spaceSpace and Physics

Comets Are Turning Into Planets In Three Star Systems


Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

clockOct 20 2017, 15:08 UTC

ALMA observations of the Fomalhaut system. ALMA (ESO/NAOJ/NRAO); M. MacGregor

Researchers have observed rings of comets in three different star systems and they think they are likely going to merge into single planets at some point in the future. The research is being presented at the American Astronomical Society’s Division for Planetary Sciences meeting.

Astronomers have observed well-defined rings in Fomalhaut, HD 32297, and HR 4796A. The first two have rings rich in ice while the latter is more ice-depleted and carbon-rich. In all three cases, the traditional view that early star systems are messy and chaotic is challenged.


These rings are well defined and structured, suggesting that multiple sizable objects are keeping them in check. The same phenomenon is observed in Saturn’s rings. So instead of having a single proto-planet slowly gaining mass, the researchers think that a swarm of minor objects are the best candidates to explain the observations.

“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,” research leader Carey Lisse, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory, said in a statement. “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.”

These rings are located between 11 and 30 billion kilometers (7 and 19 billion miles) from their parent star, which is between two and seven times the distance that Pluto is from the Sun. Each ring has enough material to make a few Earths and what they might be forming could be something akin to (the yet-to-be-found) Planet Nine.


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

Millions of comets might inhabit these rings. In all three systems, the stars have pushed the gasses away so the planets that are forming won’t look like Uranus and Neptune. And what might form in the carbon-rich system HR 4796A could be different still. The lack of ice suggests that only rocky fragments are left over in this disk.

Planetary formation remains mysterious and it could be that planets form in many different ways.

Gemini Planet Imager observations of the HR 4796A disk. Marshall Perrin / Space Telescope Science Institute, Gaspard Duchene / UC Berkeley, Max Millar-Blanchaer / University of Toronto, and the GPI Team

spaceSpace and Physics
  • tag
  • exoplanets,

  • planetary formation,

  • star system