Exoplanet reflected light detected for the first time (Forwarded)

ETH Zurich
(Swiss Federal Inst. of Technology Zurich)
Zurich, Switzerland

Contact:
Professor Svetlana Berdyugina
ETH Institute of Astronomy
Tel. +41 (0)76 325 41 43

26.12.2007

Exoplanet reflected light detected for the first time

Polarization technique focuses limelight

Zurich -- The ability to explore remote worlds in space has been enhanced
through a polarization technique that allows the first ever detection of
light reflected by extrasolar (exoplanet) planets. The study has been
accepted for publication in Astrophysical Journal Letters.

An international team of astronomers, led by Professor Svetlana Berdyugina
of ETH Zurich's Institute of Astronomy, has for the first time ever been
able to detect and monitor the visible light that is scattered in the
atmosphere of an exoplanet. Employing techniques similar to how Polaroid
sunglasses filter away reflected sunlight to reduce glare, the team of
scientists were able to extract polarized light to enhance the faint
reflected starlight 'glare' from an exoplanet. As a result, the scientists
could infer the size of its swollen atmosphere. They also directly traced
the orbit of the planet, a feat of visualization not possible using indirect
methods.

Hot Jupiter

The transiting exoplanet under study circles the dwarf star HD189733 in the
constellation Vulpecula and lies more than 60 light years from the earth.
Known as HD189733b, this exoplanet was discovered two years ago via Doppler
spectroscopy. HD189733b is so close to its parent star that its atmosphere
expands from the heat. Until now, astronomers have never seen light
reflected from an exoplanet, although they have deduced from other
observations that HD189733b probably resembles a 'hot Jupiter' -- a planet
orbiting extremely closely to its parent star. Unlike Jupiter, however,
HD189733b orbits its star in a couple of days rather than the 12 years it
takes Jupiter to make one orbit of the sun.

Two half-moon phases

The international team, consisting of Svetlana Berdyugina, Dominique Fluri
(ETH Zurich), Andrei Berdyugin and Vilppu Piirola (Tuorla Observatory,
Finland), used the 60cm KVA telescope by remote control. The telescope,
which belongs to the Royal Swedish Academy of Science, is located at La
Palma, Spain and was modernised by scientists in Finland. The researchers
obtained polarimetric measurements of the 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.

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 suggested 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.

"The polarimetric detection of the reflected light from exoplanets opens new
and vast opportunities for exploring physical conditions in their
atmospheres", Professor Svetlana Berdyugina said. "In addition, more can be
learned about radii and true masses, and thus the densities of
non-transiting planets."

Bibliography:

Svetlana V. Berdyugina, Andrei V. Berdyugin, Dominique M. Fluri, Vilppu
Piirola: First detection of polarized scattered light from an exoplanetary
atmosphere, Astrophys. J. Lett., online publication 24. December 2007.
http://www.journals.uchicago.edu/toc/apjl/0/0