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Comment by fhood | Hacker News Reader

I am an idiot who doesn't know anything about astronomy, and I always assumed that if you were looking for something like planet 9 you would attempt to deduce its position from the gravitational effects it might have on nearby known entities.

What am I missing in this context?

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The article states that's exactly what they're doing. They believe there's a planet nine based on its effect on other objects, and have a theoretical orbital range modeled from those gravitational effects.

The problem is space is big, and space is dark, so its hard to find things that don't generate their own light.

Ah ok, I guess I just overestimated the level of precision they were working with.

On the outskirts of Solar system things are far and move slowly. It prevents getting sufficient precision quickly.

Even if you could predict the exact orbit, you still don't know where on that orbit it is right now. So even best case that's a lot of space to search.

It seems like you could predict where it is based on the times the gravitational effects were recorded.

They believe they know where its orbit is because of its gravitational effect on other bodies, but they have no way to know where it currently is along its orbit.

Its orbit has between 200 and 1200 times the distance from the Sun as Earth (it's an ellipse), and it takes 10 to 20 thousand years to revolve around the Sun once.

Depending on where it is currently on this orbit, it might be that we will need to wait for better technology or for it to plainly get closer to Earth before we can find it.

If you know where the other planets are and know the effect 9 plays.

Surely you can work out where it will likely be in the sky.

It doesn't affect planets, it affects very distant objects and only over long periods of time.

https://en.wikipedia.org/wiki/Planet_Nine#/media/File:Planet...

https://en.wikipedia.org/wiki/Planet_Nine#Observations:_Orbi...

To add to this, they are detecting it based on how it has altered the orbits of other things over a long time, and not based on its direct gravitational effect. Which gives you an idea what its orbit should be, but not where it is within said orbit.

That gives them strip of sky to search through. A strip that unfortunately goes past some bright things - like the Milky Way.

You would. The problem is the gravitational effects are subtle, the planet is very far away, and dim.

To add to the problem. Objects in the outer solar system are spread out more vertically in their orbit. ( Like Pluto which dips far above and below the intermediate plane.

This means there is a relatively large area of they sky it could be in. Due to it's distance, it is not going to reflect much visible light at all either. So instead of being able to just scan across a the invariable plane looking for it we have to look in a large swath of the sky for something very dim, moving very slowly.

Re: would attempt to deduce its position from the gravitational effects it might have on nearby known entities. What am I missing...

There probably are NO "nearby known entities". Planet 9 is probably further away than most detectable objects. The only reason it would possibly be found while "nearby" objects would not is because it's bigger.

And it's probably so far away that its gravitational tug is very slight on the big planets that are easy to measure. The article mentioned it may tug the outer gas giants by a "dozen meters" off course from models having no Planet 9 in them. That requires a really powerful "ruler" to detect.

Thats how its existence is inferred but there are enough additional bodies that it's a chaotic signal. Then even if you do know where to look that doesn't guarantee you'll spot a relatively small dark object.

I'd guess there is very few nearby known entities and those that are in the area are too far to be precise enough.

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