
Why Can’t We Find Planet Nine? - qubitcoder
https://www.quantamagazine.org/why-cant-we-find-planet-nine-20180703/
======
chrispeel
Mike Brown, Konstantin Batygin, and Surhud More just spent time at the Subaru
telescope looking for Planet Nine. This thread summarizes their search and
what they'll do if they don't find it:
[https://twitter.com/plutokiller/status/1071978898458464256](https://twitter.com/plutokiller/status/1071978898458464256)

~~~
breck
Very cool, thanks.

> I didn't expect to get so much data; need new disks drives

Anyone know how much data they'd get in 7 days, and what models they'll run on
it to search for planet 9?

~~~
sandworm101
It isn't terribly complicated. They take images of a particular patch of sky
on two different days, then xor them against each other to detect differences.
Those differences are a moving object, something associated with our solar
system rather than the (relatively) non-moving stars in the background. That
gives you a detection and an angular speed, from which you can make
assumptions about distance (stuff in orbit further away moves slower, really
far away and it can seem to move in reverse).

Then you wait six months for earth to be on the other side of its orbit and
take some more pics based on your previous guesses about the orbit. That lets
you triangulate for a better distance estimate. Combine all the estimates and
you can match it to a reasonable orbit. Do that over and over and you get an
increasingly accurate estimate of the true orbit. And from
distance+brightness+composition you get a size/mass.

~~~
breck
Thanks! I was wondering if the devil is in the details--if the datasets are so
large (petabytes+?) that there are millions of these "differences", most of
which are false positives from factors like atmospheric noise or equipment
issues, and what the computing challenges were sifting through it all...Or
perhaps as you hint it is relatively straightforward after all.

~~~
sandworm101
With two images there are lots of false positives. But if you have three or
more you can look for stuff moving in strait lines. That can be easily
automated. Most near-earth object detection (bright, fast objects) is now
totally automated. Hunting for planet X is about finding a little smudge of
pixels moving a pixel or two to the right.

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vivekd
It's interesting how space research technology works, right now we can detect
planets around distant stars many light years away but we are still unable to
detect a planet several times larger than earth that is likely orbiting our
own sun.

I wonder if it's possible to send a probe out beyond the orbit of Neptune and
try and detect planet 9 using gravitational lensing the way we do with planets
in other solar systems.

~~~
vanderZwan
When surrounded by pitch dark the human eye is capable of spotting the light
of a candle from tens, possibly hundreds miles away at a clear night, and yet
under those conditions it would also be perfectly possible to hide an elephant
a few meters away from someone

~~~
TylerE
I call bullshit. The horizon is only about 3 miles away.

~~~
anoncoward111
Elevation significantly increases visibility, such that the longest
photographed distance on Earth is 443km :)

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benj111
Wasn't Pluto predicted before it was discovered? And wasn't the mass of Pluto
less than what was predicted?

Plutos wikipedia page seems to support this (under mass estimates).
[https://en.m.wikipedia.org/wiki/Pluto](https://en.m.wikipedia.org/wiki/Pluto)

Ps when its discovered, can the planet start with a P so the rhymes still
work?

~~~
jonathankoren
After the demotion of Pluto, I rather liked this mnemonic:

My Very Educated Mother Just Served Us... Nothing!

~~~
_kst_
I use the names of the planets to remember the mnemonic.

~~~
novaleaf
I recited the planet names to my kids as toddlers using the tune of Brahms'
Lullaby. I think it helped them.

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JamesUtah07
From Mike Brown's Twitter:

> My biggest fear, though, is the Milky Way galaxy. There are SO MANY stars
> that we tend to avoid even looking there. But our predicted region goes
> through the Milky Way. So we are going to have to deal. We're testing a
> little of that this week, (Dec 9)

If it is indeed in that region it may be a long time before we detect P9. But
I'm optimistic that they'll find it in their latest survey they did. Mike
Brown is extremely experienced at finding objects in the outer solar system so
if he's confident that they'll find it soon than so am I.

~~~
DoctorOetker
could it not be seen as an advantage? in reflection we have fourth power, but
in obstruction its inverse square of the distant (group of) stars?

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fhood
I am an idiot who doesn't know anything about astronomy, and I always assumed
that if you were looking for something like planet 9 you would attempt to
deduce its position from the gravitational effects it might have on nearby
known entities.

What am I missing in this context?

~~~
badloginagain
The article states that's exactly what they're doing. They believe there's a
planet nine based on its effect on other objects, and have a theoretical
orbital range modeled from those gravitational effects.

The problem is space is big, and space is dark, so its hard to find things
that don't generate their own light.

~~~
fhood
Ah ok, I guess I just overestimated the level of precision they were working
with.

~~~
wongarsu
Even if you could predict the exact orbit, you still don't know where on that
orbit it is right now. So even best case that's a lot of space to search.

~~~
lamellama
It seems like you could predict where it is based on the times the
gravitational effects were recorded.

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rishav_sharan
My layman-scifi-loving hypothesis is that its not a planet that we are looking
for but the remnants of the novae which birthed the solar system. this star
core is likely mostly iron and other heavier elements, and as such is
incredibly dense. It will probably be a super earth only a few times the
Earth's radius but with mass over 10x that of earth. While detecting gas
giants is easier due to their infrared signature, one such dense star core in
(or beyond) the Oort cloud would be practically invisible to us in the
infrared searches and using starlight occlusion (due to its much smaller
size).

~~~
aruggirello
I'm afraid there are no 'wild star cores' in the Solar System backyard. A
supernova remnant would either be a white dwarf, or a city-sized neutron star
(possibly a black hole) with a mass larger than that of the Sun. The latter
could go undetected, but can be safely excluded since in that case it would be
the Solar System actually orbiting it (or anyway being affected by it - the
minimum mass for a neutron star is ~1.4 Sol, and a black hole would be even
larger), and we already know the Sun's path around the Milky Way to a certain
extent. OTOH a white dwarf would likely take billions of years to cool off and
we would be able to spot it - so no, only planetary bodies may still be
lurking there undetected.

Edit - clarification.

~~~
KnightOfWords
> A supernova remnant would either be a white dwarf...

A small correction, white dwarfs are not produced by supernovae but by main
sequence stars running out of fuel.

------
zaroth
They’ll just find it, add it as a “planet”, then discover dozens or hundred of
others just like it that aren’t on the same orbital plane as our first 8, and
demote it again.

~~~
Rooster61
Unlikely. My understanding of the gravitational breadcrumbs we have seen
indicate a much larger body (~10x Earth's mass, like a smaller ice giant
similar to Uranus/Neptune), one that would almost certainly clear out the area
around it through accretion. It would be very surprising to see something with
that kind of mass still sharing an orbital neighborhood of other similar
bodies.

~~~
garmaine
It is in the Kuiper belt. It hasn’t cleared it’s neighborhood. (But then the
definition of “planet” you are referencing is utterly uselesss and misapplied
already anyway.)

~~~
bronson
It has to clear the Kuiper belt? That's a tall order!

~~~
garmaine
Who knows. The definition is vague to the point of uselessness. Pluto,
apparently, is not a planet because it hasn't cleared Neptune out of its
orbit. But then by symmetry neither has Neptune cleared Pluto out of its
orbit, so if we're to have any consistency then we can't call Neptune a planet
either. Oops.

The reality is that the IAU definition of "planet" was a very poor decision
made by people who aren't planetary scientists. The geophysical definition is
much better: an object large enough to assume spherical shape from its own
gravitational force acting on its constituent material, and too small to have
initiated fusion and be a star.

So Pluto is a planet, as is Ceres, the Moon, and a thousand other known
objects.

~~~
skykooler
The definition isn't exactly "cleared its orbit", but rather "gravitationally
dominates its orbit". Neptune fits these criteria, because while Pluto does
cross its orbit it is locked in a resonant orbit with Neptune. Similarly, the
Moon, as well as Cruithne and several other near-earth objects, are all locked
in a resonant orbit with the Earth. However the asteroid belt is not resonant
with Ceres.

~~~
Latteland
what if the earth was not a moon of another planet but was in an orbital
resonance with that larger planet. we'd still not be considered a planet. this
planet definition seems pretty arbitrary to me. the astronomy world feels like
it's self-defining something to justify pluto not being a planet.

------
Rooster61
This is almost certainly an oversimplified question born out of ignorance, but
I've always wondered why we have to rely on the light of the Sun to find
distant, dark objects. Why would it not be possible to fashion an extremely
powerful laser and sweep it across the sky in an area where gravitational
clues indicate a dark object may reside? Unless the object was utterly black
on its surface, I'd think we would be able to monitor wherever the beam
traverses and look for tell-tale spectra coming back in our direction due to
the laser being scattered by a solid object's surface.

Again, I know I am oversimplifying a complex problem, and I could speculate on
factors that might render this impractical, such as atmospheric effects or the
necessary power needed for a laser strong enough to travel to such a distant
point (it might not be physically possible to create a laser large enough to
exceed the light we would already see reflected back from the already
formidable output of the Sun), but I don't have the requisite knowledge of
physics to really make solid assumptions about such things.

~~~
avar
Relative to planet #9 we're right next to a giant nuclear fusion powered light
700,000 km across. If it isn't lighting up the surface of planet #9 well
enough for us to see it already, then we're not going to be able to construct
some laser that makes a difference.

That's before you get to the problem that space is ridiculously big, and
sweeping it across the entirety of the surface of the imaginary sphere that
might contain planet #9 would probably take millions of years (citation
needed).

~~~
LeifCarrotson
Regarding those two points:

A very good laser has a beam divergence of tenths of milliradians, or 0.01
degrees. At the radius of planet 9, this is a very large beam, and we'd have a
hard time putting out enough power to compete with the sun.

Furthermore, the sky has an area of 40,000 square degrees. Our little laser
beam projects a circle with an area of 0.0000785 square degrees. You'd need to
aim at 500 million spots to see the whole sky.

~~~
avar
> You'd need to aim at 500 million spots to see the whole sky.

That's smaller than I expected. To convert that into human scale sqrt(500
million) = 22360, in millimeters that's around 22 meters (72 ft).

So if we had a square wall 22 meters by 22 meters we could paint a grid of 1
mm by 1 mm squares on it and end up with around 500 million squares.

Then we'd need to stand there in front of the wall with a laser pointer and
shine the laser at each individual square and check the light bouncing back at
us.

~~~
hughes
Don't forget - due to the speed of light it takes 5-10 hours to get feedback
on if the square illuminated, but maybe more because you don't know exactly
how long you'll need to wait.

And there's uncertainty in whether your measurement is accurate.

And there's a background of other illuminated squares that can interfere with
your readings.

And you're spinning while this is going on.

And the target square is continually moving. You might have just missed it.

------
flareback
I hope that there is a Planet Nine. I like space related news and I think it
would be an amazing discovery

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chiefalchemist
Is there any possibility there's more than one one Planet 9? This is, in
searching for a single unseen planet, the calculations, estimates and such
will be wrong if there's actually two (or more?) extra planets.

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umeshunni
Relevant XKCD: [https://xkcd.com/1633/](https://xkcd.com/1633/)

~~~
zaroth
Is Pluto the black circle with the question mark or is that “Planet Nine”?

Not marking Pluto on this would ruin it for me, even if it’s in the “Dwarf
Planet” box.

~~~
Raphmedia
[https://www.explainxkcd.com/wiki/index.php/1633:_Possible_Un...](https://www.explainxkcd.com/wiki/index.php/1633:_Possible_Undiscovered_Planets)

~~~
zaroth
“Pluto, no longer considered a planet (it was the ninth until 2006), is not
marked on the chart, but it would be below Neptune just outside the pink
region (2,300 km diameter and 30-50 AU away).“

Yeah, that sucks.

~~~
krylon
Do not take this the wrong way, but I am always a little confused about people
feeling about the whole planet-dwarf_planet thing so strongly.

Pluto is still out there, just as it was a hundred years ago, when nobody knew
it existed. And just recently, NASA sent a probe Pluto's way, that managed to
amaze _me_ , an outspoken Pluto-hater[0]: Turns out that Pluto is much more
complex than people had anticipated.

Who cares what the IAU decides to call it? It is what it is. You could call a
rose a turd, and it still would not smell as badly. ;-)

[0] Okay, I do not _hate_ Pluto. I just wish NASA, ESA or whoever would send a
couple of probes to Uranus and Neptune. I feel very strongly about this.

------
jkingsbery
Since Pluto is _clearly_ Planet Nine, I'm assuming this was written before
1930.

(I'm only being a little facetious. According to
[https://www.amazon.com/Chasing-New-Horizons-Inside-
Mission-e...](https://www.amazon.com/Chasing-New-Horizons-Inside-Mission-
ebook/dp/B076H7LK8Y/ref=sr_1_1?ie=UTF8&qid=1545245406&sr=8-1&keywords=new+horizons),
plenty of scientists consider Pluto a planet still.)

~~~
OscarCunningham
Most definitions that make Pluto a planet would make it Planet _Ten_ , because
Ceres would also be a planet.

~~~
mcv
In the 19th century, Ceres, Pallas, Juno and Vesta were all considered planets
for a while. At some point the solar system had 12 planets. Until people
realised it was nuts to consider all asteroids planets. Just like what
happened with Pluto upon the discovery of more Kuiper Belt objects.

------
zulfishah
Were it to be confirmed, would "Planet 9" actually be classified as a Dwarf
Planet?

~~~
flukus
It's estimated to have 10 times the mass of earth, putting it up in the Uranus
and Neptune range:
[https://en.wikipedia.org/wiki/Planet_Nine#Size_and_compositi...](https://en.wikipedia.org/wiki/Planet_Nine#Size_and_composition)
.

~~~
ajmurmann
Size isn't sufficient though. It also needs to have cleared its orbit.

~~~
flukus
It's orbit is thought to range between 200 and 700 AU, which is well outside
the Kuiper belt at 30-50 AU and well inside the Oort cloud at 2,000-200,000
AU. So barring the discovery of more stuff in that area it seems to qualify.

~~~
skykooler
Would that explain why there's a gap between the outside of the Kuiper Belt
and the inside of the Oort Cloud?

~~~
flukus
That would be well beyond my knowledge or ability to say.

It certainly seems plausible, it may even explain the existence of the Kuiper
belt, being held in the balance between Neptune and this planet much like the
inner belt is by Mars and Jupiter.

------
infinity0
Could "planet nine" actually just be dark matter?

~~~
neolefty
If you define dark matter as matter whose gravitational influence we infer but
whose nature is still mysterious, then sure it is right now!

------
burtonator2011
TL;DR inverse square rule... it's super dark out there!

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darepublic
What if planet nine doesn't want to be found

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macspoofing
Wouldn't it be great if instead of a planet, this was a black hole with the
mass of a planet.

~~~
dyukqu
I am not a physicist and I don't know if that's possible, but seems unlikely,
if I understand this statement correctly: "After collapse to the neutron star
stage, stars with masses less than 2-3 solar masses should remain neutron
stars, gradually radiating away their energy, because there is no known
mechanism for further combination, and forces between neutrons prevent further
collapse."[0]

Assuming Jupiter turning into a black hole, the radius of it's event horizon
would be just 2.2 meters (and that's 8.7 millimeters for Earth and 3
kilometers for the Sun). [1]

That would take forever to _pinpoint_ an object with a radius of a few meters
(apart from its existence as a black hole).

[0][http://hyperphysics.phy-
astr.gsu.edu/hbase/Astro/blkhol.html](http://hyperphysics.phy-
astr.gsu.edu/hbase/Astro/blkhol.html)

[1][http://astronomy.swin.edu.au/cosmos/S/Schwarzschild+Radius](http://astronomy.swin.edu.au/cosmos/S/Schwarzschild+Radius)

~~~
macspoofing
I'm actually surprised how negatively people are taking this.

>but seems unlikely

I'm sure it's unlikely, but how great would it be to have a small black hole
in our backyard to potentially visit with a probe.

>After collapse to the neutron star stage, stars with masses less than 2-3
solar masses should remain neutron stars, gradually radiating away their
energy, because there is no known mechanism for further combination, and
forces between neutrons prevent further collapse."

We don't understand all the mechanisms that lead to black formation. Most
likely that kind of black hole would not be a result of neutron star merger or
supernova, but maybe it came about through another unknown mechanism
(primordial black hole?). How great would that be to find out!

>That would take forever to pinpoint an object with a radius of a few meters
(apart from its existence as a black hole).

Sure, but maybe we could detect it indirectly. Maybe this tiny black hole has
moons or satellites that can be detected. Or maybe we can detect it during an
asteroid or comet collision.

For a non-physicist, you sure have strong opinions about what is possible and
not possible.

~~~
dyukqu
Yeah, I do. I'm so glad that you've realized that, I feel flattered.

------
brlewis
I'm on my phone... Hope this link works. Around 9:30 there's a question about
planet 9 and the NASA scientist says Pluto is Planet 9.

[https://m.facebook.com/story.php?story_fbid=257826325089507&...](https://m.facebook.com/story.php?story_fbid=257826325089507&id=79209882917&refsrc=https%3A%2F%2Fm.facebook.com%2F&_rdr)

~~~
brlewis
OK the link works. The NASA scientist is Hal Weaver
[https://www.nasa.gov/mission_pages/newhorizons/launch/weaver...](https://www.nasa.gov/mission_pages/newhorizons/launch/weaver-
bio.html)

This in addition to Alan Stern's criticism of the IAU classification:
[https://en.wikipedia.org/wiki/Pluto#IAU_classification](https://en.wikipedia.org/wiki/Pluto#IAU_classification)

