Monthly Archives: June 2012

Twice in a Lifetime

As anyone with any contact to the outside world knows, the planet Venus transited its own star as seen from our perspective this week. The only thing separating this from being nearly identical to an exoplanet (aside from the obvious difference in distance) is that as Earth orbits the same star, our perspective keeps changing, denying us the ability to observe a transit every Venusian revolution (there are also some issues with the differences in the inclinations between the two orbits that play a vital role in making this an uncommon phenomenon for us).

The above image, taken by the JAXA/NASA Hinode spacecraft shows for Venus what happens essentially each time an exoplanet transits its star. The Kepler spacecraft watches thousands of planets do this, but not quite as dramatically, for Kepler cannot resolve the discs of its 150,000 target stars, and instead has to rely on the characteristic dimming of the star as the planet blocks some of the star’s light.

Notice the entire limb of the planet is visible, even the part that is not yet over the solar disc. Sunlight is passing through the Venusian atmosphere and is scattered every which way, some of it reaching us for us to observe. The colour of the sunlight will be changed as it filters through the atmosphere, just like on Earth where our atmosphere turns sunsets and sunrises red. This is exactly what is observed from Earth when we determine the compositions of extrasolar planet atmospheres through transmission spectroscopy.