The alternate planetary formation theory, called "tidal downsizing," works like this: Once a large gas giant planet forms out of the clumpiness of a star's protoplanetary disk, through the drag caused by the remaining dust and gas surrounding the star, it begins to get pulled toward the star. At a certain distance from the star, the star's tidal forces strip the gasses from the gas giant's atmosphere. Eventually, the solid, rocky core of the gas giant is the only thing left.
The gas giant, in effect, has been stripped naked.
Interestingly, this "top-down" theory may explain how smaller planetary bodies, that formed at the furthest-most reaches of star systems, drift into orbits closer to their stars. They were once massive gas giants (like Jupiter) that drifted too close to their stars and had their thick atmospheres sucked off - leaving only a rocky core and perhaps a thin atmospheric layer hugging the surface.
"I like to think of the mechanisms as opposites," Nayakshin said. "One is bottom up - core accretion - and the other is top down - tidal disruption."