
‘Goblin’ world found orbiting at the edges of the Solar System - okket
https://www.nature.com/articles/d41586-018-06885-1
======
autocorr
They don't explain it in detail, but the reason it's consistent with Planet IX
[1] is its orbit has specific properties that require a near pass with a large
body in the outer solar system (basically the definition of Planet XI). That
seems like it may be easy to do by other means, but if you've played Kerbal
Space Program you know how difficult it can be to change all six orbital
elements in the way you want to. For example, a close pass to Jupiter could
launch something extremely far our, but it would not change it's perihelion or
closest approach to the Sun. Like a Homann transfer, going to a larger orbit
requires at least two "kicks": one to kick it into an ellipse and a second at
the outer point. Planet XI would give the kick at a far distance to get many
of these curious Trans-Neptunian Objects out to crazy perihelion distances
like 65 au. There's also a specific effect to the inclination and argument of
perihelion, but I don't understand how these work as well.

The amount of sky we have to look for Planet IX is enormous (and given how
faint it is likely to be), and the evidence so consists of about ten of these
TNOs with these orbits (not many, but it's very hard to make these orbits).
But there are potentially hundreds of bodies with orbits like these waiting to
be found by a the soon to be constructed LSST, which is an 8 meter telescope
that will survey its entire sky every 3 days. With many more of these orbits
to predict from, we may really narrow down the search space to look for it,
and ultimately test its existence.

[1] Predicted by Konstantin Batygin and Mike Brown, the latter of which played
a big part in helping to disqualify Pluto as a planet by discovering Eris. I
know from a lunch-talk that he greatly relishs the thought of both demoting
the former Planet XI and predicting the new one.

~~~
Kapow
I think you have some XIs that should be IXs.

~~~
wmeredith
And an it’s that should be an its. This was hard to read.

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rrmm
The pre-print pdf is
[https://arxiv.org/pdf/1810.00013.pdf](https://arxiv.org/pdf/1810.00013.pdf)

Inclination of 11.669 degrees

The diameter listed in the paper is 300km assuming a moderate albedo of 15%.

The paper suggested this object provides more evidence of the existence of
Planet X/IX hypotheses of the authors (Trujillo/Shepard/et.al.) and
Brown/Batygin.

~~~
interfixus
So, we can detect - _see_ , dammit - something perhaps only 300km across at
twice the distance to Pluto, and by reflected light alone.

Every now and again, my intuition breaks utterly down.

~~~
jcims
If you trace the path of individual photons it seems implausible. That somehow
a photon leaves the sun, travels possibly weeks until it strikes some
nanoscopic face of a crystal on the surface of this object, which is rotating
and travelling tangentially at several km/sec, and departs at such a resolved
and precise an angle that it makes it almost all the way back to the sun,
passes through the atmosphere of an orbiting body travelling 30km/sec, lands
on a finely curated layer of aluminum where it is reflected yet again and
lands on a square likely smaller than an index card.

~~~
kopo
Well since we still don't have any great explaination for the 200 year old
double slit experiment, how do you know it is photons moving around? Could be
star sized tsunami waves of light trashing about collapsing into photons when
finally measured.

~~~
abdullahkhalids
In fact, we do have an explanation for it - its called quantum theory and it
explains and predicts not only the double slit experiment but many other much
more complicated phenomena.

~~~
thaumasiotes
Meh. Predicts, yes. But I expect you could poll massive support for the
proposition "the standard model is an accurate description of what happens,
but not an explanation of why things would be like that".

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Tor3
Why 'Goblin', though? There's no explanation in the article, nor on the
wikipedia page, and I can't imagine an explanation by myself.

~~~
billysielu
It's because the goblins live there, duh :)

~~~
0xdeadbeefbabe
Gollems probably live there too.

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SideburnsOfDoom
[https://en.wikipedia.org/wiki/2015_TG387](https://en.wikipedia.org/wiki/2015_TG387)

A short article, but in some ways more details. And some links

~~~
__exit__
Thank you. So many new terms there: sednoids, Oort clouds...

~~~
SideburnsOfDoom
"Sednoid" is a new term to me too, though clearly it means "an object like
e.g. Sedna".
[https://en.wikipedia.org/wiki/90377_Sedna](https://en.wikipedia.org/wiki/90377_Sedna)

Kuiper belt and Oort cloud are well-known decades-old terms

[https://en.wikipedia.org/wiki/Kuiper_belt](https://en.wikipedia.org/wiki/Kuiper_belt)

[https://en.wikipedia.org/wiki/Oort_cloud](https://en.wikipedia.org/wiki/Oort_cloud)

~~~
__exit__
First thing in my mind when reading "sednoid" was "mathematics!".

As for the rest of the terms, they are new to me due to my non-familiarity
with cosmology in general.

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wycy
I gather from this article that its orbital radius varies between 65 AU and
2300 AU, which is an extremely eccentric ellipse. I wonder if that high
eccentricity would disqualify it from "planet" status. Most other bodies
considered "planets" are far closer to circular.

It's also unclear to me if the Goblin orbits in the usual planetary plane.

~~~
pas
Probably the simple fact that it does not clear its orbit disqualifies it from
planet status. And it's probably too small to be in hydrostatic equilibrium
(not enough self-gravity to force it to a roundish shape).

~~~
schindlabua
Out of interest: I looked up "Hydrostatic equilibrium" and it says "[thing is
in equilibrium] when the flow velocity at each point is constant over time".
How exactly does that statement relate to what you said?

~~~
grigjd3
Most objects in space have some amount of spin to them. That is particles
throughout a body, each with a velocity that may be changing, but the change
in velocity with position is smooth, or the object would be ripping itself
apart. This means the object has flow. A noticeably misshapen object will not
have relatively constant speed and angular direction as a function of latitude
and radius. However, objects that gravity rounds out do have this feature (to
within an approximation, there is still weather). Thus the term hydrostatic
equilibrium. The reason we don't just say "circular" is that this does not
include oblate objects, which result from higher spin.

~~~
schindlabua
It took some additional help from IRC but I think I got it now. Thanks for
your answer!

In general, while total angular momentum is conserved, rigid bodies tend to
rotate in an unstable fashion, because rigid-body forces apply different
amounts of torque to different parts of the object. Which I kind of knew about
but I didn't connect the dots. Hence no constant speed and angular direction,
and no hydrostatic equilibrium. That was the missing link for me!

~~~
pas
Now I think I don't understand it! :o)

So, flow velocity is simply the fluid mechanical velocity vector
field/mapping. And it has to be constant, otherwise the object would not be in
equilibrium, but it'd be still flowing (as in it would have parts that are
going somewhere).

Now I think this definition you have found is not directly applicable to
rotating celestial bodies, as the point velocity is a vector, and it
constantly changes due to the rotation.

So probably a higher order derivative is zero, and that's the condition that
we should use.

Or of course we can transform to a non rotating frame.

But what the parent poster said confuses me: "a velocity that may be changing,
but the change in velocity with position is smooth, or the object would be
ripping itself apart. This means the object has flow." You can have smooth and
constant rotation but with many axes (tumbling), so I don't really see how
this gets us to roundisness.

As I understand the concept, the point is that "the object doesn't have parts
that want to fall toward its center of gravity, but can't because rigid
forces", because it's big enough that gravity creates enough pressure and heat
that everything becomes plastic over thousands of years, and thus flows. (But
this doesn't make much sense, because cold enough rock is pretty stable - as
far as I know - so the material will only allow gravity to overcome it if it
undergoes enough crystal structure faults [due to radioactive decay or
exogenous damage, such as micrometeorites] - so the flow rate is constant,
zero, even if there are stresses and forces that would increase the flow.)

~~~
grigjd3
You have a few misconceptions and you are waaaaay overthinking this. The
primary measure for this equilibrium is the time derivative of the mass-
density-velocity, or momentum-density if you like. Now imagine a cube spinning
on a primary axis. Since a cube is not constant radius, there are times where
matter exists at a given point and other times where the matter does not exist
at that point. Clearly, the time derivative of the momentum-density is not
zero. This is not the case for a sphere as any point that has mass under the
spinning sphere will have mass at all later points in time. The reason we
don't just say sphere, though, is this kind of equilibrium accounts for oblate
objects as well, and many planets are oblate.

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walrus01
This is now the third Sednoid discovered.

[https://en.m.wikipedia.org/wiki/Sednoid](https://en.m.wikipedia.org/wiki/Sednoid)

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pard68
> popularly known as Planet Nine

Poor Pluto...

~~~
zumu
It used to be popularly called Planet X. I've heard it to referred to as
Planet Nine

~~~
BuckRogers
Yeah, I can't get Planet X out of my mind. It sounds better anyway, with a
little more mystery.

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tibbon
What is the far out giant planet it mentions?

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swarnie_
Slowly we're getting closer to proving another planet is out there. I do like
it when nutters spewing out 100's of mad theories a month get one correct, in
this case its the existence of Nibiru.

~~~
rrmm
The fun thing is that Planet X/Nibiru/Nemesis are basically crazy talk (but
different from each other), and Planet IX is quite possibly true, but also
distinct from the foregoing. But they are all synonymous in internet
conspiracy land, (and in their eyes, corroborate each other's existence).

~~~
swarnie_
My understanding is the term "Planet X" came about while we still called Pluto
the 9th planet making Planet IX and X the same theory.

Maybe i'm way off here, the government mind control satellites seem to have
got to my reviewers.

~~~
rrmm
The paper this article refers to actually refers to 'Planet X' as the
theorized large planet in its section on orbital stability models.

I think the planet X moniker originally meant 'x' as unknown (it was coined
during the search that found Pluto). Nowadays, it has been used to refer to
several distinct theories including Planet IX. I dunno when exactly the X got
re-interpreted.

What I do know is WE'RE DOOOOMED! (according to the internet).

~~~
etatoby
As soon as the Annunaki come to take what's theirs, yes we are. (j/k)

