First detection of polarized, scattered light from an exoplanetary atmosphere

An international team of astronomers, led by Svetlana Berdyugina (Project Scientist at Tuorla Observatory and Professor of Astronomy at ETH, Zurich) has detected for the first time and monitored the light scattered in the atmosphere of an exoplanet. Similarly to the way polaroid sunglasses filter away reflected sunlight to reduce glare, the scientists used polarization effects to enhance the faint reflected starlight "glare" from an extrasolar planet. This allowed them to trace directly the orbit of the planet and infer the size of its swollen atmosphere, in contrast to all other exoplanets detected by various indirect methods.

The exoplanet circles a red dwarf star HD189733 in the constellation Vulpecula and lies about 60 light years from the Earth. The planet, known as HD189733b, was discovered two years ago via Doppler spectroscopy and transits. It is so close to its parent star that its atmosphere expands from the heat. Astronomers have, until now, never seen light reflected from an exoplanet although they deduce from other indirect observations that this one probably resembles a "hot Jupiter". Unlike Jupiter, it orbits its star in a couple of days rather than the 12 years it takes Jupiter to make one orbit of the Sun.

The orbit of HD189733b as projected on the sky. Solid and dashed lines indicate parts of the orbit in front of and behind the sky plane, respectively. The orange circle indicates the host star, HD189733, while the smaller blue circle shows the exoplanet itself, seen in transit across the face of the stellar disk The direction of the orbital motion is indicated by the arrows. Two possible orbits can be reconstructed from the observations, as shown in the two separate figures.

The orbit of HD189733b shown as an animation, indicating how the polarisation of the source changes as the planet circles the host star, the red dwarf HD189733.

The team, consisting of Svetlana Berdyugina, Andrei Berdyugin and Vilppu Piirola (Tuorla Observatory) and Dominique Fluri (Institut für Astronomie, Swiss Federal Institute of Technology, Zurich), used the remotely controlled 60cm KVA telescope on La Palma, Spain, to obtain polarimetric measurements of the star and its planet. They discovered two polarization maxima near the point when the star and planet are at maximum apparent separation.

The polarization maxima indicate that the scattering atmosphere is considerably (>30%) larger than the opaque body of the planet and most probably consists of particles smaller than half a micron, such as atoms, molecules, perhaps water vapor which was recently suggested for this planet, or even tiny dust grains. Such particles effectively scatter the light in the blue - in exactly the same way as in the Earth's atmosphere. The researchers are also able for the first time to recover the shape and orientation of the planet's orbit as seen on the sky plane, i.e. obtain an "image" of the orbit.

The polarimetric detection of the reflected light from exoplanets opens new vast opportunities for exploring physical conditions in their atmospheres as well as for determining radii and true masses, and hence densities, of non-transiting planets.

The study has been accepted for publication in Astrophysical Journal Letters . Further information can be obtained from Svetlana Berdyugina at +41-76-3254143 (Switzerland) or +358-44-2788525 (Finland) or email at sveta@astro.phys.ethz.ch