A Search for Spiral-Arm-Driving Planets

MWC 758: Best-fit and 3σ lower limit (solid lines) of where the major-arm-driving planets should be based on the measured pattern speed, plotted over the SPHERE data. The 1σ limit (dashed line), which is interpolated from the best-fit and the 3σ limit, is presented for the purpose of illustration and it may not represent a true 68% likelihood. The Astrophysical Journal

Planets form in gaseous and dusty protoplanetary disks around young stars that are a few million years old. Forming planets gravitationally interact with the host disk, producing structures such as gaps, spiral arms, and vortices (Kley & Nelson 2012). By comparing observations with theoretical models, spatially resolved disk structures may yield rich information about the properties of embedded planets, such as their orbits, and dynamical constraints on their masses. 

In the past decade, near-infrared imaging of disks with high spatial resolution has discovered spiral arms at tens of astronomical units in a few systems, Now This image, captured with the Very Large Telescope SPHERE adaptive optics in Chile, reveals the large-scale spiral arms visible in the MWC 758 protoplanetary disk, located less than 500 light-years away. 

The r²-scaled NICMOS, SPHERE, and NIRC2 (from left to right) observations of MWC 758 in Cartesian (top) and polar (bottom) coordinates with total flux normalized to unity. The gray dotted circles and lines mark the inner working angles. The Astrophysical Journal

Such arms are thought to be triggered by one of two mechanisms: gravitational instability, or a companion orbiting within the disk. A team of scientists led by Bin Ren (The Johns Hopkins University) has recently used observations of these arms over a decade-long baseline to track the speed of rotation of the arms. 

Since companion-driven arms corotate with their drivers, this exercise which could reveal the location of a planetary-mass, unseen companion that drives the arms. Ren and collaborators find that the most likely location for such a planet to orbit in this disk is at 89 AU, just outside of the visible spiral arms. For more information, check out the article doi:10.3847/2041-8213/aab7f5

Image of MWC 758 from ALMA (Atacama Large Millimeter/submillimeter Array). Credit: ESO/R. Dong et al.; ALMA (ESO/NAOJ/NRAO)

Sources: The Astrophysical Journal, Wikipedia, ESO,