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takyon writes:

http://jhuapl.edu/newscenter/pressreleases/2017/171019.asp

Narrow dense rings of comets are coming together to form planets on the outskirts of at least three distant solar systems, astronomers have found in data from a pair of NASA telescopes.

Estimating the mass of these rings from the amount of light they reflect shows that each of these developing planets is at least the size of a few Earths, according to Carey Lisse, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland.

Over the past few decades, using powerful NASA observatories such as the Infrared Telescope Facility in Hawaii and the Spitzer Space Telescope, scientists have found a number of young debris disk systems with thin but bright outer rings composed of comet-like bodies at 75 to 200 astronomical units from their parent stars — about two to seven times the distance of Pluto from our own Sun. The composition of the material in these rings varies from ice-rich (seen in the Fomalhaut and HD 32297 systems) to ice-depleted but carbon rich (the HR 4796A system).

[...] In Fomalhaut and HD 32297, researchers expect that millions of comets are contributing to form the cores of ice giant planets like Uranus and Neptune — although without the thick atmospheres enveloping the cores of Uranus and Neptune, since the primordial gas disks that would form such atmospheres are gone. In HR 4796A, with its warmer dust ring, even the ices normally found in the rings' comets evaporated over the last million years or so, leaving behind core building blocks that are rich only in leftover carbon and rocky materials. "These systems appear to be building planets we don't see in our solar system — large multi-Earth mass ones with variable amounts of ice, rock and refractory organics," Lisse said. "This is very much like the predicted recipe for the super-Earths seen in abundance in the Kepler planet survey."

The supposed exoplanets could also be called "massive solid planets" or "mega-Earths".

Accretion of Uranus and Neptune from inward-migrating planetary embryos blocked by Jupiter and Saturn

Infrared Spectroscopy of HR 4796A's Bright Outer Cometary Ring + Tenuous Inner Hot Dust Cloud

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