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Evidence for a Hidden Planet in IM Lupi’s Disk

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very large telescope image of the protoplanetary disk around IM Lupi
This image from the European Southern Observatory's Very Large Telescope shows the dusty disk surrounding the young star IM Lupi, the location of which is marked with a gray circle. [ESO/H. Avenhaus et al./DARTT-S collaboration]

The disk surrounding the star IM Lupi has come into clearer focus in the past few years thanks to new observations that revealed spirals, kinks, and other interesting structures. Could a hidden planet be the cause of all these features?

A Detailed Disk

radio image of the disk around IM Lupi

IM Lupi as seen at a wavelength of 1.25 mm. The white bar in the lower right-hand corner is a 10-au scale bar. [Adapted from Andrews et al. 2018]

IM Lupi is a young star with an intriguing protoplanetary disk. Observations of IM Lupi’s disk over the past few years have found that the disk doesn’t rotate evenly; there are more than a dozen “kinks” where the gas moves at a different rate than what we would expect for a smoothly rotating disk. In addition, a spiral pattern is imprinted upon the disk’s upper surface.

Previous research has suggested that these features could signal that IM Lupi’s expansive disk hides a massive planet orbiting the star at a distance of 117 au. In a new publication, a team led by Harrison Verrios (Monash University, Australia) puts that theory to the test.

On a Hunt for Planets

Verrios and collaborators used hydrodynamic modeling to understand how the presence of a planet would affect IM Lupi’s disk. In addition to modeling a planet-less disk as a control case, the team investigated the effects of a planet with a mass 2, 3, 5, or 7 times the mass of Jupiter orbiting the central star at a distance of 100–120 au.

Observed and modeled polarized intensity maps

Observed (left) and modeled (right) polarized intensity maps. The model shows the results for a planet with a mass of 2 Jupiter masses. Click to enlarge. [Verrios et al. 2022]

In order to compare against observations, the team generated images from their hydrodynamic models. Specifically, they modeled the emission from the disk at wavelengths of 1.25 millimeters (which traces the warm dust) and 1.6 microns (which shows the polarized light scattered off the disk). The team found that by including a planet in their simulations, they could reproduce all of the observed velocity kinks as well as the distinctive spiral pattern on the disk’s surface. The team’s models also predicted that the wake created by the planet’s motion should be visible in velocity maps, and observations match this prediction closely. Overall, the authors found that a 2–3-Jupiter-mass planet orbiting at a distance of roughly 110 au produced the best match to the observations.

Disk Disturbances Demystified

Comparison of observed and modeled velocity maps

Comparison of observed (top row) and modeled (bottom row) 1.25-mm images and velocity maps. The Δv at the bottom of each panel denotes the difference from the rest velocity of the 12CO J=2–1 transition. Click to enlarge. [Adapted from Verrios et al. 2022]

Does this study rule out the possibility that the structures in IM Lupi’s disk have a different cause, such as a gravitational instability? Protoplanetary disks are notorious for having features that mimic the signals of planets, and more work is needed to confirm the presence of a planet tucked away in IM Lupi’s disk — but the authors’ simulations suggest that a planet can explain all the intriguing features of the disk without invoking another cause.

Previously, researchers theorized that a planet could only disturb its disk in a small region immediately surrounding the planet, which would suggest that the more widespread disturbances in IM Lupi’s disk must have another cause. However, Verrios and coauthors found that widespread features popped up in their simulations, suggesting that planets can have more far-reaching effects than predicted.

Citation

“Kinematic Evidence for an Embedded Planet in the IM Lupi Disk,” Harrison J. Verrios et al 2022 ApJL 934 L11. doi:10.3847/2041-8213/ac7f44