HEIC0720: EMBARGOED UNTIL 10:00 (CET)/04:00 AM EST 11 December, 2007
http://www.spacetelescope.org/news/html/heic0720.html

News release: Hazy red sunset on extrasolar planet

11-December 2007 The NASA/ESA Hubble Space Telescope has given 
astronomers a fascinating new insight into the atmosphere of a planet 
in orbit around another star. The observations provide evidence 
of the presence of hazes in the atmosphere of the planet HD 189733b. 
 
A team of astronomers have used the NASA/ESA Hubble Space Telescope to 
detect, for the first time, strong evidence of hazes in the atmosphere 
of a planet orbiting a distant star. The discovery comes after 
extensive observations made recently with Hubble’s Advanced Camera for 
Surveys (ACS).  
 
The team, led by Frédéric Pont from the Geneva University Observatory 
in Switzerland, used Hubble’s ACS to make the first detection of hazes 
in the atmosphere of the giant planet. “One of the long-term goals of 
studying extrasolar planets is to measure the atmosphere of an Earth-
like planet, this present result is a step in this direction" says 
Pont."HD 189733b is the first extrasolar planet for which we are 
piecing together a complete idea of what it really looks like."

The new observations were made as the extrasolar planet, dubbed HD 
189733b, passed in front of its parent star in a transit. As the light 
from the star passes through the atmosphere around the limb of the 
giant extrasolar planet, the gases in the atmosphere stamp their unique 
signature on the starlight from HD 189733. 

The planet itself, orbiting close to its parent star, is a ‘hot-
Jupiter’ type of gas giant slightly larger than Jupiter. The proximity 
to its star results in an atmospheric temperature of roughly seven 
hundred degrees Celsius. Measurements of the way light varies as the 
planet passes in front of its parent star indicates that HD 189733b has 
neither Earth-sized moons nor any discernable Saturn-like ring system. 
 
Hubble’s ACS camera, coupled with a grism (a kind of cross between a 
prism and a diffraction grating) allowed the astronomers to make 
extremely accurate measurements of the spectrum of HD 189733b, allowing 
conclusions to be drawn about the composition of the planet’s 
atmosphere. The exquisite level of precision needed to make this 
observation can only, at the moment, be achieved from space. The 
combination of a large planet and relatively small parent star – only 
76% of the diameter of our Sun – contributes to the success of this 
delicate experiment. 
 
Where the scientists had expected to see the fingerprints of sodium, 
potassium and water there were none. This finding, combined with the 
distinct shape of the planet’s spectrum, infers that high level hazes 
(with an altitude range of roughly 1000 km) are present. So the 
atmosphere on HD 189733b would look very similar to a gorgeous red 
sunset over Athens! Venus and Saturn’s moon Titan, in our own Solar 
System, are also covered with haze. According to the scientists the 
haze probably consists of tiny particles (less than 1/1000 mm in size) 
of condensates of iron, silicates and aluminium oxide dust (the 
compound on Earth which the mineral sapphire is made of).

As part of the observations of HD 189733, the teams of astronomers also 
needed to accurately account for the variations in the star’s 
brightness during the set of observations. ‘Starspots’ like those seen 
on our own Sun may cover several percent of the star and are thought to 
be about 1000 degrees Celsius cooler than the rest of HD 189733’s 
surface. It was found that there is a starspot on the star’s surface 
which is over 80,000 km across. 

# # #

Notes for editors
The Hubble Space Telescope is a project of international cooperation 
between ESA and NASA.

The Space Telescope European Coordinating Facility in Garching near 
Munich, Germany is responsible for Hubble’s grism mode.

Image credit: ESA, NASA and Frederic Pont (Geneva University 
Observatory) 

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For more information, please contact:

Frederic Pont
Geneva University Observatory, Switzerland 
Cellular: +41-78-623-9654
Email: Frederic.Pont@obs.unige.ch
 
Lars Lindberg Christensen
Hubble/ESA, Garching, Germany
Tel: +49-(0)89-3200-6306
Cellular: +49-(0)173-3872-621
E-mail: lars@eso.org

Ray Villard
Space Telescope Science Institute, Baltimore, USA
Tel: +1-410-338-4514
E-mail: villard@stsci.edu