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This page was published on 04/01/2008
Published: 04/01/2008


Published: 4 January 2008  
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Watching a sunset, outside our solar system

The romance of watching the sun set has entranced millions around the world for eons. Imagine, however, watching the Sun set on a completely new planet. Once the realm of imagination and special effects, astronomers are doing just that. Apart from the beauty of such a spectacle, these observations are giving astronomers vital insight into the atmospheric conditions of distant planets. This allows them to determine the possibility of the existence of oxygen and water — and maybe more — on these planets.

NASA/ESA © An artist’s impression of the extrasolar planet HD 189733b, seen here with its parent star looming behind. The planet is slightly larger than our own solar system’s Jupiter. Its atmosphere is a scorching 800 °C.
An artist’s impression of the extrasolar planet HD 189733b, seen here with its parent star looming behind. The planet is slightly larger than our own solar system’s Jupiter. Its atmosphere is a scorching 800 °C.
The Hubble Space Telescope currently in orbit around the Earth is a project of international cooperation between the European Space Agency (ESA) and NASA. Since its launch in 1990, it has become one of the most important instruments in the history of astronomy. Now, a team of astronomers — led by Frederic Pont from the Geneva University Observatory in Switzerland — have used Hubble’s Advanced Camera for Surveys (ACS) to make the first detection of hazes in the atmosphere of a planet orbiting a distant star.

The planet, which is to be found outside our solar system, has been dubbed HD 189733b. 'HD 189733b is the first extrasolar planet for which we are piecing together a complete idea of what it really looks like' said Frederic Pont. '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.'

As the giant planet passed in front of its parent star, the team of astronomers were able to make careful observations and measurements of the atmospheric conditions. 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 theory behind this process is now quite commonly used, and had been formulated from Huygens to Newton. By measuring the refraction of light passing through a substance, scientists are able to determine what the substance is, based on the refraction's properties.

The astronomers utilised the ACS camera on the Hubble telescope and coupled it with a grism, a cross between a prism and a diffraction grating. This allowed them to make extremely accurate measurements of the spectrum of the planet.

What they found was surprising. The high level of haze they witnessed was reminiscent of a gorgeous red sunset over Athens, but without any traces of sodium, potassium or water, which they had expected. Instead they found condensates of iron, silicates and aluminium oxide dust (the compound on Earth constituting the mineral sapphire).

The level of precision needed to make this observation can currently only be achieved from the Hubble Telescope. This observation was also helped thanks to the fact that the large planet had a relatively small parent star, which is only 76 percent of the diameter of our Sun.

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The European homepage for the NASA/ESA Hubble Space Telescope

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