yes and no. Most exoplanet detections so far have been indirect: people just search for the dimming from the eclipse or similar other methods. So you're right about that. But recently we're starting to actually directly take sensitive enough images to be able to see planets (see http://planetimager.org). They aren't using NNs yet. But there have been discussions of using NNs for various aspects of those searches.

Detecting dimming is actually surprisingly difficult and currently partly done by humans using Mechanical Turk-like platform. https://www.planethunters.org/

Dude, stars don't have constant brigthness...

They are also very very far away, and only a few photons are making it to the lens every so often; it's quite a difficult task.