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Actual Planet Formation around Binary Stars

ALMA Images Shed More Light on Planet Formation around Binary Stars

Using the Atacama Large Millimeter/submillimeter Array (ALMA) radio telescope in Chile, an astronomer from the United States has taken a detailed look at the very early stages of planet formation around a nearby binary star.

An artist impression of HD 142527 based on ALMA data. According to astronomers, HD 142527 consists of a main star a little more than twice the mass of our Sun and a smaller companion star only about a third the mass of our Sun; they are separated by approximately one billion miles. The rendition shows a distinctive arc of dust (red) embedded in the protoplanetary disk. 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. The two dots in the center represent the two stars in the system. Image credit: B. Saxton / NRAO / AUI / NSF.

Astronomers struggle to understand how planets form in double star systems.

Early models suggested that the gravitational tug-of-war between two stellar bodies would send young planets into eccentric orbits, possibly ejecting them completely from their home system or sending them crashing into their stars.

Observational evidence, however, reveals that planets do indeed form and maintain surprisingly stable orbits around binaries.

To better understand how such systems form and evolve, Rice University astronomer Dr Andrea Isella took a new look at the planet-forming disk around a binary star system known as HD 142527.

According to the scientist, HD 142527 lies in the constellation Lupus at a distance of about 457 light-years from Earth.

The system is a member of the Scorpius-Centaurus association, a cluster of young stars containing objects similar to HL Tau, the subject of the first high-resolution images taken as part of ALMA’s long-baseline campaign in 2014.

Images of HL Tau revealed ring structures in the dust and gas cloud around the star, an indication that planet formation is under way. ALMA images of HD 142527 show a broad ring around the double star.

This composite image of HD 142527 shows a distinctive arc of dust (red) and a ring of carbon monoxide (blue and green). Image credit: Andrea Isella / Rice University / B. Saxton / NRAO / AUI / NSF / ALMA / ESO / NAOJ.

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, but there is a noticeable dearth of gases within a huge arc of dust that extends nearly a third of the way around system.

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

“The crescent-shaped dust cloud may be the result of gravitational forces unique to binaries and may also be the key to the formation of planets.”

“We’ve been studying protoplanetary disks for at least 20 years,” he 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.”

Dr. Isella reported his results February 13 at the AAAS meeting in Washington, D.C.

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