How can astronomers observe planets thousands of lightyears away in other solar systems, but yet can’t be sure that there isn’t an undiscovered “planet 9” in our own solar system?
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They look at all of space and can pick out planets passing in front of a sun or so. Imagine looking out in a field and seeing a big bird (sun) fly in the distance. You can notice it and then get binoculars on it and observe it. But the birds (planet)around you are smaller than the (sun) and unless you at that specific area as it’s there you won’t see it.
Space is huge and looking for a non glowing entity super close to us is hard. Unless you’re looking right at the planet in the vastness around us you won’t see it
We see planets around other stars when they pass in front of the star and we see the light dip a little bit. We’ve gotten quite good at it.
There are some big constraints on a possible planet 9 that make it hard to see. First, it has to be not that big. We know this from what changes we see in the orbits of the planets we know about. Secondly, it has to be pretty far from the sun. The minimum distance is probably about 100 times the distance from the Sun to the Earth and there are estimates that it is 300 or more times that distance. So being that far from the sun it will get very little light. Then, from what we know about stuff far from the sun, they tend to be pretty dark in color. We learned this from the Pluto flyby that New Horizons did. So we are trying to see something not very big, far from the sun and pretty dark colored that we don’t know where it is. That’s a tough order.
We can’t see every planet around every star. We can see a few planets around a few stars if they happen to pass between the star and us. If planet X passed between us and the Sun, we’d certainly see it. But, however, it’s out with Pluto so it’s probably not a Planet under the current “rules for planets”.
I’m not an expert but my basic understanding is we detect exoplanets by seeing how they change the light of their star as the planet passes in front of the star. The star also gives us a specific target to look at.
A 9th planet in our system will never pass in front of our star from our perspective. So instead we need to look around in the dark over a huge area looking for something minuscule compared to the sky and hope to see it reflecting some light back at us.
Imagine trying to decide if someone was in a dark room by watching for them to walk in front of a flashlight pointing at you vs trying to see them by noticing the flashlight behind you reflected off their shirt.
We can only see distant planets when they, by some chance, either block their star’s light or cause it to wiggle.
This means that most distant planets, we have not seen. The vast majority of them. We just get lucky and see a few.
We haven’t happened to see planet 9.
If you’re looking for a planet around another star, you already know to point the telescope at that star, and it’s easy to find the star again night after night because it doesn’t move around. With a planet, you need to track it over a period of time to figure out its orbit, and from that and its brightness you can estimate size. Otherwise you might see it one night, never find it again, and assume it was some unknown asteroid.
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