• Scientists using NASA telescopes have spotted narrow dense rings of comets coming together to form massive planets on the outskirts of at least three distant solar systems.
• 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. 
• 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).
• The scientists are especially intrigued by the red dust ring surrounding HR 4796A, which shows unusually tight form for an infant solar system.
• Lisse traced the extreme red colour to the burnt-out rocky organic remains of comets, a result of the system’s ring being close enough to the star that they have all boiled off.
• Comets crashing down onto these growing planet surfaces would kick up huge clouds of fast-moving, ejected ‘construction dust,’ which would spread over the system in huge cloud.
• The only apparent solution to these issues is that multiple mini-planets are coalescing in these rings, and these small bodies, with low kick-up velocities, are shepherding the rings into narrow structures – much in the same way many of the narrow rings of Saturn are focused and sharpened.

Comets in Distant Solar Systems

• 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.
• However, this is 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.

Source:Science Daily & Wiki