Planet Spotted Forming Around Binary Stars

An artist's impression of how HD 142527 might look in visible light. The reddish area has no carbon monoxide gas because it has frozen onto dust particles, assisting the process of them sticking together. B. Saxton (NRAO/AUI/NSF)

A crescent-shaped gap has been observed in the gas around a pair of stars. The astronomers who discovered it believe the missing material is coalescing into a planet, providing an unprecedented opportunity to understand how planet formation happens.

Science fiction loves the idea of planets with two suns, from Tatooine, to Magrathea. Until recently, however, it was thought that such planets would be rare, existing only where the stars were so close they could be circled like a single object, or so distant that one would be just a bright star.

Yet we now know that planets exist in a wide range of binary systems. New work presented at the American Association for the Advancement of Science meeting on Saturday may help explain how this occurs. The study was conducted with the Atacama Large Millimeter/submillimeter Array (ALMA), the collection of radio telescopes that is revolutionizing many areas of astronomy.

"This binary system has long been known to harbor a planet-forming corona of dust and gas," said Dr. Andrea Isella of Rice University in a statement. "The new ALMA images reveal previously unseen details about the physical processes that regulate the formation of planets around this and perhaps many other binary systems."

The system Isella is referring to, HD 142527, is 450 light-years away and is part of the Scorpius-Centaurus association of young stars that formed together but are now drifting apart. These stars have proven a rich resource for astronomers studying the process of planetary formation.

Composite image of the HD 142527 system showing dust (red) and carbon monoxide (blue and green). Gas in the arc has frozen onto dust particles, potentially assisting planetary formation. The central dots represent the stars' locations, but not their relative brightnesses. Andrea Isella/Rice University; B. Saxton (NRAO/AUI/NSF); ALMA (NRAO/ESO/NAOJ)

Where most protoplanetary disks are symmetric, HD 142527 has a crescent-shaped dust cloud, thought to be caused by the complex gravitational field of two stars in orbit around each other. The primary star has more than twice the Sun's mass. The secondary, which orbits at a distance greater than Saturn's from the Sun, has a third the mass of the Sun, making it hundreds of times fainter. Planets orbiting either star may be disrupted by the other's pull.

The lack of gasses within the reddish arc is probably the result of carbon monoxide – the dominant gas elsewhere in the system – freezing onto dust grains.

"The temperature is so low that the gas turns into ice and sticks to the grains," Isella said in another statement. "This is important for planet formation. The solid dust needs to stick together to form a bigger body that will eventually attract more rock and gas gravitationally.

"If you try to smash rocks together, they don't stick together very well," he said. "If you smash snowballs together, they do. So when you form an ice mantle around the grains, you increase their capability to stick together."

"Where the red in the image is brightest, the density of the dust peaks," Isella said. "And where we find a dense clump of dust, the carbon monoxide molecules disappear."

ALMA has previously spotted what may be a planet in the formation of a triple star system, but this is a far clearer example of planetary formation around multiple suns.

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