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

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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

4 Answers

Anonymous 0 Comments

It’s a bit like knowing what the moon looks like but not being sure if there is an ant somewhere in your house.

The moon is big and bright despite how far away it is, so it’s easy to find and learn about. Ants are small and hide in the darkness.

Similarly, those planets around other stars are *around other stars* so we know where to look because stars are bright and easy to see.

If there is another planet in our Solar System it will be small and dark (we have already found all the big ones) and it will very far away like Pluto.

Anonymous 0 Comments

When we look for planets in other systems, we’re looking at all the stars we can point a telescope at, and writing down which ones we see planets at. We mostly find the ones that are easy to find, because they’re big and close to their stars, so either they transit the star or they make it wobble in a detectable way. So we have a long list of whatever we happened to find that’s the easiest to find.

In our solar system, you’re talking about finding one particular planet, that’s not conveniently between us and a star that’s very close to it. The only way to see something like that is if you know exactly where to look for it, and even then it’ll be hard to see because nothing is shining very much light on it.

It’s like saying that we can throw a net in the ocean and pull out thousands of fish, so why did it take us so long to find a coelacanth? One technique is just kinda surveying a huge category of common things you happen to find, the other is looking for a very specific and rare thing when you don’t know what it looks like or where to look.

Anonymous 0 Comments

Imagine you’re sitting on top of a hill at night, next to a street lamp. You can see everything in the area illuminated by the street lamp, but past a certain distance it’s dark and you can’t see anything. Looking down the hill, you can see hundreds of other street lamps in the city, little islands of light in a sea of darkness.

You can see a lot of things going on around those other street lamps. Cars, people, buildings, etc. Even when they are far away, if you look long enough, you can make out some details. And sometimes there are objects moving in the darkness that you can’t see most of the time, but occasionally move into the light. And because you’re so far away, you can track them as they move between the lights.

But if there are objects moving in the darkness around you, it’s sometimes harder to tell. Although you are close to the light and so are getting a lot of detail, you can only see the things around you from where you’re standing. You can’t see them against another light, or from multiple angles. So you have to rely on clues like moving shadows or maybe seeing other object move a bit when the hidden objects disturb them. Because of where you are positioned, even though the object is closer, you can see less of it.

Anonymous 0 Comments

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.


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.