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Orion Hosts an Oddly Askew Protoplanetary System With Three Stars

​A cloud of dust and gas whirling around an infant star system 1,300 light-years away is so unique that researchers say it cannot be likened to anything they’ve seen so far. It has three rings, wrapped around three stars, and the rings all have different orientations, with the innermost extremely misaligned from the other two.

It is the first direct proof that such imbalance, also known as ‘disc tearing,’ and predicted in modeling, can take place in the cosmos. However, as per the observations taken by the Atacama Large Millimeter-submillimeter Array (ALMA), which the most detailed data of the system yet, it is still not clear how the disc tearing event happened.

GW Orionis​ and Its Mysterious Birth

The system, dubbed GW Orionis, is located approximately 1,300 light-years away from Earth in the constellation of Orion. It has two stars, placed in orbit around each other at a distance of about one astronomical unit (au), with a third star orbiting the pair on an irregular orbit at a distance of eight au.

Around the three stars, the massive protoplanetary cloud of dust and gas whips, with the rings at distances of 46, 185, and 340 au from the core of the system. The outer ring is the most massive seen yet in a protoplanetary system.

Protoplanetary disks, as the name implies, are the material from which planets take shape around a star. Firstly, the star has to form and grow in a stellar nursery, then a knot of material in a protostellar cloud gravitationally crashes and begins to spin. This reels a massive disk of gas and dust that accumulates into the expanding star.

When this shaping process is complete, the remaining material in the disk starts to clump together and ultimately forms planets and other minor cosmic objects. That is why, in a planetary system like our Solar System, the planets and rock belts are somehow aligned along a flat plane, orbiting the star’s equator.

However, around systems of multiple stars, the planetary plane is usually misaligned with the orbits of the stars. Analyzing the protoplanetary disks around multiple star systems can help researchers learn more about the way this misalignment takes place. The odd misalignment in the protoplanetary disk in GW Orionis was first spotted in ALMA datasets in 2017.

ALMA observation (left) and VLT (right). [Image Credit: ALMA (ESO/NAOJ/NRAO), ESO/Exeter/Kraus et al.]
“We were surprised to see the strong misalignment of the inner ring,” said astronomer Jiaqing Bi of the University of Victoria in Canada. “But the strange warp in the disk is confirmed by a twisted pattern that ALMA measured in the gas of the disk.”

A second team of astronomers also analyzed the observations, using both ALMA and the datasets collected by the European Southern Observatory’s Very Large Telescope.

“In our images, we see the shadow of the inner ring on the outer disk,” said astronomer Stefan Kraus of the University of Exeter in the U.K. “At the same time, ALMA allowed us to measure the precise shape of the ring that casts the shadow. Combining this information allows us to derive the three-dimensional orientation of the misaligned ring and of the warped disk surface.”

Gravitation is Not the Only Culprit

Fortunately, even though the misalignment was only spotted recently, GW Orionis​ has been observed since 2008, and the third star in the system was found in 2011. That gave the scientists a few years of data from which to reconstruct the orbits of the system.

Using 3D computer simulations of the system, Kraus and his colleagues discovered that the opposing gravitational influences of the stars, together with various planets, were able to create the pronounced disk tearing seen in GW Orionis​.

But Bi and his team found that the gravitational effect of the stars’ orbital double-dealing is not sufficient on its own to result in the observed rings.

“Our simulations show that the gravitational pull from the triple stars alone cannot explain the observed large misalignment. We think that the presence of a planet between these rings is needed to explain why the disk was torn apart,” said astronomer Nienke van der Marel of the University of Victoria. “This planet has likely carved a dust gap and broken the disk at the location of the current inner and outer rings.”

If there was such a planet, it would be the first one researchers would have ever discovered orbiting three stars – but it is not yet time to make such a claim. More observations of the system are currently on the way in an attempt to try and solve this interesting mystery.

A paper detailing the research has been published in The Astrophysical Journal Letters and Science.

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