You are detecting a planet (let's say bigger than Jupiter) of radius 100,000 km (1e+5 km) from (let's use Andromeda) 2.5 million light years away (2.3e+19 km).

Note that is 14 orders of magnitude.

So, let's scale that to a grain of sand (1mm or 1e-6 km) so that's 1e-11 in scale which makes the distance of 2.3e+8 km away--or roughly the distance to Mars.

So, to detect a planet in Andromeda is like detecting a single grain of sand on the Earth from Mars.

In the words of Douglas Adams; ``` Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. ...

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The first possibility of a planet outside our solar system was discovered just over 300 years after the first telescope but not confirmed until 1992, almost 400 years after telescopes.

Finding the first planet outside our galaxy just 28 years later is pretty good!

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.

Think of it this way: the best picture we have of andromeda (the galaxy closest to us) has 1.5 billion pixels. Compare that to the fact that the andromeda galaxy contains on the order of one trillion stars.

So we can't really see individual stars, except in the sense that some may be bright enough that they dominate the signal in their pixel.

I guess a lot of people could not describe what a galaxy is. Solar system - our sun and the planets that go around it. Galaxy - hundreds of billions of suns clustered in space. We call ours the Milky Way after the band of stars you can see on clear nights. Other galaxies - Andromeda is well visible from Earth, but there’s probably again hundreds of billions of them.

It's been very difficult to gather the evidence until now? Given the astronomical distances involved and our technological progress?

We are not even sure if there are planets beyond Pluto and that is just our own solar system.

Detecting planets is hard, even within our own galaxy.