Scientific Method —

Binary stars give planetary disks a twist

May also wrench planets out of the plane in which they form.

New measurements of the star system HK Tauri provide insight into the complicated environments that can govern the formation of exoplanets. Before scientists had the ability to study exosolar systems in detail, it was expected that other systems would look a lot like ours. The planets of our Solar System orbit in a plane that roughly corresponds to the Sun’s equator, occupying nearly circular orbits.

However, when exoplanets began to be discovered, that expectation of familiarity was shattered. Exoplanetary systems occupy all kinds of orbits, with inclination varying wildly. There is currently no consensus about what causes these planetary orbits to get so out of whack.

Planets form out of a disk of gas and dust, part of the same material that formed the star. Since the whole system, including the star, was essentially one spinning disk at one point in its evolution, planets should orbit roughly along the same plane as their host star’s equator. After all, that is what we observe in our own Solar System. But in exosolar systems, that’s not always the case.

There are theoretical models that can explain the misalignments. Many of these involve the presence of another star in the system (since most star systems are binaries). If the companion’s co-orbit with the first star is tilted with respect to the original plane of the gas and dust that formed planets, the companion star’s gravity could either alter the planet’s orbital inclination over time or skew the disk during planet formation.

As these models predict, protoplanetary disks have been observed to be misaligned in a few young binary star systems. But until now, such measurements have been mostly sensitive to the inner part of the disk—scientists haven’t been able to accurately measure the full, three-dimensional alignment of the bulk of the disk.

Using new data from the ALMA radio telescope, the scientists were able to more accurately measure two proto-planetary disks in the binary star system HK Tauri.

“Although there have been earlier observations indicating that this type of misaligned system existed, the new ALMA observations of HK Tauri show much more clearly what is really going on in one of these systems,” stated Rachel Akeson, one of the two scientists involved in the study.

The two stars in the HK Tauri system, HK Tauri A and B, are separated by about 386 astronomical units (AU). (For comparison, at the farthest point in its orbit, Pluto is a little under 50 AU from the Sun). Both stars have protoplanetary disks, but the disk on HK Tauri B has been easier to measure since it’s nearly edge-on from our point of view, scattering some of the star’s light.

HK Tauri A’s disk, however, is not scattering enough light to be observed at visible wavelengths because it is inclined at a dramatically different angle. This told us that the two disks have different inclinations, but without being able to see it, it wasn’t clear by how much the angles differed.

The new measurements were taken in the far infrared to extremely high frequency radio wave portion of the electromagnetic spectrum. This enabled scientists to study both disks in more detail.

The data shows a dramatic angle of ~60-68 degrees between the two disks. While the individual inclinations of the disks cannot be accurately determined yet, the angle between the two means that one or both of the disks are not aligned with the orbital plane of the stars.

This supports the idea that oddball orbits can be caused by perturbations from another star in the system. And it suggests that these perturbations may begin well before planets have formed. If it turns out that this is a common part of the binary formation process, it could account for many of the misaligned orbits seen in other exosolar systems. However, the results do not mean that this is the one-and-only cause of the misaligned orbits; only that it can occur.

“Our results show that the necessary conditions exist to modify planetary orbits and that these conditions are present at the time of planet formation, apparently due to the formation process of a binary star system,” explained Eric Jensen, the other scientist on the project. “We can’t rule other theories out, but we can certainly rule in that a second star will do the job."

While these results are suggestive, learning how many misaligned orbits are caused by this mechanism will require statistical studies of young binary systems. And it’s unlikely that it can account for all of them.

“Although understanding this mechanism is a big step forward, it can’t explain all of the weird orbits of extrasolar planets," says Jensen. “There just aren’t enough binary companions for this to be the whole answer. So that’s an interesting puzzle still to solve, too!”

Nature, 2014. DOI: 10.1038/nature13521  (About DOIs).

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