A Tale of Two Stars

To us, with our one lonely star, binary systems seem foreign and strange—like rare diamonds in the sky. But in reality, a vast number of solar systems contain more than one sun. Imagining what the sky would look like on planets in such systems is a favorite pastime of artist and astronomer alike.

But how can such worlds form to begin with?

Indeed, astronomers have long struggled to comprehend how planets are able to form around binary stars. The gravitational pull of two massive stellar bodies would likely eject young planets from the system, or drag them into one of the star's fiery infernos.

But observational evidence has shown that planets are indeed able to form (and maintain stable orbits) around binary stars.

Astronomers recently had a new, more detailed look at planetary formation around a binary star at a very early stage, and this may give us some answers to the aforementioned questions.

Ultimately, the latest view was achieved with the help of the Atacama Large Millimeter/submillimeter Array (ALMA) at the Atacama desert in northern Chile. The researchers discovered a crescent-shaped region of dust without gas in the outer reaches of a double star’s protoplanetary disk. These findings provide fresh insights into a binary system’s potential to form planets and how such events unfold.

A composite image of the HD 142527 binary star system from data captured by the Atacama Large Millimeter/submillimeter Array
Protoplanetary Disks

To study this phenomenon, astronomers took a more detailed look at binary star HD 142527 and the planet-forming disk around it. The binary system is 450 light-years away from Earth in a cluster of known as Scorpius-Centaurus Association.

The HD 142527 system consists of a large star and a smaller companion star that is separated by 1.6 billion km (1 billion miles), which is about the distance between Saturn and the Sun.

Astronomer Andrea Isella, of Rice University in Houston, notes that scientists were previously well aware of the disk surrounding the star, and that the latest image reveals important insights: "this binary system has long been known to harbor a planet-forming corona of dust and gas. 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 press release details what was seen: "ALMA's new, high-resolution images of HD 142527 show a broad elliptical ring around the double star. The disk begins incredibly far from the central star -- about 50 times the Sun-Earth distance. Most of it consists of gases, including two forms of carbon monoxide (13CO and C180), but there is a noticeable dearth of gases within a huge arc of dust that extends nearly a third of the way around the star system."

Astronomers hope to better understand the nature of planet formation across the Universe by studying a wider range of protoplanetary disks with ALMA. But as previously noted, some information has already come to light.

"The temperature is so low that the gas turns into ice and sticks to the grains," Isella said. "This process is thought to increase the capacity for dust grains to stick together, making it a strong catalyst for the formation of planetesimals, and, down the line, of planets."

The new images of HD 142527 show a crescent-shaped dust cloud that possibly results from gravitational forces that are unique to binary stars.

"There are between a few hundred and a few thousands we can look at again with ALMA to find new and surprising details. That's the beauty of ALMA. Every time you get new data, it's like opening a present. You don't know what's inside" said Isella.

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