Wide binary stars can have tilted disks

Wide binary stars can have tilted disks

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Imagine a bride and a groom, whirling around each other in a romantic dance. The guests at the wedding party are all walking around the couple in one direction, like cars on a roundabout. Both the dancing pair and their wedding guests are on the same floor. A scientist would say: they all move in the same plane.
In the universe, however, things can be different. If two stars whirl around each other (the dancing pair), you would expect that orbiting planets (the wedding guests) also orbit in the same plane. But according to new ALMA observations, that may not always be the case.
There are many binary stars in the universe: two stars orbiting each other. NASA’s Kepler space telescope has found that some of these binaries have a planetary system. If you would live on such a ‘Tatooine’ planet, you would see two suns in the sky instead of one! (Tatooine was the home world of Luke Skywalker in the Star Wars movies.)
The Tatooine planets found by Kepler all orbit so-called close binaries. In a close binary, the two stars are not very far apart, and revolve around each other in less than 40 days or so. Moreover, the planets all orbit in the same plane as the binary star, just like the wedding couple and their guests.
But ALMA made a surprising discovery. ALMA can’t see full-grown planets, but it can observe the disks of gas and dust around newborn stars from which planets will be born. In the case of close binary stars, ALMA found that these disks are indeed in the same plane as the central stellar pair, in good agreement with the Kepler observations. But for wider binaries, with orbital periods larger than a month or so, the disks are often not in the same plane.
If these ‘misaligned’ disks also will condense into planets in the future, those planets will also move in orbits that are tilted with respect to the orbit of the central binary. It’s as if the wedding guests still walk around the dancing bride and groom, but on one side of the room they climb a stair to the second floor, and on the other side they go down into the basement.
Astronomers more or less understand how planets form around single stars like our own sun. The new ALMA observations will now shed more light on the formation of planets around binary stars.


Most young stars are orbited by flat, rotating disks of gas and dust. In the future, the material in the disk will condense into planets. ALMA has observed many of these ‘proto-planetary’ disks in much detail. Some binary stars (two stars orbiting each other) also have protoplanetary disks. ALMA has now studied 19 of these ‘circumbinary’ disks, and looked at their orientation. For close binaries – two stars orbiting each other in less than a month or so – the disks lie in the same plane as the binary’s orbit. For wider binaries, however, the disks are often tilted: they show all kinds of orientations.


The ALMA observations of circumbinary disks were carried out by an American team of eight astronomers, led by Ian Czekala of the University of California at Berekeley. Ian and his colleagues have published their new results in an article in The Astrophysical Journal, a professional astronomy magazine.