Three stars, warped rings may show how planets end up moving backward
If all we knew about were our own Solar System—a condition that was largely true even two decades ago—we’d think that planets are all neat and tidy. All of the familiar ones rotate in a single plane aligned with the Sun’s equator, and move in the same direction. And that’s exactly how they should behave, considering that planets form from a single disk of material rotating around the star.
But as we’ve gotten a clearer picture of the diversity of exosolar systems out there, we’ve seen some pretty odd things, like planets that orbit in the opposite direction than they should or planets with orbits that are nowhere near the plane of their system. While some of these idiosyncrasies can be explained by gravitational interactions in systems with multiple planets, there might be conditions where planets could form in bizarre orbits.
Now, researchers have imaged an exosolar system where that seems to be happening.
Make mine a triple
The star system in question is named GW Orionis, and it’s located in the star-forming region of Orion, about 1,250 light years from Earth. The system is young, still in the process of forming, and consists of three stars. Two of them, both somewhat larger than the Sun (2.5 and 1.4 times its mass), orbit each other closely at roughly the same distance as the Earth is from the Sun. A third star, also slightly larger than the Sun, orbits these two at a distance that’s roughly eight times the distance between the Earth and Sun.
The researchers have been observing the system for 11 years, which has allowed them to get fairly precise orbital information. And already here, things are a bit awkward. The outermost star’s orbit is inclined by 13° relative to the plane of the inner two stars’ orbit. That’s not especially surprising. Multistar systems form as a large cloud of gas and other materials fragments as it collapses. Rather than occurring in a neat plane, the collapse occurs in a turbulent, three-dimensional environment that often leads to off-center orbits.
But the orbits of the three stars had consequences for the disk of gas and dust that formed around them. This disk also showed up during the imaging campaign, and, as seen above, the results were fairly complex. Images of the disk reveal a complicated pattern of bright and dark patches surrounding the stars, along with at least three different rings of dense material within the disk.
Most of the new analysis involves the research team interpreting this pattern of bright and dark material, generating a three-dimensional model of the system. (Lots of the paper consists of sentences like, “To reproduce the on-sky projected shape of R3, its off-center position with respect to the stars and the shape of shadows S1 and S2, we adopted a nonzero eccentricity (e = 0.3 ± 0.1 for ring R3), with the stars located at one of the focal points of the ellipse.”) The end result is a physically plausible arrangement of the material surrounding the stars of GW Orionis.
In the model, the outermost rings orbit in a single plane, but the plane doesn’t align with the orbital plane of any of the stars. Perhaps more significantly, their orbits are retrograde, in that they orbit the stars in a different direction than the third star orbits the inner two. The third ring, in contrast, is oriented in the same plane as that of the stars. But its center isn’t the center of the stars’ orbits.
Physically, there should be a single disk of material surrounding all three stars. Instead, there seems to be a a large outer disk that’s consistent with this. But, closer to the center of the system, the disk is skewed by the off-axis orbits of the stars. There’s either a break between the outer and inner ring or, if the material is contiguous, it’s distorted as if the inner ring had been pushed up through a thin sheet of plastic.
If the model is correct, this represents the first clear case of what’s called “disk tearing,” in which the misalignment of the material and the stars creates forces that can break up the disk. Tearing can also lead to the formation of rings that undergo precession and start wobbling around the axis of their rotation.
The authors then took their model of the location of the material and ran it forward in time under the gravitational forces produced by the stars. They found that the system broke up into a series of separate disks, each orbiting the system in a different plane, and with the inner-most disk paving a large precession around the system that takes 8,000 years to cycle. This disk starts at about 40 times the Earth-Sun distance—roughly the same distance as the innermost ring visible in GW Orionis.
The one thing that doesn’t quite match up between the model and the actual system is that there shouldn’t be ring-like structures until material is orbiting in different planes. In this case, the two outer-most rings appear to still be in the same plane. In some cases, separations like this are produced by having planets form, which can clear material out of the disk and form gaps in it. But as of yet, there are no planets visible in the observations.
Rings to planets
But that’s not to say that planets couldn’t form in this complex gravitational environment. The authors estimate that there’s about 30 Earth masses’ worth of material in the inner-most ring, more than enough to form a planet.
How much might this explain some of the oddball exosolar systems we’ve seen? The authors point out that many of the exoplanets orbiting close to their host stars (about 40 percent of them) orbit in the wrong direction, orbit in an unusual plane, or both. The disk-tearing process definitely could explain this. But it’s also possible that many of the planets that end up that close to their star get there because of gravitational interactions with other planets. And those interactions could also explain the oddball nature of these orbits.
So, how much is explained by disk tearing remains an open question. Hopefully, thanks to these observations, we’ve got a better chance to spot instances of it. More examples would be great, and identifying them without 11 years of observations would be even better.
My Lesson Planning
via Ars Technica https://arstechnica.com
September 4, 2020 at 04:44AM