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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My understanding is that you don’t generally see individual stars that far away unless something exceptional is happening (the star is crazy massive, there is gravitational micro lensing, it’s a variable star, it’s exploding, etc.)
We know Pluto is there. Years ago we found other, nearby exoplanets similar distances away from the sun and decided it made more sense to eliminate 1 planet (Pluto) than add others. We did this by changing the definition of a planet.
Simply put, stars shine and planets do not.
A star is pumping out a ton of radiation in various forms. Everything from radio waves to visible light, to gamma rays.
Planets reflect light. Which means far less light is reaching the viewer and tends to get drowned out by the light of the star they orbit (imagine trying to spot a pinhead when there is a super bright spotlight behind it…all you see is the spotlight…)
The hypothetical Planet 9 is very far out, and also may have a surface that doesn’t reflect much light. So it is literally just a shadow floating out in the blackness of space from our perspective.
Which one is easier to see at night? A small rock or a flashlight?
That is an awesome question; I’m sorry you got a bunch of “because they glow” answers.
I assume your question relates to the recent discovery of Earendel; 28 billion light years away. Or the earlier discovery of Icarus at 9 billion light years. Seeing individual stars at this distance is truly mind boggling.
To give context, if you look in the night sky:
* 0.05% of the stars are lest than 10 light years away.
* circa 19% are less than 100 light years away
* circa 81% are within 200 light years.
After 200 light years we generally can’t see them unless they are oddly bright. The furthest we can see is 16,000 light years away.
For these types of stars we are calculating the distance directly. We use a “stereo vision” by taking images 6 months apart; such that the earth orbit gives a 300 million km stereo effect. This needs to be done June/Dec to maximise distance, given the nature of our elliptical orbit.
This gives us not only the distance, but importantly the size of the nearby stars. We learnt that stars of different sizes have different spectrums.
For stars > (nominally) 1,000 light years we measure there brightness and spectrum. Then estimate size from the spectrum and calculate how far away it must be to achieve its current brightness.
OK, so that’s star watching at 1000’s of light years. What about millions?
Well the furthest thing from earth we can see by the naked eye is the Andromeda galaxy (2.5 million ly). This uses 1 trillion stars to be that bright. We need a different technique, spectroscopic redshift, to discover the distance to these objects; probably goes beyond ELI5.
OK so what if we use a giant telescope? Then we can start to see a very bright things (bright stars / quasars / galaxies) at distances of 100’s of millions of light years.
Occasionally, using the Hubble or something, you may find a galaxy or quasar at billions or even 10’s of billions of light years away.
So a large galaxy is circa 10^20m and a star is circa be 10^9m. This is a 10^11 size difference. A star is to a galaxy as a the width of a human hair (10^-4) is to the planet earth (10^7).
So seeing a single star at 10 billion light years, is like looking at mars without a telescope and happening to notice a spec of dust on one of the rovers. It’s a bonkers crazy event.
It happens with something called gravitational lensing; the effect of , large objects producing enough gravity to bend light, acting sort of like a lens (but that term is a bit misleading). Occasionally something might come into “focus” of that “lens” and you get a lucky glimpse of the impossible.
On a smaller scale, it’s why we found planets 150 light years away.
When thousands of galaxies cluster together and every thing is just right; you see a star at billions of light years.
Confirming the 9th planet is acutely well with in the realms of a large telescope. It’s (probably) much further away than pluto/neptune; so we just need to know where to point the telescope. However calculating where to point the telescope is super difficult. This is really hard to ELI5. But I will give it a go using a to scale sound problem.
Imagine you a are in the middle of an football field with your eyes closed trying to hit people with a bow & arrow. Every person around you is humming a different note to give you a clue.
* Mercury is 57cm away, Venus 1m away, Earth 1.5m away, Mars 2.2 meters away. You shoot them easily.
* Jupiter is 7.8m away, Saturn is 14m away. But they are super noisy and kinda fat; so easy to hit.
* Uranus is 28m away; Neptune is 45m away (edge of the field). These guys were a bit harder to find. Pluto was 59m way (in the stadium), so it took a while to hit him.
* Planet Nine is 375 meters away… possibly… or that noise could have been a cricket. You need to borrow the clubs biggest bows to try and hit it. However the club only has a few bows that shoot that far; and want to use them for more interesting things. So until you have a better idea of what you heard; and where it came from; you only get limited use of the big bows.
For the same reason it’s easy to count the lights in a city but very hard to count people at night. One emits data and the other only poorly reflects it.
Stars are bright, so they’re always shining. Planets are dark, and the only way we can tell if they’re there is if they block a star that we know is there.
For the same reason that, at night, you can see someone with a flashlight from far away, but can’t see, say, a nearby blue bouncy ball perhaps even if you’re pointing a flashlight right at it.
The only time we can detect an individual star that is billions of LY away, is when that star explodes—a supernova. It’s difficult to overstate how bright a supernova is.
Planets, especially the outer planets, are very cold and dark, making them very difficult to see.
I think part of the problem is that most pictures showing our Solar System aren’t to scale. If you were an outside observer looking at the Solar System, you’d have a very hard time spotting any planets. The distance between our small-to-scale planets is also very, very large. It’d be like dropping a black marble into the Marianas Trench and trying to spot it on the bottom from the surface.
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