Polarisation: From exploding stars to quiet planets

Understanding of the biggest and the smallest

Polarimetric measurements allow astronomers to learn much more about astronomical objects than only their brightness. In regular telescope only measures the intensity of an object in a specified wavelength range. However, if we would be able to measure the polarisation of the light simultaneously, we can look at characteristics of astronomical object that otherwise would be hard (or impossible) to measure.

• Massive stars end their lives with a powerful and luminous explosion that forms what is known as a supernova. Since 1970, astronomers have been trying to measure the polarisation of supernovae. This can be used to determine the intricate details of the explosion. For example, by measuring the polarisation of the emitted light, asymmetries in the shape of the expansion of the stellar remnant can be detected.


• In 2002 a team of astronomers detected polarisation of the oldest light in the universe: the cosmic microwave background. Theoretically, this polarisation was expected to be present according to the theory of inflation: the unimaginably large expansion of the size of the universe during a tiny fraction of a second after the Big Bang. By studying the polarisation of this background, astronomers are trying to understand more about the very first seconds of our universe.


• Polarimetry allows us to measure light coming from very faint sources. Black holes, the most massive objects in the universe, do not emit light. However, in 2019 a team of researchers did the unthinkable: image the shadow of a black hole in galaxy M87. This could only be done by combining many radio telescopes all over the globe into a world-wide network. Short after, in 2021, the team discovered that a significant fraction of the light in this image is polarised. This allows astronomers to map the structure of the material around the black hole.


• 2021 was a fruitful year for polarimetry. Yet another team of astronomers managed to measure polarised light from a disk of dust and gas around a gaseous exoplanet using the SPHERE instrument. These measurements eventually provide understanding of the formation of the planet and possible moons. In the future, polarisation measurements of rocky planets allow us to obtain more information regarding their atmospheres and surfaces to eventually look for possible signs of life.

How do we actually measure the polarisation of light? • Let’s have a quick recap on the observing technique behind polarimetry. To identify polarized light, we use filters that only passes through light with a particular direction of polarisation; so-called polarisers. Just to be clear: whenever we say that light behind a polariser becomes polarised, there is no physical change in the polarisation of the light. The light obtained a preferred polarisation direction because of filtering out other directions. As a direct consequence, unpolarised light passing through a polariser only leaves 50% of the intensity of the incident light.

Instruments that measure the polarisation of light use two polarisers. The incoming light is equally split into two beams. One passes through a horizontal polariser as the other goes through a vertical polariser, producing two images. By subtracting one image from the other, the remaining unpolarised light is cancelled from the resulting image. This helps astronomers to search for the polarised scattered reflection of exoplanets since the resulting image removes the unpolarised glare of the starlight.