Astronomers finally measure polarised light from exoplanet
An international team led by Leiden astronomers has, after years of searching and defying the boundaries of a telescope, for the first time directly captured polarised light from an exoplanet. From this light they can deduct that a disk of dust and gas orbits the exoplanet. In this disk moons are possibly forming.The researchers will soon publish their findings in the journal Astronomy & Astrophysics.
Heavyweight
The discovery concerns the exoplanet DH Tau b. This is a very young planet of only 2 million years old at 437 light years from Earth in the constellation Taurus. Exoplanet DH Tau b does not resemble our Earth. The planet is at least eleven times more massive than Jupiter, the most massive planet in our solar system. The planet is also located ten times further away from its star than our furthest planet Neptune. The planet is still glowing after its formation. As a result, it emits heat in the form of infrared radiation.
Dust disk
The researchers discovered that the infrared radiation of the planet is polarised. This means that the light waves vibrate in a preferential direction. And that, according to the researchers, is because the infrared radiation of the planet is scattered by a disk of dust and gas that orbits the planet. In such a disk, moons may form.
Furthermore, the disk around the planet appears to have a different orientation from the disk around the star. Such a tilted disk indicates that the planet has likely formed at a large distance from the star. This is contrary to the theory that planets are formed close to their star and then migrate outward.
Overwhelming light
For the observations, the astronomers used the SPHERE instrument on the Very Large Telescope of the European Southern Observatory (ESO) in Chile. This instrument can, among other things, block the overwhelming light of the associated star and determine the polarisation of the remaining light.
Unique insights
First author and research leader Rob van Holstein has been working with the SPHERE instrument since his university study in 2014: ‘Because we fully understood the instrument, we were able to make it perform better than it was designed for. In the end, we were able to capture the light from twenty exoplanets, one of which had polarised light.’
Co-author Frans Snik has been trying to capture polarised light from planets since 2012: ‘It's already very special that we can see a planet separated from the star around which it orbits. And now we can also deduce that material is orbiting this planet as well, and that this material does so at a completely different angle than the disk that orbits the star. This gives us unique insights into how such a planet and possible moons are formed.’
Extremely Large Telescope
In the future, the researchers aim to carry out similar research on the Extremely Large Telescope that is under construction. This telescope should make it possible to study the light of rocky, Earth-like planets. From the polarisation of the light it will be possible to obtain more information about the atmosphere of such planets and whether there are possible signs of life.
Dutch orginal news item from astronomie.nl
Scientific paper
A survey of the linear polarization of directly imaged exoplanets and brown dwarf companions with SPHERE-IRDIS – First polarimetric detections revealing disks around DH Tau B and GSC 6214-210 B. By: R.G. van Holstein et al. Accepted for publication in Astronomy & Astrophysics.