Eli5: how do we know so much about deep space, but not our own solar system?

Question

I watch a lot of science content online and I always see stuff like “scientists have found a planet 100 light years away that is twice the mass of earth made out of solid methane and rains diamonds at 400mph” and then others that say “scientists think there could be a ninth planet passed Neptune but we’re not sure, maybe, who knows?” Like, how do we know so many specifics of these incredibly far off worlds but can’t figure out what’s in our own relative backyard?

In: Planetary Science

Answer 1

Imagine you and your friends go to a big lakeside campground are you are all sitting around your campsite’s campfire on a moonless night.

You can see your friends’ faces illuminated by the fire, and you can see the ground by the fire pretty well, but its kinda gets dimmer and dimmer the further from the fire you get and you have to use a flashlight to grab something from inside your tent.

Across the lake, however, you can see other campsites’ campfires. If they aren’t too far away you can probably see other people – illuminated by their own fires – looking over their fires out over the lake just as you are looking over your fire out over the lake. Or, if the fires are very far away, maybe you can’t see peoples front-sides illuminated by their fires, but you could still see their silhouette back-sides if they walk/stand between their own fire and you.

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That’s basically what is happening with the stuff that scientist report seeing in space.

We can “see planets billions of miles away” because we can see their silhouettes when their orbit puts those planets between us and their sun (which is super easy even if its super far away).

The “possible ninth planet in our own solar system” stuff happens when we basically detect *something* rustling in the dark around our own sun (usually based on gravitational/orbital simulations getting more and more detailed/accurate and noticing something feels off) but it’s far enough away that we couldn’t see it without knowing exactly where/when to look or we’d have to go look with flashlights.