Simply to say we are looking at stars, not planets. Mostly in our galaxy

We can't see exoplanets from distant galaxies. I think you've got things a bit confused.

Because diffraction, and most exoplanets have been detected by star transits where even if the star only shines on a single pixel in the detector, we can still look for periodic brightness changes.

The exoplanets we see are from other star systems in our galaxy, not in other galaxies.

And we didn't see these exoplanets directly, we see a drop in the magnitude (brightness) of the star the exoplanet is orbiting.

1. We cannot see any exoplanets from any galaxies outside our own. All the exoplanets we’ve found are in nearby areas of the Milky Way.

2. When we do detect exoplanets, they tend to be large and orbiting close to their parent star. We cannot see them directly, but instead we observe their effect on the star, either by watching it wobble slightly as the two orbit their common center of mass, or by watching the star dim slightly as the planet passes in front. Neither of these methods is applicable to a planet in our own system orbiting far out from the Sun.

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If we knew exactly where to look, we probably could just barely see dwarf planets or even bigger ones in the Kuiper belt. It's hard to pick them out from the background noise when they're so dim and far from the sun.

We can't see exoplanets directly. We see the effect their atmospheres have on the light that passes through them, and we see the gravitational effects the planets have on their parent star in the form of the stars' "wobble". There are no direct images of exoplanets.

Edit: I stand corrected. We have directly imaged exoplanets

Well, we have imaged a couple dozen exoplanets directly. But we'd see those ones if they were in the Solar system too.

Well, we have seen explanets directly. Just not most.

https://en.wikipedia.org/wiki/List_of_directly_imaged_exoplanets

I thought that too until recently, but it's simply not true. In fact, we've been directly imaging exoplanets for over a decade.

https://www.reddit.com/r/Damnthatsinteresting/comments/q6u9bj/four_exoplanets_orbiting_the_star_hr8799_directly/

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We can't actually see the exoplanets directly.

What we see is a slight dimming of the star it's orbiting as it passes between the star and us. Imagine someone walking in front of a car with it's headlights on while at night. You can see the change in the light, but it's unlikely you can see the person. Same principle.

We can then look at how the star "wobbles" in it's orbit. That gives us an idea of how much mass the planet has.

As a bonus: With the right tools, we can look at the light just as it starts to dim and this will give us an idea of what the atmosphere is comprised up.

Note that this is because those are gigantic worlds that’s still glowing brightly from the heat they obtained from their very recent formation (these directly imaged planets are usually a few tens of million years old). We should be able to image not-visibly-glowing planets soon, but not as of today.

The Solar System is too old for anything but the Sun to glow (except collisions which are extremely rare too), and brightness of Solar System objects scale to distance to the power of four (2 from dimming sunlight, 2 from the distance itself), so any cold planet rapidly fades into invisibility.

Map showing location of exoplanets. Most are within 5000 light years. Gravitational lensing allows astronomers to see a few in the direction of the galactic center.

As I recall, astronomers may have detected one exoplanet in the Magellanic Clouds.