The dip in brightness is so small because the planet is so faint - the often used analogy is that it's like trying to see the change in overall brightness when a firefly disappears behind a searchlight. Nevertheless, over the last few years we have honed our techniques and we are starting to routinely measure the brightness of exoplanets in optical light - like the example below.
|Light vs time for the exoplanet system WASP-12. The dip (a fraction of one percent) is caused by the planet disappearing behind the star.|
|Brightness of planet as a function of wavelength. The furthest left points are two measurements of the optical brightness|
That's what I should have been doing tomorrow night, but looking at the weather it looks like I might be watching movies on iTunes instead. Now, Sydney is a long way to come to work one night and find it is cloudy, but I'm not too depressed because I'm taking a two week climbing holiday afterwards, so if the science doesn't play ball I've got this to look forward to...