system
Given the following post, respond as if you're commenting on Reddit:
human
Why can we discover planets thousands light years away from us, but "miss" Neptun-sized planets in our Solar System?
gpt
The key is that the planets we are discovering around other stars are **much** closer to their star than this new hypothesized planet is. 

This new planet is about >20x further away from the sun than Neptune is. Even from Neptune, the Sun wouldn't appear to your eye to be very different than a bright star. It would be about the same size in your vision as a racquetball held 2 football fields away. Neptune itself would be a little smaller than the short axis of an uncooked grain of rice at that distance. 

That's just Neptune. Light from the sun when you get out as far is this other hypothesized planet is so faint, and the amount reflected back to Earth again fainter still, that there's no real chance of seeing it without looking at just the right spot for a long time with a big telescope. It's also so far away that it has very little effect on our sun's motion, and what effect it would have would take place over it's **15,000 year orbit**. No observational campaign on human timescales, even one started at the same time as human agriculture, would be able to find and confirm the planet's existence by watching the sun.

By contrast, the exoplanets we find are generally very close to their star, with short orbital periods (days, weeks, and months instead of thousands of years). Because they are closer, the effects on the star's motion are much more pronounced, and they occur regularly so we can actually observe multiple orbits. Kepler finds planets by watching dips in the amount of light instead of the star's motion, but the same effect applies. We can't find exoplanets like Saturn (yet), much less ones like the hypothesized ninth one.

**TL;DR:** It's very, very, very far from the sun with a correspondingly long orbital period, while the exoplanets we can find are very much not.