How big are the rocky/icy cores of Jovian planets? The size of Earth? Or bigger? I'd think they'd have to be pretty big to attract all that gas when the planets were being formed.

edit I'm writing a short story and the setting consists of late eighties/early nineties type of technology and knowledge of astronomy, so a rough estimate as an answer is fine
10-30 Earth masses is an estimate from this abstract, but 12-45 is a more accurate answer.