New discoveries

Triple star stirs up its own protoplanetary disk

Read time: 3 minutes

If a single star has planets, the planets usually orbit the star in a single plane. Such is the case in our
own solar system: the orbits of the planets are ‘aligned’, as astronomers say.

If a close binary star has planets, the situation is different. The two stars can orbit each other in one
plane, but the planets can move around the binary in another plane. The two planes can be tilted
with respect to each other – they are misaligned.

Now can you image what happens in the case of a triple star? Astronomers got their first clue when
they studied the triple star GW Orionis using ALMA. GW Orionis is a young triple star, surrounded by
a huge disk of gas and dust. In the future, the gas and dust in this disk may clump together into
planets.

It turns out that the disk of GW Orionis is very special. First of all, it is huge: the outer edge of the disk
is located at some 50 billion kilometers from the star. Next, the disk consists of three separate rings.
But most remarkably, the inner ring is warped, and tilted with respect to the outer two – it is strongly
misaligned.

Observations with other telescopes have revealed that the orbits of the three stars – in the very
center of the disk – are also misaligned.

According to one team of astronomers, the misaligned rings must have formed by the gravitational
effect of one or more massive planets. If they are right, GW Orionis would be the first triple star
known to have a planetary system.

Another team believes that the system of misaligned rings is the result of the varying gravitational
pull of the three stars in the center.

In any case, the ALMA observations of GW Orionis reveal that triple stars may end up with a system
of planets in very unusual orbits, very much unlike our own solar system.

What?

GW Orionis is a young triple star in the constellation Orion, at a distance of 1,330 light-years. It probably formed less than a million years ago. Two of the three stars orbit each other every 240 days at a distance of some 150 million kilometers – the distance between the Sun and the Earth. The third star is orbiting this binary every 11.5 years, at 1.2 billion kilometers, in a tilted orbit. Much farther out are the three rings of the triple star’s huge protoplanetary disk. The rings are located at 7, 28, and 50 billion kilometers from the central triple star. The new ALMA observations show that the inner ring is strongly misaligned with the outer two rings.

Who?

GW Orionis was observed with ALMA by two different teams of astronomers. The first team, led by
Stefan Kraus from the University of Exeter (United Kingdom) published their results in Science
magazine. Stefan and his colleagues believe that the weird properties of the protoplanetary disk of
GW Orionis can be explained by the varying gravitational pull of the three stars. The second team, led by Jiaqing Bi of the University of Victoria (Canada), published their findings in The Astrophysical
Journal Letters. They believe that the observed misalignment can only be explained if at least one
massive planet has already formed in the disk.

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