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First reflected light from an exoplanet

European astronomers have for the first time ever been able to detect and monitor the visible light that is scattered in the atmosphere of an exoplanet, HD 189733b, which is also known as a 'hot Jupiter,' and orbits a star slightly cooler and less massive than the Sun located about 60 light-years from Earth. According to this ETH Zurich news release, 'Polarization technique focuses limelight,' the researchers used 'techniques similar to how Polaroid sunglasses filter away reflected sunlight to reduce glare. They also directly traced the orbit of the planet, a feat of visualization not possible using indirect methods.' The team thinks that their findings are opening new opportunities for exploring physical conditions on exoplanets.

European astronomers have for the first time ever been able to detect and monitor the visible light that is scattered in the atmosphere of an exoplanet, HD 189733b, which is also known as a 'hot Jupiter,' and orbits a star slightly cooler and less massive than the Sun located about 60 light-years from Earth. According to this ETH Zurich news release, 'Polarization technique focuses limelight,' the researchers used 'techniques similar to how Polaroid sunglasses filter away reflected sunlight to reduce glare. They also directly traced the orbit of the planet, a feat of visualization not possible using indirect methods.' The team thinks that their findings are opening new opportunities for exploring physical conditions on exoplanets.

An artist's conception of HD 189733b

The team focused on HD 189733b, an exoplanet discovered in 2005. Before going further, here is "an artist's conception of HD 189733b, which some have dubbed the 'Bulls-eye' planet because of the bright 'hot spot' shown here." (Credit: David A. Aguilar, Harvard-Smithsonian Center for Astrophysics (CfA)) You'll find this picture and others in an image gallery provided by the CfA to accompany a press release dated from May 9, 2007, "First Map of an Extrasolar Planet."

The international team was led by Svetlana Berdyugina of the ETH Zurich Institute of Astronomy. She was helped by Dominique Fluri, also from ETH Zurich, and by two other researchers from the Tuorla Observatory in Finland, Andrei Berdyugin and Vilppu Piirola.

According to the ETHZ news release, these researchers used the remotely controlled 60cm KVA telescope belonging to the Royal Swedish Academy of Science and located at La Palma, Spain to obtain polarimetric measurements of the HD 189733 star and its planet. "They discovered that polarization peaks near the moments when half of the planet is illuminated by the star as seen from the earth. Such events occur twice during the orbit, similar to half-moon phases."

What conclusions did they draw from these measurements? "The polarization indicates that the scattering atmosphere is considerably larger (>30%) than the opaque body of the planet seen during transits and most probably consists of particles smaller than half a micron, for example atoms, molecules, tiny dust grains or perhaps water vapour, which was recently sug-gested to be present in the atmosphere. Such particles effectively scatter blue light -- in exactly the same scattering process that creates the blue sky of the earth’s atmosphere. The scientists were also able for the first time to recover the orientation of the planet’s orbit and trace its motion in the sky."

You'll find more details by reading two pages on Svetlana Berdyugina's website, "Polarimetric studies of extrasolar planets" and "First reflected light from an extrasolar planet." On this last page, the team states that their "our findings open new vast opportunities for exploring physical conditions on exoplanets as well as for determining radii and true masses, and hence densities, of non-transiting planets."

This research work will be published in The Astrophysical Journal Letters in its January 20, 2008 issue under the title "First detection of polarized scattered light from an exoplanetary atmosphere." But the article is already available online. Here are two links to the abstract and to the full paper (PDF format, 4 pages, 132 KB).

Here is the beginning of the abstract. "We report the first direct detection of an exoplanet in the visible polarized light. The transiting planet HD 189733b is one of the very hot Jupiters with shortest periods and, thus, smallest orbits, which makes them ideal candidates for polarimetric detections. We obtained polarimetric measurements of HD 189733 in the B band well distributed over the orbital period and detected two polarization maxima near planetary elongations."

Sources: ETH Zurich/Swiss Federal Institute of Technology news release, December 26, 2007; and various websites

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