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Comet rings are combining to form new planets

Narrow dense rings of comets are coming together to form planets on the outskirts of at least three distant solar systems, astronomers say.

Estimating from the amount of light the rings reflect, each developing planet is at least the size of a few Earths, says Carey Lisse, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory.

Fomalhaut star system debris disk
The debris disk in the Fomalhaut star system. The ring is approximately 20 billion kilometers (12.4 billion miles) from the central star and about 2 billion kilometers (1.2 billion miles) wide. The central dot is the unresolved emission from the star, which is about twice the mass of our sun.
(Credit: M. MacGregor/ALMA (ESO/NAOJ/NRAO))

“The narrow confines of these rings is still a great puzzle—you don’t typically see this kind of tight order in such a young system. 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.”

Over the past few decades, powerful NASA observatories such as the Infrared Telescope Facility in Hawaii and the Spitzer Space Telescope have found young debris disk systems with thin but bright outer rings.

Comet-like bodies make up the rings, which are far from their parent stars, about two to seven times the distance of Pluto from our own sun. The rings vary from ice-rich (seen in the Fomalhaut and HD 32297 systems) to ice-depleted but carbon rich (the HR 4796A system).

 The tight structure of the rings in the three systems means that multiple coalescing “mini-planets” are “shepherding” material through the rings.

Scientists are especially intrigued by the red dust ring surrounding HR 4796A, which shows unusually tight form for an infant solar system, Lisse says. He attributes the extreme red color to the burnt-out rocky organic remains of comets, a result of the system’s ring being close enough to the star that their ice has boiled off.

Researchers don’t see red ring dust in Fomalhaut or HD 32297, but a normal bluish comet dust containing ices instead—because these systems’ rings are far enough out that their comets are cold and mostly stable.

After eliminating other possibilities, researchers say the tight structure of the rings in the three systems means that multiple coalescing “mini-planets” are “shepherding” material through the rings.

Usually, comets crashing down onto a growing planet surface would kick up huge clouds of fast-moving, ejected “construction dust,” which would spread over the system in huge clouds, Lisse says.

Since that isn’t happening in these systems, “the only apparent solution…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.”

In other words, instead of one big construction site, the emerging planets are coming from many small ones, which will eventually merge into the final product.

“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.”

Recent studies have yielded similar theories about the formation of gas giants Uranus and Neptune, that each had multiple “subcores” that were eventually covered by thick atmospheres.

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 their cores, 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.

Aging stars could host Earth-like planets in outer regions

“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. This is very much like the predicted recipe for the super-Earths seen in abundance in the Kepler planet survey.”

“Much still has to happen, though, before these rings could become planets the size of the gas giants,” he says. “Why it’s taking so long to make outer planets in these systems—after their primordial gas disks have been stripped away—is a big mystery.”

Lisse recently presented his findings at an American Astronomical Society Division for Planetary Sciences meeting.

Source: Johns Hopkins University

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