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Comment by croutonwagon | Hacker News Reader

Question. Does it have to be a planet out there?

Could it be a small black hole?

Or is it just a matter of us not spending the time/effort to find it. I know time on instruments like Hubble is very, very tight and scheduled out years in Advance. But they have been talking about Planet X since like the 80s at least, and probably earlier.

Even Pluto was detectable relatively early and it’s on a crazy orbital plane and elliptical orbit.

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> Could it be a small black hole?

It could, the relevant part of the whole thing is the object's mass (and therefore its gravitational influence) and its orbit.

Lex Fridman did a podcast interview a few months ago with Konstantin Batygin [0] about the topic of Planet Nine and he poses this exact question of whether or not it could be a black hole [1].

More specifically, he asks if it could be a primordial black hole which would make it a double discovery since primordial black holes are still only theoretical.

[0] https://www.youtube.com/watch?v=tm7poMupE8k

[1] https://www.youtube.com/watch?v=tm7poMupE8k&t=4757s

The odds are incredibly tiny for it to be anything other than a planet.

It could be, but it would be the size of a tennis ball. See https://news.ycombinator.com/item?id=28167058

According to my calculations it would take more than 10^53 years to decay.

It won't even start to decay until space expands enough that it's warmer than the cosmological background, not that that's more than a rounding error to your calculation.

Counter argument against the black hole hypothesis includes the lack of powerful x-ray activity where the black hole should be. Also, its size would be quite unusual, which in that case could be a primordial black hole, which in itself is theoretical and never been observed.

If it were a black hole, which would necessarily mean a primordial black hole, we would never spot it. We would look, and look, and eventually give up and say it must be a statistical fluke. A coal-black body would also be hard to spot. Surprisingly many asteroids are black, for poorly-understood reasons.

We might not spot it from earth, but if we’re confident enough of where we’ll find it, its orbital parameters, its mass, a probe could be sent, and it could be detected through its SOI, and perhaps even direct observation.

Sure, ~300AU is a long way, but we have the technology, and it’s our closest black hole candidate by many orders of magnitude

If it is a choice between, (1) Spend $500M just to see whether there really is a black hole there, or (2) it is probably a just statistical fluke -- it is not hard to guess which will be chosen.

Could yes. But it certainly is not the easiest explanation.

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