William B. Hubbard é professor de ciências planetárias no Lunar and Planetary Laboratory da Universidade do Arizona. Escreveu um capítulo inteiro para o New Solar System sobre os interiores dos planetas gigantes, onde diz que Júpiter tem núcleo. Na resposta parece que tem mais dúvidas do que no livro.

"Standard models for the formation of Jupiter require the presence of
a rock-ice core of about 10 Earth masses (ME) to trigger collapse of
nebular hydrogen-helium gas. For solar composition a planet the mass
of Jupiter would include about 5 ME of rock and ice in any case.

The basic problem is that these numbers are only at the level of 1 to
3 percent of the mass of Jupiter, and our uncertainties in the equation
of state of hydrogen are at least as large as this. So it is very
difficult to demonstrate directly from available data on Jupiter that
it has a core. The moment of inertia of Jupiter would not change a
great deal with or without a 10-ME core.

The main evidence that Jupiter must have a 10-ME core is from the
model for its origin. A great deal of effort is being directed at
present to designing geophysical experiments which will give a direct
detection of the core, because the current situation is so uncertain".