Sedna and Sednoids aren’t this month’s only science news. But I saw two exciting, for me, developments; and that’s what I started talking about last week.
This week I’ll wrap up most of what I was going to say about Sednoids and “Planet X”.
Results of the James Webb Space Telescope’s observation of Sedna and two other distant dwarf planets will wait for another time: when I’m not running a fever.
Which I have been, and that’s why this was teeming with typos. I think I’ve fixed them. But if you find some, I’d appreciate it if you’d let me know in a comment.
Okay. Here’s what I was going to talk about last week:
‘As You Recall, In Our Last Exciting(?) Episode’…
Recapping part of what I said last week, we’ve been learning a great deal about the Solar System.
Pluto has been relabeled as a dwarf planet and trans-Neptunian object.
As a term, “trans-Neptunian object” isn’t something I remember from my youth. On the other hand, I haven’t learned who coined it, or when.
My guess is that it’s a fairly recent addition to science-speak, since sometimes it’s spelled “transneptunian object”. And sometimes trans-Neptunian object and Kuiper belt object are used as synonyms.
This strongly suggests its spelling and usage hasn’t been standardized yet.
Anyway, objects like Pluto and Sedna, although they’re far from the inner planets, aren’t at the edge of the Solar System.1
Oort Cloud
The Oort cloud is the Solar System’s outermost zone.
Unlike the inner Solar System, asteroid belt, outer Solar System, and Kuiper belt, the Oort cloud is a theoretical thing.
We haven’t observed an Oort cloud object yet: not one that’s been identified as such.
But we have seen and tracked long-period comets, which are coming from somewhere.
The Oort cloud, or something very much like it, is the least-unlikely source.
For now, I’m assuming that the Oort cloud is real; since that means I needn’t put words like “theoretical” in every other paragraph.
The Oort cloud is stuff left over from when the Solar System formed, some four and a half billion years back.
Objects in the Oort cloud are mostly small (yards to miles / meters to kilometers across) planetesimals: made mostly of materials that are gas or liquid here on Earth, like methane or water.
Some of them likely have cores that are rock or metal. That’s one reason NASA is sending a probe to Psyche. The asteroid Psyche may be the core of a planetesimal that had most of its outer ice knocked off. And that’s another topic.
Two more things about the Oort cloud.
Observations of comets and lots of math say that the outer Oort cloud should be roughly spherical, with an inner part that’s a comparatively dense disk.
Depending on context and who’s talking, the Oort cloud’s in-the-ecliptic disk is the Hills cloud, inner Oort cloud, or inner cloud.
Finally; the Oort cloud’s outer, roughly spherical, part is three light-years across.2 Give or take a bit.
Kuiper Belt and the “Inner Solar System”
The Oort cloud’s inner disk — theoretical, so far — is in the ecliptic.
The ecliptic is the plane of Earth’s orbit, which is pretty close to the Solar System’s invariable plane.
The Solar System’s invariable plane is an average of the planets’ orbital planes. Roughly. I’m leaving out a whole bunch of stuff about barycenters and angular momentum vectors.
It’s been one of those weeks.
The Kuiper belt is another disk of material, also in the ecliptic. It was theoretical, too, until we started charting Kuiper belt objects: and realized that Pluto was one of them.
Then we discovered the Kuiper cliff, 47.8 a.u. — astronomical units, the distance between Earth and our star — from the Sun. I talked about that last week.
One of the reasons, I gather, that Pluto was reclassified as a dwarf planet is that’s got more in common with (other) Kuiper belt objects than it does with the Solar System’s planets.
Let’s see, what else? The “inner Solar System” almost always means the inner four Solar planets: Mercury, Venus, Earth, and Mars. “Outer Solar System” often refers to our planetary system’s four giant outer planets.3
Unless someone’s talking about things outside Neptune’s orbit.
That brings me to Sedna and Sednoids, tidal forces and orbits, and the (theoretical) Oort cloud’s outer reaches.
Or, rather, that almost brings me to what I was going to talk about this week.
Our Star’s Sphere of Influence
Before astronomers found objects like Sedna, the most famous outer Solar System object was probably Nemesis.
Nemesis was (and is) a hypothetical red dwarf star or brown dwarf, orbiting the Sun every 26,000,000 years. Each time it swooped through the right (or wrong) part of the Solar System, it sent comets and asteroids hurtling toward Earth.
There are a few problems with that idea.
First, although extinction events happen, they’re nowhere near regular enough for Nemesis to be the cause. The only cause.
Second, we’re running out of places where Nemesis could be. Recent wide-field surveys have looked for Nemesis, among other objects: and so far have come up with nothing. Nothing fitting the Nemesis profile, that is.
Third, its 26,000,000 year orbit would have taken Nemesis about 1.5 light-years out from the Sun: about where scientists figure the Oort cloud’s outer edge is.
Finally, barring wildly improbable luck, Nemesis would have long since been tugged out of the Solar System.4
That’s because something about 1.5 light-years away from the Sun is near the edge our star’s gravitational sphere of influence.
“…Nothing Stands Still”: Heraclitus and the Solar System’s Shifting Border
“τὰ ὄντα ἰέναι τε πάντα καὶ μένειν οὐδέν”
“All entities move and nothing remains still.”
“πάντα χωρεῖ καὶ οὐδὲν μένει”
“Everything changes and nothing stands still.”
(Heraclitus, quoted by Plato in “Cratylus”)
I think Heraclitus had a point, at least where things in this universe are concerned.
Take the exact location of the Sun’s sphere of influence, for example. I’ve seen it described as being ‘a few’, or about 1.5, light-years out.
But what I’ve learned about astrodynamic spheres of influence very strongly suggests that the Solar System’s gravitational border keeps shifting.
That’s because what ancient astronomers called the “fixed” stars — aren’t; although for practical purposes, on the scale of human lifetimes, the stars we see in Earth’s sky stay put.
But they’re all moving in their orbits around our galaxy’s center, and so is our Sun.
Right now, Alpha and Proxima Centauri are the closest stars. Some 28,000 years from now, they’ll be about three light-years away: their closest approach.
About 10,000 years after that, Ross 248 will be at roughly the same distance.
There’s nothing magic about “three light-years”. Some stars come much closer.
Take Gliese 710, for example. The star will, astronomers figure, be just over an eighth of a light-year away in 1,290,000 years. That’s 10,520 astronomical units: close, very roughly as far away as the currently-theoretical Hills cloud.
So I figure the Sun’s sphere of influence, the Oort cloud’s outer edge, is a shifting surface; where our star’s gravity and that of neighboring stars cancel each other out.
And that, on average, it’s about 1.5 light-years out: the top of the blue zone in the “Stars Near to the Sun” chart.
I was going to talk about this sort of thing more, but it involves a lot of math that’s beyond what I’ve learned.5 And, like I said before, it’s been one of those weeks.
Sedna and Sednoids —
Backing up a bit, the Kuiper belt starts around Neptune’s orbit.
Based on mathematical models, scientists expected the Kuiper belt to extend well beyond the Solar System’s planets.
Instead, they found the Kuiper cliff, 47.8 a.u. from the Sun.
Maybe it’s just a broad gap in the Kuiper belt. If so, we haven’t found that gap’s outer edge.
Now, finally, Sedna and the Sednoids.
Sedna’s current classification is dwarf planet, it’s diameter is very roughly half Pluto’s, and it’s about as close to the Sun now as it ever gets. Its perihelion, its closest approach to the Sun, will be in July, 2076.
Sedna’s orbit keeps it well beyond the Kuiper cliff, but it’s not alone. We’ve found three other Sednoids: objects with similar orbits.
| Sednoid name | Semimajor axis | Perihelion | Inclination | |
| 90377 Sedna | 506 a.u. | 76 a.u. | 12∘ | |
| 2012 VP113 | 262 a.u. | 81 a.u. | 24.1∘ | |
| 2015 TG387 Leleakuhonua | 1090 a.u. | 65 a.u. | 11.7∘ | |
| 2021 RR205 | 990.9 a.u. | 55 a.u. | 7.6∘ |
The Sky & Telescope article I started talking about last week only mentions Sedna and three other Sednoids. I’m pretty sure 2021 RR205 got left out because its perihelion is less than 60 a.u., and maybe because it’s got the smallest inclination.
Inclination: in this context, that’s how much an orbit is tilted out of the ecliptic.6
— Galactic Tides, Time, and Rewinding the Solar System
I’ve been looking for a diagram of all three Sednoids that were mentioned in that Sky & Telescope article. But so far, I’ve only found two: one for Sedna, that’s the one heading this section; and another for VP113, above.
It’s now Friday afternoon. So I’ll put that graphic quest on a back burner, and share another excerpt:
“…Huang asked: What if there is no undiscovered planet in the Kuiper belt? In that case, the orbits of the three Sednoids should have been stable over billions of years.
“‘Planet X’ May Have Left Our Solar System Billions of Years Ago“
Emily Lakdawalla, Sky and Telescope (October 4, 2023)
This is where I was going to talk about the Solar System’s orbit around our galaxy’s center, the search for solar siblings — stars which formed with ours — and, more to the point, how stars passing by ours in the course of the Sun’s 20.4 laps around the Milky Way.
That’s not going to happen. Not this week. Which may be just as well. I tend to ramble, and that’s yet another topic.
At any rate, objects in the Oort cloud’s outer reaches feel the gravitational tug of passing stars; an anthropomorphism that I’ll let slide.
Sometimes the objects get pulled into orbits that take them to the inner Solar System. Sometimes they’re pulled out into the void between stars.
The Sednoids, far away from the planets as they are, are far enough inside the Solar System to be safe from the ebb and flow of gravitational tides.
But they’re also too far from the Solar System’s giant worlds to have the shape of their orbits bent by close encounters. Gravitational effects of the Solar System’s giant planets would rotate the Sendoids’ orbits, and that’s about it.
Science-speak for ‘rotate their orbits’ is precession.7
One more excerpt:
“…Using a computer simulation, Huang ran the solar system backward in time for billions of years. He found that the orbits of the three known Sednoids shared some remarkably similar properties just once, in the distant past: Not long after the birth of the solar system, their perihelia clustered at the same solar longitude, and their apsidal lines (the line passing through perihelion, the Sun, and aphelion) were also nearly coincident.
“This orbital clustering is a telltale sign that a single event put the Sednoids onto their present paths, an event that happened during the solar system’s youth more than 4 billion years ago. It’s also a sign that nothing has disturbed the slow evolution of those orbits for 4 billion years. In other words, there is no undiscovered planet to be found today….”
“‘Planet X’ May Have Left Our Solar System Billions of Years Ago“
Emily Lakdawalla, Sky and Telescope (October 4, 2023)
I do not think this ends the search for planet-size objects in the Solar System’s borderland. But I do think it may add a page or two, at least, to the early chapters of our home’s continuing story.
That’s it for this week, apart from — you guessed it — links:
- “Sednoids and the Mysterious Missing Planet X“
(October 14, 2023) - “OSIRIS-REx’s SRC is Back: Snapshots from Utah“
(September 24, 2023) - “My Top 10 Science News Stories For 2020“
(December 29, 2020) - “‘Oumuamua: Data, Questions“
(November 10, 2018) - “Planet 9, Maybe; Nibiru, No“
(September 29, 2017)
1 The Solar System’s outer reaches:
2 Oort cloud, mostly; and an asteroid:
- Wikipedia
- NASA
- “Where is the Edge of the Solar System?” (released September 5th, 2017; updated May 3rd, 2023) video (57 seconds) and text
- “Asteroid Psyche” (A Unique, Metal Rich World)
- Oort Cloud: Facts
- “SP-345 Evolution of the Solar System“, Chapter 12; Hannes Alfvén, Gustaf Arrhenius; Scientific and Technical Information Office (1976) via NASA History Office
- Wikipedia
- Asteroid belt
- Ecliptic
- Giant planet
- Invariable plane (AKA the Laplacian, Laplace plane, invariable plane of Laplace; which isn’t the Laplace plane, so let’s just call it the invariable plane)
- Kuiper belt
- Pluto
- Solar System
- Terrestrial planet
- NASA
- Last week’s post
4 A death star that probably isn’t there:
- Wikipedia
- “Is a Death Star Coming at Us? Study Says It’s Possible, but Don’t Panic“
NBC News (December 31, 2014)
5 More than I’m going to talk about this week:
- Wikipedia
- NASA
- “Basics of Space Flight: The Environment of
Space.”, Chapter 1. The Solar System;Dave Doody, Caltech Jet Propulsion Laboratory (2011)
- “Basics of Space Flight: The Environment of
- Wikipedia
- 2012 VP113 (Sednoid, discovered 2012)
- 2021 RR205 (Sednoid, discovered 2021)
- 541132 Leleākūhonua (Sednoid, discovered 2015) (another designation, 2015 TG387; nicknamed The Goblin)
- 90377 Sedna (Sednoid, discovered 2003)
- Dwarf planet
- Kuiper belt
- Orbital inclination
- Scattered disc
- Sednoid
7 The main points are “nodal precession”, “orbital plane”, and “apsidal precession”; the rest are related topics:
- Wikipedia
- “A Rogue Planet Helps Populate the Distant Kuiper Belt“
Yukun Huang (黄宇坤), Brett Gladman Matthew Beaudoin Kevin Zhang (draft version September 21, 2022) via arXiv - “‘Planet X’ May Have Left Our Solar System Billions of Years Ago“
Emily Lakdawalla, Sky and Telescope (October 4, 2023) - “Dynamics of Transneptunian Objects under the Influence of a Rogue Planet“
Yukun Huang, doctoral thesis; graduate and postdoctoral studies (Astronomy), The University of British Columbia (Vancouver) (July 2023)
