Light from the star HD 163296 passes through the surrounding gas-and-dust disk like a flashlight beam through murky water. The disk's particles scatter the light, giving it a special property — polarization: the waves begin to oscillate in a single plane, like a rope shaken strictly up and down.
If there are axion-like particles, lightweight dark matter candidates, in the space between us and the star, they could slightly rotate the plane of polarization, just as sugar syrup does in a well-known experiment. The astronomers checked archival near-infrared images of the disk from six different dates. The polarization angle barely changed: the scatter ranged from 1.6 to 6.4 degrees, which is within measurement errors.
This scatter is partly due to the disk being dense, so light is scattered multiple times inside it, smearing the original angle. For the first time, scientists have set an upper limit on the strength of the interaction between axion-like particles and photons using data from a protoplanetary disk: for a particle mass of about 10⁻²² eV, the coupling is weaker than 7.5 × 10⁻¹² GeV⁻¹. If in the future the polarization angle can be measured with an accuracy of 0.1 degrees, this method will become one of the most sensitive in the world.
The polarization of light from a protoplanetary disk changes not only because of dark matter: the disk itself scatters light so that the angle 'wanders' by several degrees. So searching for an axion signal is like listening for a whisper in a noisy café.