Misaligned protoplanetary disks in a young binary star system

Observations show that one or both of the protoplanetary disks in the young binary system HK Tauri are significantly inclined to the binary orbital plane, demonstrating that the necessary conditions exist for the misalignment-driven mechanisms thought to produce the unusual orbits of some extrasolar planets. Genesis of eccentric exoplanet orbits Solar System planets all have nearly coplanar, circular orbits. So it has been surprising to find that many extrasolar planets follow very elliptical orbits, or orbits that tilt at an angle from the host star's equator. With a view to establishing a mechanism that could explain how such orbits arise, Eric Jensen and Rachel Akeson studied the young binary system HK Tauri. They obtained images of planet-forming disks misaligned by 60° or more, such that one or both of the disks are significantly inclined to the binary orbital plane. The results demonstrate that the necessary conditions exist for planetary orbits to be modified by misalignment-driven mechanisms, and that these conditions are present at the time of planet formation, possibly a result of the binary formation process. Many extrasolar planets follow orbits that differ from the nearly coplanar and circular orbits found in our Solar System; their orbits may be eccentric 1 or inclined with respect to the host star’s equator 2 , 3 , and the population of giant planets orbiting close to their host stars suggests appreciable orbital migration 4 . There is at present no consensus on what produces such orbits. Theoretical explanations often invoke interactions with a binary companion star in an orbit that is inclined relative to the planet’s orbital plane 4 , 5 . Such mechanisms require significant mutual inclinations between the planetary and binary star orbital planes. The protoplanetary disks in a few young binaries are misaligned 6 , 7 , 8 , 9 , 10 , 11 , 12 , but often the measurements of these misalignments are sensitive only to a small portion of the inner disk, and the three-dimensional misalignment of the bulk of the planet-forming disk mass has hitherto not been determined. Here we report that the protoplanetary disks in the young binary system HK Tauri are misaligned by 60 to 68 degrees, such that one or both of the disks are significantly inclined to the binary orbital plane. Our results demonstrate that the necessary conditions exist for misalignment-driven mechanisms to modify planetary orbits, and that these conditions are present at the time of pl