ALMA reveals how planets form around stellar twins

ALMA reveals how planets form around stellar twins

Read time: 3 minutes

Our Sun is a single star, but many other stars in our Milky Way galaxy belong to binary systems – two suns orbiting each other. In the past, astronomers believed that binary stars could not have planets. More recently, however, quite a number of planets orbiting binary stars have been discovered. New observations by ALMA shed light on their formation.

Young stars are surrounded by disks of gas and dust. Dust particles in the disk stick together and slowly grow into pebbles, boulders, asteroids and, finally, planets. It was thought that this could never happen in the disk surrounding a binary star. The reason: the orbiting stars would cause gravitational disturbances in the disk. The resulting turbulence would prevent dust particles to stick together.

However, using NASA’s Kepler space telescope, astronomers have discovered a number of exoplanets in orbit around binary stars. Apparently, they do form. The question is: how? The ALMA observations may eventually provide the answer.

ALMA studied the broad ring of gas and dust surrounding the binary star HD 142527 in the Scorpius-Centaurus association – a large cluster of young stars at a distance of 450 light-years. The ring contains a large, arc-shaped region where the gas appears to be missing.

For some reason, gas molecules in this region have ‘frozen out’, just like molecules of water vapor in the Earth’s atmosphere can freeze into ice crystals on the windshield of a car during a winter night. The gas freezes out onto dust particles, coating them with a very thin layer of ice – at least, that’s what astronomers think is happening.

Dust particles stick together more easily when they are coated with snow or ice – that’s the reason why it is easy to make snowballs. This means that the first steps toward the formation of planets around binary star HD 142527 may occur in the arc-shaped region in the disk.

In the distant future, full-fledged planets may orbit the binary star. Imagine living on one of those planets – you would see two suns in the daytime sky, and you would witness double sunrises and sunsets!

HD 142527 is a new-born binary star. The primary star of the binary is a bit more massive than our own sun; the companion star weighs in at only a third of the mass of the Sun. The two stars orbit each other at a mutual distance of some 1.5 billion kilometers – a bit larger than the distance between the Sun and the planet Saturn. The gas-poor, arc-shaped region in the disk of HD 142527 lies at a much larger distance of some 7.5 billion kilometers – fifty times the distance between the Sun and the Earth.

The arc-shaped region in the disk of binary star HD142527 was discovered by a team of astronomers led by Andrea Isella or Rice University in Houston, Texas, and including Yann Boehler. Andrea presented the new results at the 2016 annual meeting of the American Association for the Advancement of Sciences (AAAS) in Washington, D.C., while Yann will be the first author of an upcoming article about the results in a professional journal.