Darwin to Hunt for Life on Extrasolar Planets
British astronomers and engineers are playing an active role in the development of a new telescope which may revolutionise our ideas about whether life exists elsewhere in the Universe. The European Space Agency (ESA) has recently initiated preliminary studies of the Darwin Space Infrared Interferometer, a space telescope capable of finding Earth-like planets around nearby stars, and perhaps even detecting signs of life on them. It could also be used as a giant, 21st century space telescope, like the present Hubble, to look back to the time when galaxies began to form early in the history of the Universe.
A presentation about the Darwin proposal will be made at the UK's National Astronomy Meeting at the University of Southampton on Friday 11th April by Dr Alan Penny of the Rutherford Appleton Laboratory.
Detecting Life On Other Planets
With the discovery in the last two years of planets around nearby stars, astronomers are becoming increasingly active in searching for more of these objects. Those discovered already are the 'easy' ones, giant planets like Jupiter which orbit close into their parent stars. Future searches will look for smaller planets at all distances from their stars.
Darwin will make a unique contribution to this search since it will be able not only to look for planets as small as the Earth but also to study their atmospheres. The search would concentrate on 300 Sun-like stars, many of them visible with the naked eye, which lie within 50 light-years of Earth - next door in astronomical terms.
One exciting possibility would be the detection of large amounts of water in the atmosphere of an Earth-like planet, as this would indicate that the planet has oceans like the Earth. An even more exciting prospect would be the detection of considerable amounts of ozone. Such ozone could only be produced from plentiful oxygen, which in turn could only be generated by life. The detection of both water and ozone would mean that there was a significant amount of life presently alive on the planet. This life would be carbon-based oxygen-producing life, like the algae in the Earth's oceans or the vegetation on the Earth's continents.
To achieve this performance, Darwin would have to be a huge one hundred metres across, 40 times larger than the Hubble Space Telescope. It would consist of half-a-dozen small telescopes, 50 metres apart. Each of them would send its light to a central station in a technique known as interferometry. This method, already used by astronomers on the ground, would enable the small telescopes to combine the light they receive so that they mimic a single telescope 100 metres in diameter.
Darwin's six telescopes would either be joined together by long arms in a rigid structure, or would each be mounted on individual spacecraft. In the former case, the rigid structure would rotate to build up the image. In the latter case, the individual spacecraft would have their own rocket motors, and dance around each other to build up the image. It is only recently that space technology has progressed to the stage where such a large telescope is feasible.
Overcoming the problems
The problem with detecting extrasolar planets is that they are very faint and lie very close to their parent stars. Since stars shine a billion times brighter than the light reflected by a nearby planet, any worlds orbiting a star are lost in its glare. Darwin's large size and its location above the turbulence of our atmosphere are important factors in enabling it to separate planets and stars. In addition, the telescope will collect infrared light (heat) rather than visible light. This allows it to take advantage of the fact that a typical star is only a million times brighter than a planet at infrared wavelengths, making detection of a dim planet relatively easy.
Another unique aspect of the Darwin Project is that it has to be sent deep into the solar system, somewhere between Mars and Jupiter, where it would be four times further away from the Sun than the Earth. This is to avoid the dust floating around in the inner Solar System. Known as 'zodiacal dust', this debris can actually be seen with the naked eye in rural areas at sunset and sunrise. Seen in the infrared, the zodiacal light is bright enough to drown out the faint light from planets. However, the dust does not extend beyond Mars, so it is possible to escape from it by locating the telescope further from the Sun.
The idea of using such a telescope deep in the solar system to avoid the zodiacal dust is the brainchild of Dr Alain Leger of the Institut d'Astrophysique Spatiale in Paris. Dr Leger is the Principal Investigator of the Darwin Project team.
Darwin is one of two alternative projects competing for selection as a future ESA 'Cornerstone' mission. Scientists and engineers will examine the technological and scientific aspects of both competitors before coming to a decision. If Darwin is selected, it would be launched around the year 2015. The winning project will not be chosen for several years.
IllustrationsThe following illustrations are available via the WWW at the following location: http://www.esa.int/esa-mmg/mmg.pl?collection=Space+Science&type=I
Dr Jacqueline Mitton, RAS Public Relations Officer