A weirdly warped planet-forming disk circles a distant trio of stars
Three rings of gas and dust circle a trio of stars in one of the most complicated cosmic dances yet observed by astronomers.
The star system GW Orionis is around 1,300 light-years away in the constellation Orion, and it has a pair of young stars trapped in a close do-si-do, with a third star making loops around both. A broken-up disk of dust and gas surrounds all three stars, where planets might one day form. Unlike our solar system’s flat disk, GW Orionis’ disk is made up of three loops, with a warped middle ring and an inner ring much more twisted at a jaunty angle to the other two.
The unusual geometry of this system, the first of its kind found, has been described in two recent studies by two groups of astronomers. But how GW Orionis came to be is a mystery, with the two teams offering different theories for the creation of the triple-star-and-ring system.
Astronomer Stefan Kraus of the University of Exeter in England and colleagues suggest that gravitational tugs and torques from the triple-star ballet tore apart and deformed the primordial disk in a paper published in Science. However, Jiaqing Bi of the University of Victoria in Canada and colleagues believe that a newborn planet is to blame in a research published in the Astrophysical Journal Letters.
“The question is how do you actually form such systems,” says theoretical physicist Giuseppe Lodato of the University of Milan, who was not on either team. “There could be different mechanisms that could do that.”
Astronomers have seen tilted disks of gas and dust around binary star systems, but not systems of more than two stars. Around half of the stars in the galaxy have at least one stellar partner, and their planets frequently have tilted orbits relative to their stars, resembling a jump rope rather than a Hula-Hoop. That misalignment might have been caused by the disk in which the planets were born: if the disk was just off, so would the planets.
Astronomers originally discovered that GW Orionis contains three stars and a planet-forming disk around a decade ago, and they rushed to get a closer look. (At the time, it was impossible to tell whether or not the disk was a single loop.) Bi’s team and Kraus’ team aimed the Atacama Large Millimeter/submillimeter Array in Chile at the triple-star system.
Both groups spotted the trio of stars: one around 2.5 times the mass of the sun and another about 1.4 times the mass of the sun orbiting each other once every 242 days, and another 1.4 solar mass star orbiting the inner pair every 11 years.
The studies also revealed three separate dust and gas rings surrounding the stars. The ring closest to the star trio is approximately 46 times the distance from Earth to the sun; the middle ring is around 185 times that distance; and the farthest ring is approximately 340 times that distance. Neptune is around 30 times the distance between Earth and the sun.
The researchers discovered that the innermost ring is substantially misaligned with respect to the other rings and stars. Kraus’ group added observations from the European Southern Observatory’s Very Large Telescope to show the shadow of the inner ring on the inside of the middle loop. The middle ring is twisted, swooping up on one side and down on the other, as revealed by the shadow.
Astronomers used the ALMA telescope array (left, blue) and the SPHERE instrument on the Very Large Telescope (right, red) in Chile to study GW Orionis. The ALMA observations revealed the disk’s tri-ringed structure, while the SPHERE photos revealed the innermost ring’s shadow, allowing scientists to characterize the rings’ deformed forms in detail. LEFT IMAGE: ALMA/ESO, NAOJ, NRAO; RIGHT IMAGE: ESO, S. KRAUS ET AL, UNIV. OF EXETER
Next, both groups ran computer simulations to figure out how the system formed. This is where their results diverge, according to Bi. His team believes a newly created, unknown planet cleared its orbit of gas and dust, separating the inner ring from the rest of the disk. The inner ring was free to swing around the stars once the disk was split, settling into its skewed alignment.
However, Kraus’ team discovered that the chaotic gravity from the triple stars’ orbital dance was enough to break up the disk, a process known as disk tearing. Each star tends to maintain the disk aligned with itself, and the tug-of-war warped and sheared the disk, as well as further twisted the inner ring. Theoretical studies have shown that disk tearing may occur in multiple star systems, but Kraus claims that this is the first time it has been observed in real life.
“I think it’s plausible that there could be planets somewhere in the system, but they’re not needed to explain the misalignment,” he says. “We don’t need to invoke undiscovered planets to explain what we see.”
New observations show that a trio of stars in GW Orionis are surrounded by a massive, twisted disk of gas and dust. Based on computer models and observational data, this animation shows the complex geometry of the distorted and broken-apart disk.
According to astronomer Nienke van der Marel, Bi’s colleague at the University of Victoria, the difference may be due to the assumptions that the groups made about the disk’s properties, particularly its viscosity. A more viscous disk would tear as Kraus and colleagues predict, but a less viscous disk, she argues, requires a planet to break apart. Based on studies of other star systems, she believes her team’s work is more feasible. But with current technology, there’s no way to tell what the properties of GW Orionis’ disk are really like.
And neither group could explain why the disk had broken into three pieces. “We don’t really know what’s causing the outer ring,” Klaus says.
Lodato, who predicted the disk-tearing effect in 2013, believes GW Orionis is confirmation that it exists. Back then, Lodato and colleagues were “very worried” that their simulations showed an effect that was introduced by the computations, not real physics, he says. “Now observations tell us that it does happen in reality.”
Van der Marel believes that future telescopes will be able to detect the planet if it exists.