Because they are so distant, planets outside our solar system cannot be distinguished optically from their host stars, even with the best telescopes. An international research team led by Svetlana Berdyugina, Professor at the ETH Zurich Institute of Astronomy, has now made the first ever successful observation of the visible light scattered by an extra-solar planet. The results of their work were published on 20.12.2007 in the on-line edition of the scientific journal ‘Astrophysical Journal Letters’.

Until now the determination of what are known as exoplanets was possible in most cases only by indirect means: for example the light of a star is examined and is found to vary slightly. From this it is concluded that there must be a planet with a corresponding attractive force present nearby (the so-called Doppler spectroscopy method). However, the research team has now used the polarisation of reflected light to track down the exoplanet directly.

Hot Jupiter

The exoplanet being studied is in the constellation of the fox, and its distance from the Earth is about 60 light-years. It orbits the dwarf star HD189733 in such a way that – when seen from the earth – it passes in front of the star during every orbit. Astronomers call this event a transit. The exoplanet, whose name is HD189733b because of its proximity to the host star, was discovered two years ago. It was already known from other observations that HD189733b is a “hot Jupiter”, i.e. a gas planet orbiting extremely close to the host star, which causes its atmosphere to expand because of the great heat. In contrast to the planet Jupiter, which orbits the sun once every 12 years, the planet HD189733b circles its host star in slightly more than two days.

Polarisation as a new method

The research team consisting of Svetlana Berdyugina, Dominique Fluri (ETH Zurich), Andrei Berdyugin and Vilppu Piirola (Tuorla Observatory, Turku University, Finland) used the remotely controlled 60-centimetre telescope on La Palma (Spain), which belongs to the Royal Swedish Academy of Sciences and was modernised by Finnish scientists. The researchers made polarimetric measurements of the star and its planet. As a rule reflected light is polarised light. By measuring the polarised light, the team of scientists extracted the weak light reflected by the exoplanet from the much brighter starlight, similar to the way Polaroid sun-glasses filter out dazzling reflected sunlight. This allowed the scientists to determine not only the size of the exoplanet and its atmosphere, but also to describe its orbit directly.

Two half-moon phases

The intensity of polarisation depends on the respective scattering angle. The light is most strongly polarised at an angle of 90 degrees. The researchers discovered that the polarisation reaches a maximum when the planet is about half illuminated as seen from the earth – analogous to half-moon phases – which occurs twice during each orbit.

The measured polarisation reveals that the atmosphere is about 30 percent larger than the opaque component of the planet which was observed during the transits. It probably consists of particles measuring less than half a micron, for example atoms and molecules, or perhaps even of water molecules or small particles of dust. Svetlana Berdyugina says “The polarimetric discovery of reflected light from exoplanets opens up new and far-reaching opportunities to research the physical properties of their atmospheres. In addition we can learn more about the radii and masses and thus about the densities of planets, even in the case of other exoplanets without transits.”