Extrasolar planet hunters will set their sights close to home on Tuesday when Venus passes in front of the Sun for the first time since 1882.
About 120 planets have been discovered orbiting other stars. Three of these were revealed because they dimmed their stars鈥� light during transits, which also gave information about the planets鈥� masses. And future space missions such as NASA鈥檚 Kepler, due to launch in 2007, aim to find many more transits by monitoring 100,000 Sun-like stars.
The transit of Venus will dim the Sun鈥檚 light by just a tenth of one per cent but the data astronomers hope to gather will help interpret future extrasolar searches. 鈥淓xtrasolar planets are unique among astronomical objects because there are local counterparts to study,鈥� says Sara Seager, who models the atmospheres of extrasolar planets at the Carnegie Institution in Washington, DC.
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Seager鈥檚 colleague Dan Kiselman will use the Swedish one-metre Solar Telescope in the Canary Islands to look for carbon dioxide in Venus鈥檚 atmosphere during the transit. The gas comprises 97 per cent of the planet鈥檚 atmosphere, but Seager says: 鈥淲e designed the experiment to detect something we know is there. If we鈥檙e going to interpret extrasolar planets, we want to make sure the models are correct.鈥�
The team will also look for trace amounts of sodium or potassium in the planet鈥檚 atmosphere. These elements 鈥� the remains of burned-up meteorites 鈥� float in Earth鈥檚 upper atmosphere but have never been seen on Venus. Seager hopes backlighting by the Sun may reveal the elements鈥� faint absorption signal.
Moon shine
The Sun will not have risen in Arizona during the transit, but the University of Arizona鈥檚 Glenn Schneider has devised a creative alternative to observe the event. He will look at sunlight reflected by the Moon to search for overall dimming and to see how the Sun鈥檚 spectrum changes during the approximately six-hour event.
The Sun鈥檚 rotation and the Doppler effect causes its light to be shifted towards bluer wavelengths on the side spinning towards Earth and towards redder wavelengths on the receding side. So as Venus moves across the Sun鈥檚 disc, it will block different parts of the spectrum.
鈥淥ne thing you could tell for extrasolar planets is if the planet moves in the same direction as the star rotates,鈥� Seager told New 杏吧原创. 鈥淲e have been surprised by the extrasolar planets many times and one always wants to verify even obvious ideas. If the planet was rotating in the opposite direction it would signify a massive collision had taken place.鈥�
She says Schneider鈥檚 experiment 鈥� in which Venus鈥檚 shadow will not be visible on the Moon 鈥� most closely mimics extrasolar planet observations, where the transiting planet cannot be resolved against its point-like host star.
Tilted plane
Venus transits are rare, occurring just four times every 243 years or so, because the planet鈥檚 orbital plane is tilted slightly to Earth鈥檚.
Astronomers have observed only five previous transits, but these led to the discovery of Venus鈥檚 atmosphere and to estimates of the distance between the Earth and Sun. The next transit will occur in 2012, but the one after that will be in 2117.
In 2004, the TRACE and SOHO satellites will image the event from space, and observers in Europe, Africa and Asia should get the longest ground-based views.
Those in eastern North and South America should see part of the transit around sunrise, while those in East Asia and Australia should see part of the transit around sunset. Ophthalmologists say it is essential that people use solar filters when observing the Sun to avoid eye damage.