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Comment by lupire | Hacker News Reader

I don't know MOND at all, but is it still highly plausible that decades of modern astronomy haven't found better more direct evidence for Planet Nine existing?

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The evidence from clustered orbits points towards planet nine likely being right in the middle of the busiest part of the night sky, nestled between millions of stars of the milky way. The assumption is that we already have pictures of it, it's just that when you photograph that part of the sky you are not likely to notice one very faint point of light in the middle of the ten million brighter points of light on the same plate.

Detecting planets or other cold objects is hard, when they are far from the sun. Illumination from the sun falls with inverse square law (~1/r^2). When the reflected light does the return trip, it also falls with (~1/r^2). The effect compounds on both legs of the trip, and overall the brightness of a planet falls proportional to ~1/r^4, quickly getting lost amidst all the noise in the night sky as distance increases.

Could you build an uber radar for detecting the planet, possibly working together with the radiotelescopes we already have? Given the article, it should be ~150 light hours from sun, which round trip time wise sounds doable.

Radar faces the same inverse fourth power effect.

Not sure. But to map polar ice in mercury it was necessary to use the Arecibo radio telescope as radar[1], with the benefit we knew previously where to point the instrument, exactly. Given planet nine is hundreds of times more distant, it would probably take a humungous radio telescope and a lot of power to conduct the search.

[1] https://www.nature.com/articles/369213a0

My suspicion is that you'd have a really hard time doing that. For one, if so little of the light from the sun (notably the brightest thing in our solar system) is being reflected, we'd be hard pressed to build something capable of spraying the whole search area and getting back enough particles to detect and count.

But even more than that, the power requirements would be immense. We have retro reflectors on the moon, and even with knowing where the moon is and where they are on the moon, we get almost no photons back. From Wikipedia:

> Out of a pulse of 3×1017 photons[25] aimed at the reflector, only about 1–5 are received back on Earth, even under good conditions.

That's with a dedicated reflector positioned well on the moon's surface, about 0.002au away. You're talking about hitting a rocky lump in front of a bright background 400-800au away. The power requirements wouldn't be five orders of magnitude more, it would be perhaps hundreds of orders of magnitude more.

Is it fair to say it is easier to detect planets in other solar systems?

Yes! We often detect planets in other solar systems because they pass in front of their stars, causing the brightness to dip. You can also look for Doppler shifts in the light you see from stars, which is caused by the planet pulling the star slightly. You can also look for the sorts of brightness changes you'd expect as a result of gravity lensing.

In almost all cases, you're looking for changes in something you can already see. The problem with a planet 9 is that so little light is getting to it that it's going to be very very dim.

Wouldn’t it still be passing in front of something, or is it likely to be comparatively ‘stationary’ against a similarly stationary backdrop of stars and other objects?

When a star is occluded by a planet, it'll dip in brightness at a pretty regular interval (because of the orbit). So you can look look for a series of dips, which gives you a candidate. That's then looked at more closely.

Since a planet 9 wouldn't transit a star at any regular interval, it wouldn't show up as a candidate for that search technique.

So we would need to potentially deliver the telescope far outside of our solar system and have it look back at us, kind of like a very expensive selfie stick?

That's not possible. The furthest things we've ever sent out are the voyager probes, and they're 135 and 162au away, each. We'd have to get twice that distance to even be at the distance that we expect a planet 9 to be.

Even if we managed to get many multiples of the distance planet 9 is from the sun to take pictures backward and send them home to earth, it's _so far_ from the sun that a) the amount it would occlude the sun is ridiculously small, if it ever even lined up. At that distance, the sun just looks like another star in the sky. That's like waiting for one particular asteroid to block out the North Star. And b) the orbit period would still need to be short enough that you could see it transit a few times to use the technique that scientists use to detect planets elsewhere. Just seeing something block the sun doesn't tell you what it was that blocked it, especially at 400-800au distance. Plus, then you're talking about latencies of about a light month, which makes it super hard to even know to look for the thing you saw (a month ago).

With our current, but especially decades ago capabilities, it's (was) similar to Russell's teapot problem.

It would be incredibly dark. The sun would just be a bright star out there.

There’s also some interesting speculation that planet nine could be a primordial black hole, in which case it could only be located by its indirect effects. It likely wouldn’t radiate much at all, making it basically invisible.

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