As we get better at spying worlds orbiting other stars, we find there's an increasingly diverse menagerie of exoplanets out there. But despite their differences, all worlds are thought to be built the same way.
The long-standing planetary formation theory assumes the protoplanetary dust and gas surrounding baby stars begins to clump together, and as these clumps grow in size, they collapse under gravity to form the worlds we know and love.
This is known as "core accretion." The more material in the protoplanetary disk, the larger the planets. Lots of material = massive gas giants (like Jupiter and Saturn). Little material = small terrestrial, rocky worlds (like Earth, Mars and Venus). Simple.
However, this "bottom-up" core accretion theory has been challenged (or at least modified) by a new paper that appeared in August's Monthly Notices of the Royal Astronomical Society.
According to Seung-Hoon Cha and Sergei Nayakshin of the University of Leicester, UK, for small rocky worlds - such as the numerous "super-Earths" the likes of which the Kepler space telescope is currently turning up - perhaps they were once much larger worlds with a decidedly gas giant-like flavor.
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."
Now that we are finding dozens of "super-Earths," some of which are knocking around the habitable zones of stars - the region surrounding a star with a temperature "just right" for liquid water to exist on their surfaces - this theory may help us identify how life-giving worlds like our own are formed in the first place.
Image: About to get naked? A young gas giant exoplanet drifting toward its parent star. Credit: NASA/ JPL-Caltech via Astrobiology Magazine