Why do tilted/eccentric orbits form?

For all its mysteries, the Solar System is uniquely ideal in many ways. For one, while it has rocky inner planets and giant, gassy outer ones, astronomers have found that elsewhere, massive exoplanets often orbit close to their stars, as if they formed at a greater distance and then moved in. For another, the orbits of the planets around the Sun are nearly circular and on the same plane. This is unlike what the Kepler space telescope has found in other star systems, where exoplanets often have eccentric orbits and orbits that are tilted with respect to the star’s equator. A July 31 paper in Nature adds one more system to this difference, but this time with some information about why eccentric/tilted orbits could form. Eric Jensen (Swarthmore College) and Rachel Akeson (NASA) discuss the binary star system HK Tauri, whose stars are one and four million years old with 386 AU between them. More importantly, one star, HK Tau B, has a disk of gases and dust around itself that, when viewed from Earth, appears to be edge-on, masking the starlight and making for easier imaging in visible light. Such a disk is called a protoplanetary disk because it contains the materials from which planets form.

The other star, HK Tau A, is also inclined as seen from Earth but not edge-on, so the bright starlight bleaches observations made in visible light. So Jensen and Akeson observed it in millimeter-wavelength light using the ALMA telescope in Chile and calculated its disk’s rotation rate (specifically, by observing Doppler effects on the carbon-monoxide emission lines). HK Tau A is inclined at 43° ± 5°; HK Tau B is inclined at 85° ± 1°. Depending on which direction the disks are rotating in, Jensen & Akeson find that they’re inclined relative to each other at at least 60° and at most 68° (± 3° for both). So planets forming in their protoplanetary disks could end up with eccentric and tilted orbits around their individual stars as a result of being perturbed by gravitational effects from their neighbor. As Jensen told Centauri Dreams: “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. We can’t rule other theories out, but we can certainly rule in that a second star will do the job.”


Jensen, E. & Akeson, R., Misaligned protoplanetary disks in a young binary star system, Nature 511, 567–569 (31 July 2014). doi:10.1038/nature13521