Debris disks of comets are coming together to form exoplanets

Planets are forming from multiple sites instead of growing from just one site.
By Laurel Kornfeld | Oct 24, 2017
Debris disks composed of comets surrounding stars in at least three stellar systems are coming together to form large exoplanets, planetary scientist Carey Lisse of the Johns Hopkins University Applied Physics Laboratory (JHUAPL) reported at the American Astronomical Society's Division for Planetary Sciences meeting in Utah.

The narrow, dense rings were spotted in several star systems with NASA's Spitzer Space Telescope and the Infrared Telescope Facility in Hawaii.

Located between 75 and 200 astronomical units (or AU, with one AU equal to the average Earth-Sun distance, or 93 million miles) from their stars, the bright debris rings have a variety of compositions. Some are rich in ice, such as Formalhaut and HD 32297, while others, such as HR 4796A, are rich in carbon but have little ice.

HR 4796A's ring is unusually red, likely due to cometary remains. Because the ring is relatively close to the star, the comets likely burned away and left behind rocky organic materials.

No red dust is visible in the rings of Formalhaut or HD 32297. These rings are far enough from the star that comets within them remain intact, resulting in the rings being icy.

"The narrow confines of these rings is still a great puzzle--you don't typically see this in such a young system," Lisse said regarding the debris rings in these three systems.

"Usually, material is moving every which way before an exoplanetary system gets cleaned out and settles down so that planetary bodies rarely cross each other's path, like in our present-day solar system."

Based on the amount of light the ring systems reflect, the researchers were able to estimate their masses. Those estimates show the planets being formed are several times the size of the Earth.

Objects coming together within the debris rings appear to be "shepherding" other material through the rings.

"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 clouds. 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," Lisse explained.

Millions of comet cores are believed to be coming together to form the cores of ice giant planets in Formalhaut and in HD 32297, much like the cores of Uranus and Neptune.

However, the planets in these systems will not have atmospheres, as the systems lack the primordial gas that creates those atmospheres.

In addition to ice giants, these systems also appear to be building super-Earths composed of rock, ice, and organic materials, Lisse said.




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