Could Modified Gravity Kill Planet Nine? • Hacker News Board

Re: Could Modified Gravity Kill Planet Nine?

#121

Post by dragonwriter » Sat, Mar 02, 2024, 7:31 PM UTC

lamontcg wrote: post #119

Izkata wrote: post #108

Earlier quoted context omitted.

> 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.

Somewhat related are "Nemesis", the 1984 hypothesis of a distant companion red or brown dwarf to the Sun, and "Tyche", the 1999 hypothesis of a gas giant in the Oort Cloud.

On "Planet X" specifically, the effect it was offered to explain was determined to have been measurement error after recalculation of Neptune's mass and consequently effect on Uranus's orbit in 1993 based on Voyager 2 data.

Re: Could Modified Gravity Kill Planet Nine?

#122

Post by dragonwriter » Sat, Mar 02, 2024, 7:35 PM UTC

vld_chk wrote: post #73

Whenever I read about Planet Nine search, I have a very naive (as a non-physicist) childish question: if we detect anomaly in our Solar System just less than a decade ago and can’t find anything visual which explains it, assuming there is giant planet/piece of ice floating somewhere at the edge of Solar System, how we are sure that there are not a lot of such objects and real space is not as “empty” as we think it is…

> how we are sure that there are not a lot of such objects and real space is not as “empty” as we think it is?

There could be lots of such objects without meaningfully denting how empty we think space is.

Re: Could Modified Gravity Kill Planet Nine?

#123

Post by antognini » Sat, Mar 02, 2024, 7:48 PM UTC

sigmoid10 wrote: post #75

vikingerik wrote: post #46

Earlier quoted context omitted.

And since readers may find this interesting: The various writeups all say that relativity explained the precession of Mercury's orbit, but they never say exactly what factor was accounted for by relativity. The answer: Mercury is heavier when it is at perihelion in its elliptical orbit, because it's moving faster. The increased relativistic mass makes for increased momentum at perihelion, which carries the planet a l…

This might sound true if you have a superficial understanding of relativity (and I would not be surprised to see people repeating this stuff on the internet), but it's completely false. The correction to Mercury's orbit precession from General Relativity has nothing to do with Mercury itself (its mass, speed or otherwise). It is purely based on the sun's gravitational field. If you want to interpret it as a potential…

Yes, this is correct. The orbital precession occurs even for a point mass.

Re: Could Modified Gravity Kill Planet Nine?

#124

Post by isthatafact » Sat, Mar 02, 2024, 7:55 PM UTC

AnimalMuppet wrote: post #89

mr_mitm wrote: post #54

Earlier quoted context omitted.

> DM lets you add a new parameter (the DM density) at each point in space. That's effectively an infinite number of parameters, whereas MOND has very few. No one is doing that, though. What cosmologists do is parameterize the statistics of the DM distribution. That's one or two parameters. Then we compare observations to simulations to determine how likely the observed distribution is given the statistical properties…

That's at the universe-sized level. At the galaxy level, as you state, we say "oh, that galaxy has almost no dark matter". That's a per-galaxy parameter. At the Bullet Cluster, we say "the dark matter must be here and here". That's a point-by-point distribution.

> "That's a per-galaxy parameter."

No, variation in galaxy properties is an output, not an input, of the model.

You could decide to quantify and catalog different galaxies with one or more parameters that describe their properties. You could then compare whether that catalog is statistically consistent with the output of the model (and must take into account all uncertainties in the model and the observations).

By analogy, you can measure that different people have different heights, but it does not mean that the specific height of each individual person is a unique input parameter in any fundamental model of biology.

Re: Could Modified Gravity Kill Planet Nine?

#125

Post by Izkata » Sat, Mar 02, 2024, 8:23 PM UTC

lamontcg wrote: post #119

Izkata wrote: post #108

Earlier quoted context omitted.

> 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.

The general theory was called planet X before pluto was demoted; X just means 10.

https://science.nasa.gov/solar-system/planet-x/

Re: Could Modified Gravity Kill Planet Nine?

#126

Post by plank » Sat, Mar 02, 2024, 9:07 PM UTC

antognini wrote: post #123

sigmoid10 wrote: post #75

Earlier quoted context omitted.

This might sound true if you have a superficial understanding of relativity (and I would not be surprised to see people repeating this stuff on the internet), but it's completely false. The correction to Mercury's orbit precession from General Relativity has nothing to do with Mercury itself (its mass, speed or otherwise). It is purely based on the sun's gravitational field. If you want to interpret it as a potential…

Yes, this is correct. The orbital precession occurs even for a point mass.

Former physicist myself: Not sure I understand your (and parents) argument.

Given the Suns gravitational field, and the orbit that Mercurius has, the speed of the planet can be determined. So the argument that a point mass would have the same precession does not dispute the argument that it is the relativistic mass of the point mass/planet that determines the precession.

I will not venture into arguments* whether grandparents explanation is ‘correct’, ‘best’ or ‘useful’, there is an equivalence between ‘gravitational field + orbit’ being the reason and ‘(relativistic) speed + orbit’ being the reason. * A sybling comment states that relativistic mass is avoided in modern physics. As someone who did physics 30 years ago, I can not deny or corroborate this statement. And indeed, the explanation of grandparent is not the way I myself think about the perihelium precession. But that does not make me certain enough to say it is ‘wrong’.

Edit: typo ‘of’ —> ‘or’

Re: Could Modified Gravity Kill Planet Nine?

#127

Post by AnimalMuppet » Sat, Mar 02, 2024, 9:39 PM UTC

isthatafact wrote: post #124

AnimalMuppet wrote: post #89

Earlier quoted context omitted.

That's at the universe-sized level. At the galaxy level, as you state, we say "oh, that galaxy has almost no dark matter". That's a per-galaxy parameter. At the Bullet Cluster, we say "the dark matter must be here and here". That's a point-by-point distribution.

> "That's a per-galaxy parameter." No, variation in galaxy properties is an output, not an input, of the model. You could decide to quantify and catalog different galaxies with one or more parameters that describe their properties. You could then compare whether that catalog is statistically consistent with the output of the model (and must take into account all uncertainties in the model and the observations). By an…

Let me change your analogy. You take each person, and measure their height. You also "measure" how tall they "should be". You then show that the differences between their actual height and the height they should have had fits a model. That's nice, but for each person, you still assigned a value for the difference between how tall they are and how tall they should have been.

That's what I mean by "it's a per-galaxy parameter". For each galaxy, to explain the behavior of that galaxy, you're saying "it must have X amount of dark matter".

Re: Could Modified Gravity Kill Planet Nine?

#128

Post by throwawaymaths » Sat, Mar 02, 2024, 9:47 PM UTC

ryandrake wrote: post #94

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.

...or an upper bound of how big they might be. For all we know, the galaxy might be full of really dark bowling ball sized objects. We'd have no way of observing them, right?

Really dark bowling balls would still exhibit blackbody radiation

Re: Could Modified Gravity Kill Planet Nine?

#129

Post by bradley13 » Sat, Mar 02, 2024, 9:48 PM UTC

Dark matter is the modern ether. There is zero evidence that it exists. What we have are distant phenomena that we can't explain.

If not MOND, then something like it.

Re: Could Modified Gravity Kill Planet Nine?

#130

Post by jiggawatts » Sat, Mar 02, 2024, 9:49 PM UTC

semi-extrinsic wrote: post #107

mike_hock wrote: post #98

Earlier quoted context omitted.

What if dark matter is primordial bowling balls!

So these are called "snowballs" and are excluded as dark matter candidates because we would then see a much larger number of extra-solar asteroids/meteorites. Cf. e.g. section 2 of the reference below. https://web.archive.org/web/20180723032406/http://iopscience...

I love how every time someone comes up with some random notion, a scientist can point at a chart and say “we’ve thought of that already and excluded it with experiment CoolName.”