Planets are on average very small compared to their host stars, so you need extremely precise measurements to be able to detect a drop in luminosity caused by t

Planets are on average very small compared to their host stars, so you need extremely precise measurements to be able to detect a drop in luminosity caused by the transit of a very small object in front of a huge one. It's already hard to do within our galaxy (~200k light years across), so outside of it (here 23M ly) is quite impressive.