Do planets gradually grow around a rocky core, or can gas giants form spontaneously from lumps in the disc?

ECONOMIST: Astronomers may have stumbled on the birth of a solar system

Despite the fact that it likely contains water and a rocky core, that water is almost certainly in the form of steam or a supercritical fluid.

FORBES: NASA Detects The Light Of A 'Super-Earth'

Dr Marcy suspects that his new discovery, which has about 45 times the mass of the Earth, is composed of a rocky core surrounded by a thick envelope of hydrogen and helium gas.

ECONOMIST: Extrasolar planets

During this violent time, much of the debris would have resembled comets - dirty snowballs thought to be mostly slushy water surrounding a rocky core - slamming into Earth at velocities greater than 25 km per second (16 miles per second).

BBC: Comets could have seeded life on Earth