Why we can accurately detect stars billions of light years away, but we can’t confirm if we have a 9th planet in our solar system?
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Stars emit light and pump billions of photons in all directions. Planets only reflect light, and how much light they reflect is based on the number of photons that hit them from a star, and what percentage they reflect. It’s much, much, much harder to see a planet than a star.
whats easier to see, a 100000 watt searching light at 1 km or a black 1 cm stone at 10 meters distance.
All in the dark
With respect to the one (and AFIK only) star we have seen billions of light years away, it is pure happenstance that the universe magnified its light such that we could detect it.
Secondly, we have disproved many proposed additional planets.
Finally, some notes on planet hunting. You can see a star because it is bright. Planets aren’t bright, so when you look at them (within our solar system) you only see them because they reflect light from the Sun. The further away the planet, the less sunlight reaches it to be reflected, and the dimmer the reflected light is when it reaches Earth. This makes direct observation of distant planets difficult to impossible. The planets beyond Neptune were initially detected because of the gravitational effect they have on planets we can see. However, like with light, the smaller and more distant the body is, the smaller its influence on other planets and therefore the harder it is to detect. So while it is easy to disprove the existence of a large planet close to Neptune, it is difficult to disprove a small planet which is much further out.
The light is an issue, as others have said, but also, what counts as a planet? They changed the definition to exclude Pluto a few years ago because as telescopes get more sophisticated, they were finding things bigger than Pluto orbiting the Sun. They had to choose between eight planets and an indeterminate number, probably dozens, maybe hundreds.
Big, bright things are big and bright, makes them easy to see.
Small dark things are small and dark, makes them hard to see.
There was a recent scientific paper exploring the possibility that Planet 9 is a primordial black hole (one formed in the early universe) that got caught up in the Sun’s gravity. A black hole about the size of a golf ball would weigh several times the mass of the earth and would be able to produce the gravitational effects that make us suspect planet 9 exists.
I suspect that finding a golfball with an orbit larger than Pluto’s would be hard enough, now make it one that absorbs all light. Tricky tricky.
They say it’s because they are dark and don’t radiate light. But we have chatted and mapped the course of hundreds of thousands of objects in our solar system that are miniscule compared to a planet. Its mind boggling how many of these things we know about now, I think it’s near a million actually. Some are near-earth objects, but there are others, like Sedna, which go waaaay out into the black. So the question still stands.
We’ve known for decades there is a 9th “planet”. The only debate is over what’s considered a “planet”. There’s no right or wrong answer, but we discovered a lot more objects that are more similar to Pluto than Pluto is similar to the other 8. Mainly based on size and proximity to the sun. There’s no correlation with our ability to “detect stars”.
Alot of people are explainung the fact that stars are brighter, therefore easier to see. I’ll go one further. We can even detect planets around other stars because of this light. We mostly find planets by measuring the light from a star and seeing if it gets blocked and/or dimmer at a certain rate. By the timing/rate of the dimness of the star at certain points, smart math space science people can determine/estimate how many/what size/how far planets are from their star. We don’t have that advantage looking out from our star in our point of view from inside the same solar system, so we would have to get a direct view of it. We’re stuck measuring gravity fluctuations etc to determine if there’s a ninth planet mainly because there’s no back light to measure against.
Edit:typo
The same reason we can see a lighthouse in the dark, but can’t see the rocks. Once is a giant source of light. The other is a dark object in a dark field.
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