Durante toda a vida aprendi que Júpiter tinha núcleo. Basicamente por duas razões. Uma é o momento de inércia do planeta que nos diz se há ou não material denso concentrado no centro do planeta. E no caso de Júpiter diz que sim. A outra é o modelo de formação do planeta que nos diz que se formou por acreção a partir de um núcleo de gelo e rocha.

Mas há dias que ando numa polémica com o meu amigo Carlos Oliveira sobre este assunto. Um artigo na Space.Com semeou a dúvida. Neste artigo, Alan Boss lança dúvidas sobre a possibilidade de Júpiter ter núcleo, pois este homem é defensor de uma teoria diferente da acreção para a formação dos planetas, onde o núcleo de Júpiter não encaixa muito bem.

Para tirar as minhas dúvidas fiz uma coisa que qualquer divulgador atento deve fazer que é escrever 20 mails a quem sabe do assunto e ver o que eles dizem. Começo pelo John A. Wood do Smithsonian Astrophysical Observatory. Este homem tem um livro interessante sobre o sistema solar (The Solar System da Prentice Hall). E sobre o momento de inércia deu-me uma boa lição. É que apesar do momento de inércia ser um bom indicador não nos diz realmente se existe ou não um núcleo diferenciado.

Hi José,

"A "core" is a central planetary region that is distinctly denser than the
material lying above it. When you descend into the core you cross a sharp
boundary and take a step up in density.

The moment-of-inertia factor I cited means there is denser material at the
core of Jupiter than at smaller depths, but it cannot distinguish between a
core with a well-defined boundary, where there is a sharp change in density,
and a planet where the density simply increases gradually toward the center.
Both models can give you the same moment-of-inertia factor.

The sharp-boundary model has always been more or less assumed, and is cited
in my book, but Alan Boss is saying maybe it's the other way-- density
increases gradually and there is no distinct core boundary. By hindsight,
maybe I should have mentioned that possibility".

John Wood

Perguntei-lhe depois até que ponto a teoria do Alan Boss tinha adeptos entre a comunidade. Disse que acha a teoria simpática embora seja uma teoria com poucos adeptos.

"You have to go back to why there are these two models. It's because there
are two models for the formation of Jupiter. They are what is driving the
debate about a Jupiter structure model.

See p. 153 of my (2000) book. Here a model is described in which the core of
Jupiter accreted first, of solid materials; then when the core was massive
enough, it "grabbed" gas from the solar nebula. This model predicts in a
pretty natural way that Jupiter still has a core that is distinctly more
dense than the overlying material. I think most people accept this model of
Jupiter origin, and therefore the distinct-core structure for the planet.

On p. 154, an alternative model is given for the origin of Jupiter: it
formed all at once, from more or less homogeneous nebular material, by
gravitational instability. If that happened, settling-out might have
produced a core with a distinct boundary, but it probably would not have,
leaving Jupiter with an interior that was dense only because pressure has
packed it down, and without a sharp core boundary.

Alan Boss has always (well, for 10 years or more) been the most vocal
advocate of this second formation model. In arguing that Jupiter might have
no core boundary, he is simply stating a consequence of his formation model,
and being consistent with what he has always said. I think his is a minority
opinion, but I personally like it."

Uma simpatia este homem.