Binary stars give planetary disks a twist

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.