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.
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.