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Headlines Published on 29 January 2008

Title European scientists focus on exoplanets

Light reflected from planets is polarised. This basic property has enabled scientists to observe exoplanets (also called extra-solar), which are found outside the realm of our own solar system. For the first time ever, a team of European astronomers has been able to detect and monitor the visible light that is scattered in the atmosphere of an exoplanet. Researchers used a technique similar to that employed by Polaroid sunglasses for filtering out reflected sunlight to reduce glare.

The polarisation technique allows scientists to study the atmospheres of exoplanets. © Shutterstock
The polarisation technique allows scientists to study the atmospheres of exoplanets.
©  Shutterstock

The team extracted polarised light to enhance the faint reflected starlight "glare" from an exoplanet and to estimate the size of its distended atmosphere. They also directly traced its orbit, a feat of visualisation which would not have been possible using indirect methods. The exoplanet studied circles the dwarf star HD189733 in the constellation Vulpecula, more than 60 light years from earth.

The exoplanet was discovered two years ago using Doppler spectroscopy and is known as HD189733b. It lies so close to its parent star that its atmosphere expands from the heat. Until now, scientists had never witnessed light reflected from an exoplanet, although they had calculated from other observations that HD189733b would resemble a "hot" Jupiter. However, unlike Jupiter, HD189733b travels around its star every two days, compared to Jupiterís 12-year orbit around the sun.

Project leader Professor Svetlana Berdyugina from ETH Zurich (the Swiss Federal Institute of Technology Zurich), says: 'The polarimetric detection of the reflected light from exoplanets opens new and vast opportunities for exploring physical conditions in their atmospheres. In addition, more can be learned about radii and true masses, and thus the densities of non-transiting planets.'

The research team comprised ETH Zurichís Institute of Astronomy and the Tuorla Observatory, Finland, who used the Royal Swedish Academyís 60cm KVA telescope located at La Palma, Spain. The telescope was modernised by scientists in Finland before being used to obtain polarimetric measurements of the star and its accompanying planet. Astronomers found that polarisation peaks near the point where half of the planet is illuminated by the star as viewed from the earth. This occurs twice during the orbit, like half-moon phases.

The polarisation indicates that the scattering atmosphere is considerably larger (>30%) than the opaque body of the planet viewed during transits. The atmosphere is believed to consist of particles smaller than half a micron, for example atoms, molecules, tiny grains of dust or possibly water vapour. These particles effectively scatter blue light in the same way that the blue sky of the earthís atmosphere is created. For the first time, the scientists were also able to determine the orientation of the planetís orbit and trace its motion in the sky.

More information:

  • ETH Zurich, Institute of Astronomy
  • Royal Swedish Academy
  • Born from dust and gas

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