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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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.
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