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The tides of Venus

An upcoming transit

It’s not every day that a once-in-a-lifetime opportunity comes up, but on Wednesday you have the opportunity to see the silhouette of the planet of Venus move across the disk of the sun. To see this happen again you would have to live another 105 years.

I should correct myself, as for me it’s going to be a twice-in-a-lifetime event as I was lucky enough to see the last transit in 2004 from London. And, by lucky chance, I’ve gone and plonked myself right in the corridor that sees the whole transit once again.

Where on Earth the 2012 transit of Venus can be view from. Melbourne right in the full-transit area! PD-USGOV-NASA

If you are planning to try and see this great event, which will start in Melbourne on June 6 at 8.15am, PLEASE DO NOT LOOK DIRECTLY AT THE SUN. There are plenty of websites that explain how you can view the transit safely.

There’s been a lot of overview about the science history behind previous Venus transits. Transits currently occur in pairs about 120 years apart, so the transit before the last one in 2004 was in 1882. The 1769 transit which saw James Cook journey to Tahiti, has been touted as the first global scientific event, with international scientists collaborating to find the distance between the earth and the sun.

But here we are in the 21st century and we know the distance between the earth and the Sun quite well enough now (it’s, on average, 149.6 million kilometres should you need to know). But there is a group of scientists using the upcoming transit of Venus to further our understanding of the universe. Can it help us learn more about exoplanets?

The transit on June 6 will be old-hat for the Kepler spacecraft. Orbiting alongside Earth since 2009, Kepler is monitoring hundreds of thousands of stars looking for similar events – small planets passing in front of their stars. In reality, all Kepler sees is a slight dimming of the light from the host star, but this has enabled it to find to date 61 exoplanets orbiting other stars, even identifying other solar systems.

Diagram of the planetary systems found by the Kepler spacecraft. NASA Ames/Dan Fabrycky, UC Santa Cruz

So knowing that there are planets out there is one thing, but what are these planets made up of? Do they have atmospheres? For this researchers are now turning to spectroscopy, where the light that has travelled past a transiting planet holds signatures of chemicals on the planet itself.

The method, called transmission spectroscopy, has already been shown to see chemicals in the atmospheres of exoplanets. In 2008 NASA’s Hubble telescope detected methane in the atmosphere of an exoplanet 63 light years away, catchily called HD 189733b. Though this planet is too close to its star to support life, and probably a gas giant like Jupiter, it was an important step forward for exploring the universe.

Kepler has a harder task; it’s looking for smaller, Earth-like planets, which would have much smaller atmospheres. If we are going to be able to filter out chemical signals from such small bodies we need to have a good idea of what to look for.

This is when the potential of Wednesday’s transit comes in. We have a transit of a planet that, thanks to the work of the Venus Express and Magellan missions, we know a lot about. Venus has quite a dense atmosphere, dominated by carbon dioxide. A group have calculated what they expect the transmission signal from Venus to be, and hopefully will be able to couple this with accurate observations to be taken on Wednesday.

From the (perhaps not so) humble start discovering the distance between our earth and the sun, to helping us explore planets light years away, the transit of Venus continues to help build scientific collaborations across the globe.

Now all we need is for the clouds to stay away.

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