Astronomers go planet hunting as Venus lines up with the sun

In a live link-up between UCL's Mullard Space Science Laboratory (MSSL) and the CAB-INTA Observatory, Madrid, the change in brightness of the sun during the transit of Venus on Tuesday 8 June 2004 will be monitored. Future space missions will use such dips in stellar radiation to pinpoint the presence of previously undetected planets orbiting distant stars.

Dr Dave Walton of UCL's MSSL will lead the experiment from the CAB-INTA Observatory. Journalists are invited to join Dr Andrew Coates and colleagues at MSSL near Dorking, Surrey to watch the transit and go planet hunting.

To date over 100 planets have been found outside the solar system. Most are thought to be Jupiter-sized with a few as small as Saturn and were detected using a less sensitive technique. Known as the Doppler shift, it measures disruption of a star's smooth, elliptical orbit by the presence of orbiting planets.

Researchers believe there could be planets around up to half the stars in our galaxy alone and the transit technique has the capacity to detect smaller Earth-sized planets.

Dr Andrew Coates of UCL's Mullard Space Science Laboratory explains:

"Complementary to space missions in our own solar system the detection of Earth-like planets elsewhere will be a significant step in the search for life anywhere beyond Earth.

"So far the planets that have been found elsewhere using the orbital 'wobble' method have been at least Saturn-sized and mostly closer to their parent stars than our 'own' gas giants. No Earth-sized planets have been detected due to the limitations of the technique used so far.

"Space missions using the transit method are planned for 2007. These will continuously 'stare' at 100,000 to half a million stars looking for transit-induced dips in intensity but until then the transit of Venus offers our best opportunity of trying out the technique."

A unique view of the transit will also be offered by two spacecraft presently in orbit. TRACE will have a similar view to that on Earth because it is in a low orbit but SOHO which is 1.5 million km above the Earth will see the Venus 'shadow' over the sun's corona rather than disk.

"TRACE could provide some exciting new information on limb darkening, which is thought to be the reason for the 'black drop' effect seen during transits," explains Dr Coates.

"SOHO could provide some information on the Venus atmosphere as it will be back-lit by the corona. Really it's quite a nice follow-up to Lomonosov's work with the 1761 transit as a result of which he correctly predicted that Venus would have an atmosphere. But no spacecraft has ever seen a Venus transit with their sensitive diagnostic techniques, so who knows what we will learn on the day."

Dr Dave Walton says: "The transit provides a unique opportunity to demonstrate this method. A large-format CCD, made by e2v technologies for ESA's proposed Eddington mission will measure the very small decrease in brightness, dipping by less than a tenth of one percent, during the transit. This will be an interesting test of the technique for future space missions."

"The rare transit offers a remarkable link between the past, present and future," added Dr Coates.

"In the past, one of the transits in 1761 provided the first hint that Venus may have an atmosphere. In the present, we now know that Venus has an atmosphere of 90 times the pressure of Earth's atmosphere - equivalent to the pressure hundreds of metres under Earth's ocean surface. In the future, spacecraft will explore Venus to study some of the remaining puzzles about the planet, and others will use the transit technique for planet hunting around other sun-like stars."

Notes to Editors

For further information please contact:

Judith H Moore
Media Relations Manager
University College London

Tel: +44 (0)20 7679 7678
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Additional information about the transit of Venus can be found at: www.transit-of-venus.org.uk

About the Mullard Space Science Laboratory's continuing projects:

MSSL are currently working on an experiment for the European Space Agency's Venus Express mission. Due for launch in November 2005, the Venus Express mission will reach Earth's sister planet five months later. It will make important new measurements on the atmosphere, the surface and the escape of atmosphere to space. The mission will reveal the greatest detail yet about Earth's hellish twin - where the clouds are made of sulphuric acid, the surface temperature is 460 degrees Celsius and a runaway greenhouse effect has been active for billions of years.

Venus Express will use newly-discovered 'windows' in infrared light to probe the atmosphere under the clouds, and the surface. This will help to explain why the thick Venus atmosphere rotates so quickly above the slowly rotating surface, producing the chevron-shaped clouds seen in pictures from earlier spacecraft. A ground-penetrating radar should reveal whether volcanism is present on the planet and, if so, whether they are still active.

The Venus Express instrument MSSL are working on is ASPERA, which will measure in detail, for the first time, how the outer parts of the Venus atmosphere are stripped away by the solar wind and lost to space. Similar to Mars, Venus has no magnetic field to protect its atmosphere from this effect. In contrast, our Earth has a magnetic shield which protects us from the solar wind and from cosmic rays.