was originated by the evaders mechanism and thus had a low mass from the beginning, it must be considered a coincidence that the cold population was eroded from a much larger mass to about the same mass of the hot population. Also consider Petit and Mousis (2004) that strengthens the case for a dynamical cold population implantation.

Mais argumentos para local

As to point L1, it must be noted that the eccentricities of the cold population are in average a little too high for an undisturbed primordial population. But the excitation of the eccentricities could be achieved by any migration scenario, including the Nice model. In this case, the temporary passage of Neptune’s aphelion at the Kuiper region could do the job. As to point L2, it must be noted that the Nice model (Levison et al. 2008a) naturally brings the inner portion of the planetesimal disk as a hot population and the outer portion as a cold population. This might account for the differences in sizes and colors between both populations. The correlations are not, however, quite nicely established as in the case of the locally formed cold population.

Mis argumentos para transporte

As to point T1, one can counter-argue with the idea that when Neptune approached its present position, the Kuiper region was already sufficiently depleted so as not to induce any further outer displacement for Neptune. This may need a vigorous mass erosion through collisions, which may be difficult to accept, unless two hypothesis are assumed: (1) the Nice model, that would give time for the erosion in the Kuiper belt to take place before Neptune reached 30AU; (2) a size distribution of objects concentrated in smaller ones, to make the grinding process efficient enough to deplete the Kuiper region of mass so as to stop Neptune when it reached the KB inner border (see Kenyon et al. 2008, The Solar System beyond Neptune, pp. 293–313.). This size distribution would have to be quite different from the one in the inner portion on the planetesimal disk, where a more evolved size distribution must be assumed so as to create big enough bodies to populate the Oort cloud and the trans-Neptunian region.