Eli5: Why can’t we figure out the planet after Neptune in our Solar System but we are able to find out galaxies thousands of light years away from us ?


Recently have been binge-ing on planets and cosmos, and I saw this youtuber mentioned that we might discover the next planet after Neptune.

However from my understanding, humanity has discovered black holes, magnetars, stars and so on that are hundreds, thousands or even more than that in terms of the measurement of light years.

Our solar system is not in the measurement of light years, yet we can’t discover the next planet after Neptune. Why ?

In: Technology

Galaxies, etc. have the benefit of being extremely large and extremely bright. It is sort of like how you have trouble spotting your keys dropped in your lawn in the middle of the night, but it is very easy to spot a forest fire from even miles away in the night.

A planet for example might only be a few thousands of kilometers across, while galaxies are more like a hundred thousand light years. They are also blasting light out from the ongoing fusion reactions of their constituent stars, while planets are just sort of floating in the pitch black, only radiating back the little heat they absorb from the sun.

What we discover is based on the photons we receive. Stars emit photons, so we know a fair bit about stars. Planets pass in front of stars and so block photons. So we’ve learned a fair bit about those planets. Some bodies in our solar system are close enough to us and our sun that they reflect the sun’s photons towards us, so we know about those objects.

Planet X fits none of those profiles. We know something is there not because of photons but because of the gravitational effect it has on us and the objects we can observe.

Once we have better gravitational sensors in space, we’ll probably learn more.

Using radio waves or some sort of device we can see stars and just stars. We can also see voids which are dark holes clustered with stars which is pretty much galaxies. What we cant see are satelites (planets).

You go out to the edge of the solar system with a sun-sized flashlight and you’ll find that planet real quick.

A common way we detect exoplanets is by seeing stars dim just a tiny bit as their planets pass between the star and our telescope. Planets in our own solar system beyond our orbit will never be between us and the sun, so all we have to go on is the absolutely tiny amount of light that gets all the way out there and then back to us, and that’s hard to see.

Stars are bigger and make their own light. Easy to see.

Planets are smaller, and only reflect light from it’s nearby star. Harder to see.

Also, the principle way of discovering planets, and other bodies, in our own solar system is taking multiple pictures of the same part of the sky back to back and looking for moving points of light. Stars are so far away from us that they don’t appear to be moving compared to everything else, so something that is almost certainly is orbiting the sun closer by. Thing is, due to the way orbital mechanics work, the farther away from the sun you are, the slower your orbit it, as well as wider. So this means a planet beyond Neptune (and the one they are predicting is far, FAR beyond Neptune) would be moving not only very slowly, but covering an incredibly tiny distance visually from Earth.

Think of a circle with two points on it, an inch apart from each other. You sit at the middle, and the circle radius is three inches. Visually, from the center, the two points are roughly 20 degrees apart from each other. If a point of light moved from one point to the other, it’s be incredibly visible. Now imagine the circle is 100ft in radius, and the two points are a millimeter apart. The distance visually would be .00~1 degrees apart, much harder to notice, especially if the light reflecting off of it is dim and at risk of being drowned out by background light. Even if it is spotted, it can easily be mistaken for a star.

The sky is also flippin huge, so doing such a scan over the entire sky is just no feasible, and would take a hundred years to do. This is why you sort of have to predict where the Planet’s going to be before you go looking for it. This is actually how Neptune was discovered. Galileo was thought to map Neptune twice, but it moved so slowly he mistook it for a fixed star. Le Verrie, like many astronomers at the time, found that Uranus’ orbit didn’t make sense, and hypothesized that another planet was exerting gravity on it, messing with its orbit. Using Uranus’ orbital data, he calculated where this planet should be, and sent it to an observatory to look for the Planet. They found it the next night, right where it was predicted.

The same thing is happening now with the search for Planet Nine, but using the orbits of Oort Cloud comets instead of a planet, which unfortunately doesn’t narrow down the predicted location enough to find the planet in one night and the orbits are possibly explained by measurement bias in how we detect and track Oort cloud comets.

The only things we actually see far away from us is stars. Lots and lots of stars. Sometimes there’s a giant cluster of stars outside our galaxy, and that’s another galaxy. Sometimes, we look at stars closer to us, and see a shadow in front of them. That’s a planet passing by.

Within our solar system, faraway objects are much harder to spot as they get further from the sun. Less and less light reflects off the planet and less and less light makes it back to Earth for us to see.

A galaxy is 100,000 light years cross and millions of lights years away.

Planets are 1000 kms in diameter and *billions* of kms away.

Just look at the ratios and also take into account planets *reflect* light while galaxies *emit* their own.

Another thing worth considering is we don’t look for a specific galaxy, we take a picture of the sky, and say “hey, look, there’s a galaxy there!”.