Researchers have discovered how worlds form in two-sun systems

By using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, an international team of researchers has managed to see the earliest stages of planetary formation around a binary star and uncovered new insight into how worlds form in such two-sun systems.

Andrea Isella, an assistant professor of physics and astronomy at Rice University who presented the findings Saturday at the annual meeting of the American Association for the Advancement of Science (AAAS) in Washington DC, and his colleagues used the ALMA radio telescopes to take a closer look at HD 142527, a binary star located about 450 light-years from Earth.

This system, which is located in a young star cluster called the Scorpius-Centaurus Association, consists of a main star slightly more than twice as massive as our Sun, and a smaller companion roughly one-third the mass of our Sun. The two stars are separated by about one billion miles, or nearly the same distance separating the Sun and Saturn, the researchers said in a statement.

“This binary system has long been known to harbor a planet-forming corona of dust and gas,” explained Isella. “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.”

Frozen gases could help dust grains aggregate more easily

While studying HD 142527, Isella and his fellow astronomers discovered a striking crescent-shaped region of dust that is surprisingly devoid of gas in the outskirts of its protoplanetary disk. These disk typically contain the tiny grains of dust and small pockets of gas which are eventually pulled together by gravity to form increasingly larger agglomerations, and ultimately planets.

The details of this process are not well understood, which is why astronomers have been using ALMA to study a wide variety of protoplanetary disks all across the universe – including the one forming around HD 142527. High-resolution images of this binary system show that is possesses a broad elliptical ring that surrounds both stars.

A composite image of the HD 142527 binary star system from data captured by the Atacama Large Millimeter/submillimeter Array shows a distinctive arc of dust (red) and a ring of carbon monoxide (blue and green). The red arc is free of gas, suggesting the carbon monoxide has “frozen out,” forming a layer of frost on the dust grains in that region. Astronomers speculate this frost provides a boost to planet formation. Credit: Andrea Isella/Rice University; B. Saxton (NRAO/AUI/NSF); ALMA (NRAO/ESO/NAOJ)

This disk begins at a considerable distance from the central star – approximately 50 times further away than the Earth is from the Sun, the researchers said. It is comprised primarily of gases such as two forms of carbon monoxide (13CO and C180), but there is a significant lack of these gases in one large dust arc that extends nearly one-third of the way around the system. Isella speculated that this may be the result of gravitational forces that are unique to binary systems.

Furthermore, he and his colleagues believe that this could be the key to planet formation in two-sun systems. The absence of gases may be due to them freezing and forming ice on the grains of dust, which could increase the likelihood that the dust grains stick together and aggregate into a planetesimal, and eventually into a full-grown planet.

“We’ve been studying protoplanetary disks for at least 20 years,” the Rice professor said. “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.”

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