system
Given the following post, respond as if you're commenting on Reddit:
human
How are we able to locate and measure planets hundreds of light years away yet unable to see the hypothesized planet beyond Pluto?

   Based on gravitational pull some scientists have proposed a possible 10th planet that lies beyond Pluto which should have a sizable mass. Given this technique has been used successfully in the past to discover Neptune before we had been able to see it, that would suggest it is somewhat plausible there is something additional in our solar system. 

Given this information, how is that we can detail, measure, and name planets in other solar systems, when we can’t even see the end of our own? 

Any help would be greatly appreciated!
gpt
The way we observe exoplanets is quite different from how we observe planets in our own solar system.

For a planet in our own solar system, we either have to observe it directly by registering the light coming from it, or deduce its presence because of the impact its gravity has on objects that we can see directly.

The problem with direct observation is that planets don't emit much light on their own and we rely on the planet reflecting sunlight to be able to see it. The intensity of light falls off with the square of the distance, so objects twice as far away only receive a quarter of the light. So take Pluto, for example. On average it's about 40 times further away from the Sun than the Earth, so it receives 1600 times less light per unit of surface area (and Pluto is quite a lot smaller too). But to see Pluto directly, its light needs to make its way back to us after being reflected by the surface of this little dwarf planet. Objects that are further out than Pluto will end up being much dimmer.

A second problem is where do you look for such an incredibly faint spec of light? To observe it directly, you'd have to point your hypersensitive telescope at the exact right spot in the sky.

So objects that are at a distance of Pluto or beyond will be both incredibly dim and will take up only a tiny point in the sky. These things combined make them very hard to spot.

So how do we spot exoplanets?

One way that we see an exoplanet is not by seeing it directly, but rather by seeing its shadow. As the exoplanet orbits its star, it will sometimes pass directly in front of the star (when seen from Earth). When that happens, it blocks a little bit of the light coming from the star. So by keeping a telescope pointed at a star and keeping track of the intensity of light that's coming from it, we can detect whether a planet is moving in front of it and briefly casting a shadow.

But there's another way to find exoplanets, for those that don't happen to orbit in a way that lets them cast a shadow our way. While a star exerts gravity on a planet, the planet also pulls at the star, albeit in a greatly reduced way. Nevertheless, this pull causes the star to "wobble" a bit and this wobble introduces a periodic Doppler shift in the light coming from the star. Detecting such a Doppler shift is an indication that an exoplanet may be present.

If one observes a star long enough, it's possible to determine the period of either of the aforementioned phenomena and this determines the length of a "year" on the exoplanet. This, in turn, is linked to the distance between the planet and the star. Finally, the magnitude of the distortion caused by the planet is a measure for its size.

With this information, a fairly complete picture can be formed of the exoplanet. We can determine if it's a likely to be gas giant or a rocky planet and if it's in the stars habitable zone or not.

So to conclude: We observe things in our solar system by looking at them directly and seeing the sunlight reflected by the object. Things that are beyond Pluto are simply too dim to easily spot this way. Exoplanets on the other hand are detected and studied by the effect their presence has on the star they orbit. These two different approaches are why we can spot exoplanets more easily than distant objects in our solar system.