Sednoids and the Mysterious Missing Planet X

1958 Solar System poster, 1888 wood engraving for Flammarion's pop science book, B movies, Superman comics. (
Remembering yesteryear: a Solar System poster, the Flammarion picture, movies, and atomic angst.

As I’ve said before, this isn’t the world I grew up in.

Holf Weiher's Euler Diagram: IAU Executive Committee labels for objects in the Solar System. (October 2022)Back then, the Solar System had nine planets, assorted moons, and asteroids. Plus, of course, the sun.

Now we’ve got planets, dwarf planets, minor planets, natural satellites, trans-Neptunian objects, plutoids, comets, centaurs, and small Solar System bodies.

Just to keep things interesting, definitions for the new labels overlap. Some labels, like plutoids, didn’t catch on; and it all keeps changing as we collect more data.

This week I’ll be talking about Sednoids, another subset of trans-Neptunian object;1 along with whatever else comes to mind.

“All the News That’s Fit to Print” — and Some That Isn’t

E. Weiss drawing: 'Great Comet of 1861' (C/1861 J1, comet Tebbutt). From 'Bilderatlas der Sternenwelt', E. Weiß. (1888) via Wikipedia, used w/o permission.
Great Comet of 1861, drawing by E. Weiß. (1888)

Evening Post's 'A Startling Scientific Prediction.' (Volume LVI, Issue 26, July 30, 1898, Page 4 (Supplement)) via Papers Past“Planet X” popped up in my news feed a few times this summer, with the usual mix of science and silliness:

About that “Scientist admits” one: the Daily Express piece starts with “Known as the Nibiru cataclysm…” and ends with a link to an article in BBC Science Focus:

Robert Matthews said the odds of something big from the outer Solar System hitting Earth were “mercifully small”: with no numbers given.

My guess is that I’ve got a better chance of winning Minnesota’s Powerball® and the Irish Sweepstake on the same day. Without entering either. Speaking of which, flipping a coin apparently doesn’t yield 50-50 odds. A recent study says it’s more like 51-49.2

A few scientists say “Planet X” left the Solar System long ago. I think they’ve got a point, and that’s part of what I’m talking about this week.

But I also suspect that Planets Y and Z may be lurking in the Solar System’s borderland.


Anonymous(?) French(?) artist's cartoon of a destructive comet. (1857)Possible planets Y, Z, and whatever would come after Planet Z, don’t fill me with existential dread.

That may take a little explaining.

Or maybe not.

Either way, let’s (briefly) recall a still-famous apocalyptic prognostication.

The year was 1910. Scientists, using recently-upgraded spectroscopy tech, detected cyanogen in the tail of Halley’s Comet.

So far, so dull.

But wait! There’s more! A famous newspaper quoted a famous scientist, and cyanogen metaphorically hit the fan.

“…The New York Times reported that the noted French astronomer, Camille Flammarion believed the gas ‘would impregnate that atmosphere and possibly snuff out all life on the planet.’

“Most scientists sought to reassure the public. The famed astronomer Percival Lowell explained that the gases making up Halley’s tail were ‘so rarefied as to be thinner than any vacuum.’

“But the damage had already been done. People rushed to purchase gas masks and ‘comet pills.’ The New York Times reported that ‘terror occasioned by the near approach of Halley’s comet has seized hold of a large part of the population of Chicago.’…”
(“Ten Notable Apocalypses That (Obviously) Didn’t Happen“, Mark Strauss, Smithsonian Magazine (November 12, 2009))

Time passed. But not the enduring appeal of scientific-sounding looming dooms and the more traditional End Times Bible Prophecies.3

Both of which I find tiresome, and that’s another topic for a week when there’s either nothing else going on: or an outstandingly silly example has been getting attention.

Sedna, Sednoids, and Orbits: Traces of a Missing World?

Yukun Huang (University of British Columbia, Canada)'s illustration: Sednoids; orbits of Sedna, 2012 VP113 ('Biden'), 2015 TG387 (541132 Leleākūhonua). (2023) via Sky and Telescope, used w/o permission.
Yukun Huang’s illustration: Sednoids; orbits of Sedna, 2012 VP113, 2015 TG387 (541132 Leleākūhonua).

‘Planet X’ May Have Left Our Solar System Billions of Years Ago
Emily Lakdawalla, Sky and Telescope (October 4, 2023)

“When Sedna was discovered more than a decade ago, its orbit — far beyond that of Neptune — baffled astronomers. Unlike most such far-out objects, Sedna never comes anywhere near Neptune. Its closest approach to the Sun is 76 a.u., more than twice Neptune’s average distance….”

“…Now, new work presented by Yukun Huang (University of British Columbia, Canada) at the 55th meeting of the Division for Planetary Sciences of the American Astronomical Society suggests that there is no such planet — at least, not anymore. While encounters with an outer-outer planet could have established the orbits of Sedna and two other, more recently discovered ‘Sednoids,’ that larger world must have been ejected from the solar system during its chaotic childhood 4.5 billion years ago….”

So far, we know of four Sednoids. I’m pretty sure 2021 RR205 got left out of this article because its perihelion is less than 60 a.u. — astronomical units, the distance between Earth and our star.

Sednoid nameSemimajor axisPerihelionInclination 
90377 Sedna506 a.u.76 a.u.12 
2012 VP113262 a.u.81 a.u.24.1 
2015 TG387 Leleakuhonua1090 a.u.65 a.u.11.7 
2021 RR205990.9 a.u.55 a.u.7.6 
Sednoid orbits, expanded from table in Sky and Telescope. (October 4, 2023)

Beyond the Kuiper Cliff: An Unexpected Void and Wandering Worlds

S. Sheppard / Carnegie Inst. of Science's diagram: Sedna, 2012 VP113, Kuiper belt and Solar System planet orbits. via Sky and Telescope. (2014 )One thing that’s distinctive about the Sednoids is that they’re well outside the Kuiper cliff.

The Kuiper cliff is 47.8 a.u. from the Sun. That’s where objects have a 1:2 resonance with Neptune.

Orbital resonance. Yeah. That’s almost technobabble, so here’s a quick definition. If something goes around the Sun once while Neptune goes around twice, it has a 1:2 resonance with Neptune.

Orbital resonances matter, since they affect the orbits of planets — and everything else on the roster of Solar System objects.

The Kuiper cliff was unexpected, since mathematical models said there should be a mess more objects beyond that 1:2 resonance. Maybe there are; but if so, they’re well beyond the ‘cliff’.

The Kuiper belt, on the other hand, wasn’t unexpected.

After Clyde Tombaugh discovered Pluto, that was in 1930, Frederick Leonard and Armin Otto Leuschner said there were probably many more objects beyond Neptune’s orbit.

WilyD's chart of the outer Solar System, from Jupiter's orbit to 60 astronomical units (AU) from the Sun. Epoch January 1, 2015.And, perhaps more to the point, in 1951 Gerard P. Kuiper published “On the Origin of the Solar System”.

He described an area where we’d expect to find comet-like objects.

“…The outermost region of the solar nebula, from 38 to 50 astr. units (i.e., just outside proto-Neptune), must have had a surface density below the limit set by equation (7). The temperature must have been about 5-10’K. when the solar nebula was still in existence (before the proto-planets were full grown), and about 40°K. thereafter. Condensation products (ices of H20, NH3, CH4, etc.) must have formed, and the flakes must have slowly collected and formed larger aggregates, estimated to range up to 1 km. or more in size. The total condensable mass is about 1029 g., but not all of this could be collected. These condensations appear to account for the comets, in size, number and composition.

“The planet Pluto, which sweeps through the whole zone from 30 to 50 astr. units, is held responsible for having started the scattering of the comets throughout the solar system. Pluto’s perturbations will have caused initial, near-circular, cometary orbits to become moderately elliptical; thereupon stronger perturbations by Neptune and the other major planets will have scattered them even more broadly….”
(“On the Origin of the Solar System“, Gerard P. Kuiper, Proceedings of the National Academy of Sciences (January 15, 1951))

Julio Ángel Fernández published a paper in 1980, saying there should be a belt of comets just outside Neptune’s orbit.

And that since for every comet falling in from the Oort cloud, 600 (probably, based on simulations) headed out into interstellar space, the just-outside-Neptune’s-orbit belt was where most observed comets came from.

A sizable roster of other scientists published other research before and after. Then, in 1992, still another researcher spotted 1992 QB1 — renamed 15760 Albion in 2018 — and so far we’ve charted upward of a thousand other objects in and beyond the Kuiper belt.4

Next: the Oort cloud, then back to Sednoids.

Charting the Borderlands of Sol

NASA/Caltech's illustration PIA05569: Sedna Orbit Comparisons: four panels showing the location of the newly discovered (illustration released 2004) planet-like object Sedna. Moving clockwise from upper left, each panel zooms out. The first panel shows orbits of the inner planets, asteroid belt and Jupiter's orbit. The second panel shows orbits of the Solar System's giant planets, Pluto and the Kuiper belt. Below that, at lower right, are orbits of the giant planets, Pluto, and Sedna (red ellipse). Finally, at lower left, Sedna's orbit and the (probable) inner part of the Oort cloud. NASA/Caltech image released March 15, 2004, via JPL/NASA, used w/o permission.
Solar System orbits, size comparison from NASA/Caltech.

The Oort cloud — when I started writing this bit, I remembered one of my favorite quotes:

“When we try to pick out anything by itself, we find it hitched to everything else in the universe”
(John Muir, quoted in a book, article, essay, letter, or something Terry Gifford (wrote?). Possibly associated with Muir’s “My First Summer in the Sierra” (1911).)

I also remembered that I have to wrap up “Sednoids and the Mysterious Missing Planet X” on Thursday. So I’ll make this brief. Brief or me, that is.

Anyway, we’ve been seeing comets — very likely since the first of us looked up as one was passing by. With no street lights dimming our night vision, comets can be spectacular.

Someone left us the first written record of a comet, but I haven’t learned who or when.

My guess is that it’s someone who worked in Sumeria, about five and a half millennia back. That’s when cuneiform was the latest thing in informational storage and retrieval tech, and when folks started systematically tracking lights in Earth’s sky and recording their data.

Sumerians were the first astronomers (and astrologers). The first we know of, at any rate.

About two centuries back, we developed new statistical analysis tools — realized that positions of the stars and planets lack the significance astrologists had assumed, and that’s yet again another topic.

Where was I?

The Oort cloud.

Anything, everything, and the universe.



Somewhere along the line, we accumulated a mildly-complete record of showy comets. We also learned that whatever the things were, they were well outside Earth’s atmosphere.

Edmond Halley dug through available records and said that that the comets of 1531, 1607, and 1682 were the same comet. He also said that the thing would show up again in 1759, which it did.

Halley’s Comet appears in Earth’s sky every 75 to 79 years.

We’ve since learned that it’s a short-period comet. Today’s “short-period comet” label covers comets which take less than two centuries to go around the sun.

Some, like X/1991 G1, go around once every four or five years; but those tend to be so faint, it takes a decent telescope to spot them.

The bright ones tend to come through at intervals on the scale of a human lifetime, so it’s no wonder it took folks a while to realize the things were predictable.

Oddly enough, a list of numbered comets has orbital periods running from a little over three to over 365 years, and includes comet 1P/Halley. But a list of Halley-type comets doesn’t include Halley’s Comet.5

There may be an interesting story behind that. But if there is, I don’t know it: and don’t have time to dig through humanity’s archives to find it. Not this week.

Out of the Ecliptic, Beyond the Kuiper Belt

ESO/K. Meech et al.'s illustration of 'Oumuamua's path through the Solar System. (2017) via Wikimedia Commons, used w/o permission.Fast-forward from 1759 to now.

Scientists have learned that short-period comets like Halley’s generally orbit in or near the ecliptic.

That’s the plane of Earth’s orbit. It’s also pretty close to the average orbital plane of the Solar System’s planets and asteroids.

That gets us back to the Kuiper belt, which is also more-or-less in the ecliptic.

Tracking short-period comets back to their origin, we’ve learned that they come from the Kuiper belt. For the most part.

But not all comets are short-period comets. Some come in on orbits that make sense only if their origin is well beyond the Kuiper cliff. And their orbits aren’t necessarily lined up with the ecliptic.

Then there are things from the depths of interstellar space, but that’s for another time.

Many or most scientists figure there’s something they’ve been calling the Oort cloud: out beyond the Kuiper cliff, and the orbits of known trans-Neptunian objects, including Sedna.

I think the odds are good that we’ll find “Oort objects”, or whatever the label will be: but until we do, the Oort cloud is a theoretical object.

One of the reasons I strongly suspect the theoretical Oort cloud model describes something that’s real is that very-long-period comets, those with orbital periods on the order of a millennium, exist.

They come from somewhere: and a roughly-spherical cloud of leftover material from the Solar System’s formation seems like a least-unlikely explanation.6

To be Continued

NASA's illustration: the Kuiper Belt and Oort Cloud in relation to inner solar system. (2016)There’s more I was going to talk about this week: including what Yukun Huang and other scientists had to say about Sednoids, and why Planet X may be long gone.

But this is all I have time for. So this story is “to be continued” until next Saturday.

Meanwhile, the usual links; slightly focused on the Solar System’s smaller objects:

1 Labels for assorted Solar System objects:

2 What are the odds:

  • Wikipedia
  • Powerball — Minnesota Lottery
  • Fair coins tend to land on the same side they started: Evidence from 350,757 flips
    František Bartoš, Alexandra Sarafoglou, Henrik R. Godmann, Amir Sahrani, David Klein Leunk, Pierre Y. Gui, David Voss, Kaleem Ullah, Malte J. Zoubek, Franziska Nippold, Frederik Aust, Felipe F. Vieira, Chris-Gabriel Islam, Anton J. Zoubek, Sara Shabani, Jonas Petter, Ingeborg B. Roos, Adam Finnemann, Aaron B. Lob, Madlen F. Hoffstadt, Jason Nak, Jill de Ron, Koen Derks, Karoline Huth, Sjoerd Terpstra, Thomas Bastelica, Magda Matetovici, Vincent L. Ott, Andreea S. Zetea, Katharina Karnbach, Michelle C. Donzallaz, Arne John, Roy M. Moore, Franziska Assion, Riet van Bork, Theresa E. Leidinger, Xiaochang Zhao, Adrian Karami Motaghi, Ting Pan, Hannah Armstrong, Tianqi Peng, Mara Bialas, Joyce Y.-C. Pang, Bohan Fu, Shujun Yang, Xiaoyi Lin, Dana Sleiffer, Miklos Bognar, Balazs Aczel, Eric-Jan Wagenmakers (submitted October 6, 2023; (v1), revised 10 October 10, 2023 [version cited is v2]) via arXiv

3 The non-apocalypse of 1910, mostly:

4 Sedna, Sednoids, scientists, and comets:

5 Please note: astronomy is not astrology:

6 Stuff I had time for this week, and some I didn’t:

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About Brian H. Gill

I was born in 1951. I'm a husband, father and grandfather. One of the kids graduated from college in December, 2008, and is helping her husband run businesses and raise my granddaughter; another is a cartoonist and artist; #3 daughter is a writer; my son is developing a digital game with #3 and #1 daughters. I'm also a writer and artist.
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