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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There is a number that tells you “can you see it” called Apparent Magnitude. The lower or more negative the number the brighter the thing to a person on earth. For every 1 lower, the object is 2.5x brighter. For 3 lower in magnitude the object is 2.5×2.5×2.5=15.6x brighter.
On this scale the Sun is -26.7, the Moon is -12.7, and Mars is -2.9. So if you are standing on the Earth looking up into the sky, the sun is 380,000x brighter than the Moon, and the Moon is 9,500x brighter than Mars. The Moon and Mars are only bright at all because they are close to the Sun and the light reflects off them to the Earth. The further something is from the Earth and Sun, the less bright the reflected light will be, unless the thing is bright on its own.
Stars like the Sun are bright on their own. The sun puts out around 10^26 Watts of light power. If every bit of electricity on earth was powering LED lights pointing up to the sky, and there were a trillion Earth copies all doing this, the Sun would still be 100x brighter.
Edit: 100x, not 10,000x
The Star is very bright, so it’s very easy to see.
The Planet doesn’t produce light of its own, so it needs to reflect someone else’s light back at us for us to see it.
The sun is basically just an unusually large Star when you’re that far away… so there’s not much light to hit it and bounce back. For us to see it, we’ve got to catch and focus a lot of the returning light.
The lighting. You can notice a candle light from two miles away, but I doubt you’ll notice a dime at that range.
Imagine being in an Airplane hanger at night. That’s space.
It’s pitch black except for a candle way over at the other end. This is a star.
There’s also a grain of sand somewhere a few feet away. This is the planet.
Which is easier to see?
Stars emit light and other electromagnetic frequencies that we can detect.
Planets reflect those from stars.
So if the 9th planet is too far away or non-reflective, we can’t see it.
We can only know it’s there because it has gravity and it’s gravity impacts other objects.
Same reason it’s easy to see when the basement light is in when standing at the top of the stairs but difficult to find the cat in the dark when the light is off.
You ever lose a mosquito that’s right in front of you?
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