Could Modified Gravity Kill Planet Nine? • Hacker News Board

Re: Could Modified Gravity Kill Planet Nine?

#111

Post by adastra22 » Sat, Mar 02, 2024, 6:39 PM UTC

jfengel wrote: post #76

GordonS wrote: post #8

If I understand correctly, MOND is suggesting that gravity behaves differently at sufficiently large scale (galactic), similar to how we observe different physics at a quantum level?

I wouldn't have put it that way, since the quantum/classical transition is an emergent phenomenon without additional parameters, while MOND proposes a new term being a added to the physics. Still, at a broad level there are similarities, yes. In QM scale practically has a life of its own -- made explicit in the Copenhagen interpretation.

What? Quantum physics definitely adds new terms…

Re: Could Modified Gravity Kill Planet Nine?

#112

Post by adastra22 » Sat, Mar 02, 2024, 6:40 PM UTC

pfdietz wrote: post #3

If MOND were a single theory such ideas would be more interesting.

You could say the same about dark matter.

Re: Could Modified Gravity Kill Planet Nine?

#113

Post by canjobear » Sat, Mar 02, 2024, 6:41 PM UTC

catlifeonmars wrote: post #105

canjobear wrote: post #100

Earlier quoted context omitted.

The first step is to propose a model that explains stuff. Then you test it on new predictions.

God did it. In all seriousness, I think the bar is higher than just “here is an explanation that makes sense”. The new model has to explain stuff better than existing established models.

Sometimes it's nontrivial to show that a theory predicts something, even if the thing is already observed.

Re: Could Modified Gravity Kill Planet Nine?

#114

Post by adastra22 » Sat, Mar 02, 2024, 6:44 PM UTC

nubinetwork wrote: post #4

Pluto is a planet ( /s but not really)

Especially when you consider that whatever planet is found won’t have cleared its orbital path (it’s in the Kuiper Belt) and so wouldn’t be Planet 9 by the IAU’s ridiculous definition. The headline is doubly wrong.

Re: Could Modified Gravity Kill Planet Nine?

#115

Post by tgv » Sat, Mar 02, 2024, 6:56 PM UTC

cratermoon wrote: post #83

Milgrom's theory has a problem. There's no theoretical basis for it. It arises from observations, but nobody can say why or how it fits in cosmology.

TBF: doesn't the same apply to dark matter? It originated because something didn't fit, and it was an easy explanation, without further evidence.

Re: Could Modified Gravity Kill Planet Nine?

#116

Post by bastawhiz » Sat, Mar 02, 2024, 7:03 PM UTC

catlifeonmars wrote: post #103

Qem wrote: post #33

Earlier quoted context omitted.

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.

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.

Re: Could Modified Gravity Kill Planet Nine?

#117

Post by lamontcg » Sat, Mar 02, 2024, 7:11 PM UTC

perihelions wrote: post #84

ano-ther wrote: post #78

Earlier quoted context omitted.

Very good question. As a non-astronomer, my guess is that we don’t observe a lot of transient black outs for example of the milky way. Which would happen if there’s a lot of these around. That gives you an upper bound of how many objects like that exist.

Microlensing events! (to be slightly more precise) https://en.wikipedia.org/wiki/Rogue_planet#Microlensing - "They found 474 incidents of microlensing, ten of which were brief enough to be planets of around Jupiter's size with no associated star in the immediate vicinity. The researchers estimated from their observations that there are nearly two Jupiter-mass rogue planets for every star in the Milky Way.[26][27][28]…

And "MACHOs":

https://en.wikipedia.org/wiki/Massive_compact_halo_object

In the 1990s I went to a talk which suggested that "WIMPs" (Weakly Interacting Massive Particles aka supersymmetric particles that would give rise to dark matter) was an acronym for "Well It Might be Physics" while MACHOs was "Maybe Astrophysics Can Help Out".

The upper bound on MACHOs has looong since eliminated them as the explanation for all of dark matter though...

Re: Could Modified Gravity Kill Planet Nine?

#118

Post by Izkata » Sat, Mar 02, 2024, 7:14 PM UTC

vundercind wrote: post #97

FrustratedMonky wrote: post #12

Earlier quoted context omitted.

Pluto should have had some grand-father clause. Keep it a planet for historical sake, but with an asterisk. So we wouldn't keep arguing about the new objects that are same size or bigger, yes, Pluto isn't a planet. But lets just go ahead and acknowledge that we are going to call Pluto a planet in 'name-only' for the memories.

Colloquial language doesn't have to follow scientific language. No facts about the physical universe changed with the name change. We can just keep calling Pluto the ninth planet, which lets the hypothesized distant planet be "Planet X" which is way the hell cooler than "planet 9". There's no need for any permission to do this.

NASA still has a page where that's the primary term: https://science.nasa.gov/solar-system/planet-x/

Re: Could Modified Gravity Kill Planet Nine?

#119

Post by lamontcg » Sat, Mar 02, 2024, 7:19 PM UTC

Izkata wrote: post #108

Arech wrote: post #37

Earlier quoted context omitted.

MOND was there years before Batygin (co-author of planet Nine hypothesis) was born.

> co-author of planet Nine hypothesis Are you referring to what he published in 2016? Because it's much older than that, we used to call it Planet X.

"Planet Nine" is a term of art that refers specifically to the 2016 theory.

"Planet X" refers specifically to a theory proposed by Percival Lowell in 1906 about the orbit of Uranus which was disproven in 1993 by Voyager 2 flyby data.

They are both more specific theories than just "a trans-Neptunian planet we haven't discovered" which is overly broad.

Re: Could Modified Gravity Kill Planet Nine?

#120

Post by russdill » Sat, Mar 02, 2024, 7:28 PM UTC

eigenspace wrote: post #25

Blows my mind that MOND still gets attention.

There's still plenty of room to study MOND. There's a lot of people outside the scientific community that seem to misjudge the general scientific consensus on it. But doing publishing research on things like MOND is still important.