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Caltech Researchers Find Evidence of a Planet Beyond Pluto

caltech.edu

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Re: Caltech Researchers Find Evidence of a Planet Beyond Pluto

#71

Post by logicallee »

School-Cotton wrote: post #70
logicallee wrote: post #54

Earlier quoted context omitted.

So I might not have a great enough understanding but isn't 3.8 sigma not high for this kind of announcement (with the current title)? if someone wants to report finding a planet in the solar system ten times the size of Earth wouldn't they do so at more than @3.8 sigma. In particular, the prior is quite low because it would affect a bunch of other astronomical calculations, wouldn't it? So you would want the confiden…

I wish people wouldn't downvote you without explaining why you're wrong. I have zero education in statistics and your comment makes intuitive sense to me so I'd like to learn more.

Before your comment, I edited it to be much more civil and less presumptuous, my original phrasing had included "3.8 sigma is fuck-all", I was briefer, more resolute, and had just included my first link. (You can see evidence of my original phrasing if you remove the hedging I added.)

But the downvotes meant I'm probably wrong - rather than delete I edited it to be much nicer so I could find out why I'm wrong.

Re: Caltech Researchers Find Evidence of a Planet Beyond Pluto

#73

Post by scott_s »

logicallee wrote: post #71
School-Cotton wrote: post #70

Earlier quoted context omitted.

I wish people wouldn't downvote you without explaining why you're wrong. I have zero education in statistics and your comment makes intuitive sense to me so I'd like to learn more.

Before your comment, I edited it to be much more civil and less presumptuous, my original phrasing had included "3.8 sigma is fuck-all", I was briefer, more resolute, and had just included my first link. (You can see evidence of my original phrasing if you remove the hedging I added.) But the downvotes meant I'm probably wrong - rather than delete I edited it to be much nicer so I could find out why I'm wrong.

Perhaps the downvotes are coming because their arguments do not rely just on the improbability of that outcome.

Re: Caltech Researchers Find Evidence of a Planet Beyond Pluto

#74

Post by scott_s »

tdaltonc wrote: post #25

> Batygin and Brown inferred its presence from the peculiar clustering of six previously known objects that orbit beyond Neptune. This raised a big red flag in my mind. This must produce a literally astronomical multiple comparisons problem. Yes they reported sigma = 3.8, but if they didn't do their multiple comparisons correction right (which I am in no position to determine), they're basically reading tea leaves. I…

The authors are not relying just on the improbability of that arrangement, but also on its stability. From one of the authors, on http://www.findplanetnine.com/:

"Mike and I were genuinely perplexed. Could the confinement of the orbits be due to an observational bias or a mere coincidence (after all, we are talking about six objects here - not exactly “big data”)? Thankfully, the probability of the observed alignment being fortuitous can be assessed in a statistically rigorous manner, and clocks in at right around 0.007%. Not a great gamble. Moreover, application of simple perturbation theory (or direct numerical integration) demonstrates that if allowed to evolve under the gravitational influence of Jupiter, Saturn, Uranus and Neptune, the orbits would become randomly oriented on timescales much shorter than than the multi-billion year lifetime of the solar system. So the dynamical origin of the peculiar structure of the Kuiper belt cannot be outsourced to the distant past - something is holding the orbits together right now."

Re: Caltech Researchers Find Evidence of a Planet Beyond Pluto

#75

Post by bzbarsky »

logingone wrote: post #15

I don't seem to have a grasp of our visibility of our solar system. We can see a number of the planets with the naked eye, a number of the moons with binoculars and a few hundred $/£/€/.. telescope. Yet even with these great big radio telescopes, antenna arrays, Hubble, etc, we seem to be quite unaware of what's in our neighbourhood. Anyone have figures of how much we've surveyed?

It's all about distances. The planets we can see with the naked eye are at most 11AU from us (Saturn is at 10AU at aphelion).

For comparison, Neptune is never closer than about 29AU to us. Brightness goes down as the square of the distance. But it gets worse, because that's brightness at given source brightness. Planets don't radiate intrinsically; they reflect sunlight. And sunlight brightness drops off as the square of distance from the sun. Which is to say that even if they were the same size and albedo (they're not), Neptune would appear about 81 times dimmer than Saturn to us. Oh, also (linear) angular size goes down linearly with distance. So if you want to see the two planets as disks of the same size, you need to have a field of view three times narrower, which means you need 9 times as many of them to cover the sky. And if you want the same amount of light gathered, you have to spend 81 times longer gathering it, per the above brightness calculation.

All of which is to say that the time needed to do an "equivalent" sky survey N times further away (within the solar system, where all the illumination is coming from the sun) scales as N^6. And planets do move, so you might still miss one if you survey a place where it's not yet, then take a while to get to where it used to be, such that it has moved. This is why Neptune was found via its gravitational interactions followed by a survey of a small part of the sky, not brute-force observation.

Anyway, back to the topic at hand, the proposed Planet Nine orbits 20 times farther out than Neptune. 20^6 is 64 million. So if we assume we've pretty much surveyed everything out to the radius of Neptune's orbit at some resolution, and then spent about 64000 times as much time surveying stuff out to the distance Planet Nine is proposed to be at, at the same resolution (I doubt we have), then we've probably surveyed about 0.1% of the stuff out there.

I can't tell you what the actual number is, unfortunately, but I suspect the answer is we haven't really done very good systematic surveys out at that distance.

Re: Caltech Researchers Find Evidence of a Planet Beyond Pluto

#76

Post by scott_s »

gregwtmtno wrote: post #28
nkoren wrote: post #12

If it holds up, this is fantastic. Given how interesting the Pluto system has turned out to be, I can only fantasise about a potential super-earth system relatively nearby. However, to be completely pedantic: would this actually be a planet ? Or still a dwarf planet , despite its massive size? Keep in mind that the definition of planethood is not only that it's large enough to be rounded by its own gravity, but that…

> However, to be completely pedantic: would this actually be a planet? This is one of the paper's author's answer to that question. http://www.findplanetnine.com/2016/01/is-planet-nine-planet....

For those that won't click through, a very short explanation given by the author: "Is Planet Nine gravitationally dominant? I think it is safe to say that any planet whose existence is inferred by its gravitational effects on a huge area of the solar system is gravitationally dominant."

He then goes on to give a more technical argument.

Re: Caltech Researchers Find Evidence of a Planet Beyond Pluto

#77

Post by bzbarsky »

qb45 wrote: post #43
antognini wrote: post #9

Earlier quoted context omitted.

They do, and at the end of the press release it says that Brown has begun an observing campaign to find the planet. Unfortunately detecting these very distant objects is very difficult. The light we see from them is reflected light from the Sun, so their apparent brightness goes as 1/r^4, not 1/r^2 like most objects. And because they are so close, there is a strong parallax from the motion of the Earth. THe consequen…

Are you sure it's r^4 , not 4r^2 ?

Yes, it's definitely r^4. If we assume that the planet is far from the sun compared to the earth, such that the earth-planet distance and the sun-planet distance are both approximately r, then the amount of light the planet gets from the sun is proportional to 1/r^2. Then the planet reflects that light in all directions, and the fraction of it that comes back to us on earth is also proportional to 1/r^2. 1/r^2 * 1/r^2 = 1/r^4.

Re: Caltech Researchers Find Evidence of a Planet Beyond Pluto

#78

Post by daveguy »

logicallee wrote: post #71
School-Cotton wrote: post #70

Earlier quoted context omitted.

I wish people wouldn't downvote you without explaining why you're wrong. I have zero education in statistics and your comment makes intuitive sense to me so I'd like to learn more.

Before your comment, I edited it to be much more civil and less presumptuous, my original phrasing had included "3.8 sigma is fuck-all", I was briefer, more resolute, and had just included my first link. (You can see evidence of my original phrasing if you remove the hedging I added.) But the downvotes meant I'm probably wrong - rather than delete I edited it to be much nicer so I could find out why I'm wrong.

Upvoted for tempering the aggression and because I agree with you. As far as I can tell your interpretations of the statistics are correct. I don't see how an argument can include information stronger than the statistics of indirect detection of the object (without direct detection).

Re: Caltech Researchers Find Evidence of a Planet Beyond Pluto

#79

Post by mkrfox »

jschwartzi wrote: post #42
daveguy wrote: post #13

Earlier quoted context omitted.

I would expect a smear with characteristics of the planet would be as effective to identify it as a single snapshot. Wouldn't the direction be well identified from indirect detection? Is the blur also less effective because of increased background exposure?

Well, if the smear takes the light level below the noise floor of the sensor then you can't even say that there's a smear.

That's why you don't see any cars in long exposures of highways.

Re: Caltech Researchers Find Evidence of a Planet Beyond Pluto

#80

Post by speeder »

SolarNet wrote: post #35
theptip wrote: post #29

Did anyone else notice that the rendering of the orbits in the article looks strikingly similar to Kerbal Space Program's orbit UI?

Or is it perhaps the other way around. Did Kerbel use the same UI as a popular rendering program used by astronomers.

I know that the main author of KSP was a big fan of Orbiter before making KSP, and I think he was aware of other astronomy software.
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