Trans-Neptunian Objects, or TNOs, sound simple enough, the term being descriptive of objects moving beyond the orbit of Neptune, which means objects with a semimajor axis greater than 30 AU. It makes sense that such objects would be out there as remnants of planet formation, but they’re highly useful today in telling us about what the outer system consists of. Part of the reason for that is that TNOs come in a variety of types, and the motions of these objects can point to things we have yet to discover.
Thus the cottage industry in finding a ninth planet in the Solar System, with all the intrigue that provides. The current ‘Planet 9 Model’ points to a super-Earth five to ten times as massive as our planet located beyond 400 AU. It’s a topic we’ve discussed often in these pages. I can recall the feeling I had long ago when I first learned that little Pluto really didn’t explain everything we were discovering about the system beyond Neptune. It simply wasn’t big enough. That pointed to something else, but what? New planets are exciting stuff, especially when they are nearby, as the possibility of flybys and landings in a foreseeable future is real.
Image: An artist’s illustration of a possible ninth planet in our solar system, hovering at the edge of our solar system. Neptune’s orbit is show as a bright ring around the Sun. Credit: ESO/Tom Ruen/nagualdesign.
But a different solution for a trans-Neptunian planet is possible, as found in the paper under discussion today. Among the active researchers into the more than 1000 known TNOs and their movements are Patryk Sofia Lykawka (Kobe University) and Takashi Ito (National Astronomical Observatory of Japan), who lay out the basics about these objects in a new paper that has just appeared in The Astronomical Journal. The astronomers point out that we can use them to learn about the formation and evolution of the giant planets, including possible migrations and the makeup of the protoplanetary disk that spawned them. But it’s noteworthy that no single evolutionary model exists that would explain all known TNO orbits in a unified way.
So the paper goes to work on such a model, and while concentrating on the Kuiper Belt and objects with semimajor axes between 50 and 1500 AU, the authors pin down four populations, and thus four constraints that any successful model of the population must explain. The first of these are detached TNOs, meaning objects that are outside the gravitational influence of Neptune and thus not locked into any mean motion resonance with it. The second is a population the authors consider statistically significant, TNOs with orbital inclinations to the ecliptic above 45 degrees. This population is not predicted by existing models of early Neptune migration.
The paper is generous with details about these and the following two populations, but here I’ll just give the basics. The third group consists of TNOs considered to be on extreme orbits. Here we fall back on the useful Sedna and other objects with large values for perihelion of 60 AU or more. These demand that we find a cause for perturbations beyond the four giant planets, possibly even a rogue planet’s passing. Any model of the outer system must be able to explain the orbits of these extreme objects, an open question because interactions with a migrating Neptune do not suffice.
Finally, we have a population of TNOs that are stable in various mean motion resonances with Neptune over billions of years – the authors call these “stable resonance TNOs” – and it’s interesting to note that most of them are locked in a resonance of approximately 4 billion years, which points to their early origin. The point is that we have to be able to explain all four of these populations as we evolve a theory on what kind of objects could produce the observable result.
What a wild place the outer Solar System would have been during the period of planet formation. The authors believe it likely that several thousand dwarf planets with mass in the range of Pluto and “several tens” of sub-Earth and Earth-class planets would have formed in this era, most of them lost through gravitational scattering or collisions. We’ve been looking at trans-Neptunian planet explanations for today’s Kuiper Belt intensely for several decades, but of the various suggested possibilities, none explained the orbital structure of the Kuiper Belt to the satisfaction of Lykawka and Ito. The existing Planet 9 model here gives way to a somewhat closer, somewhat smaller world deduced from computer simulations examining the dynamical evolution of TNOs.
What the authors introduce, then, is the possibility of a super-Earth perhaps as little as 1.5 times Earth’s mass with a semimajor axis in the range of 250 to 500 AU. The closest perihelion works out to 195 au in these calculations, with an orbit that is inclined 30 degrees to the ecliptic. If we plug such a world into this paper’s simulations, we find that it explains TNOs that are decoupled from Neptune and also many of the high-inclination TNOs, while being compatible with resonant orbit TNOs stable for billions of years. The model thus broadly fits observed TNO populations.
But there is a useful addition. In the passage below, the italics are mine:
…the results of the KBP scenario support the existence of a yet-undiscovered planet in the far outer solar system. Furthermore, this scenario also predicts the existence of new TNO populations located beyond 150 au generated by the KBP’s perturbations that can serve as observationally testable signatures of the existence of this planet. More detailed knowledge of the orbital structure in the distant Kuiper Belt can reveal or rule out the existence of any hypothetical planet in the outer solar system. The existence of a KBP may also offer new constraints on planet formation and dynamical evolution in the trans-Jovian region.
A testable signature is the gold standard for a credible hypothesis, which is not to say that we will necessarily find it. But if we do locate such a world, or a planet corresponding to the more conventional Planet 9 scenarios, we will have ramped up our incentive to explore beyond the Kuiper Belt, an incentive already given further impetus by our growing knowledge of the heliosphere and its interactions with the interstellar medium. But to the general public, interstellar dust is theoretical. An actual planet – a place that can be photographed and one day landed upon – awakens curiosity and the innate human drive to explore with a target that pushes all our current limits.
The paper is Lykawka & Ito, “Is There an Earth-like Planet in the Distant Kuiper Belt?,” Astronomical Journal Vol. 166, No. 3 (25 August 2023) 118 (full text).
100s of AU distant implies very long mission times using currently envisioned technology – solar sail (with sundiver maneuver), a chemical rocket with a gravitational assist, nuclear thermal, and nuclear electric.
If we have a defined target, it would make a good mission with a larger beamed solar sail than that envisioned by Breakthrough Starshot.
for 400 AU, at 1% c the trip would take 1 year.
at 0.1% c the trip would be a reasonable 10 years
Assuming the proposed ground-based laser in Chile is used, that suggests a 0.1 – 1kg craft (including sail). Very small, but it would be interesting to know if that was sufficient for a useful flyby with miniaturized instruments that include a camera. My iPhone weighs 200g, so possibly this could be done if a tiny radioisotope battery was deliverable, or perhaps using the beam to periodically recharge a conventional battery.
Given the distance is not far off the gravitational focal line starts at around 550 AU, could such an approach be a way to get a telescope to start imaging exoplanets with the SGL with a shorter timeline than currently envisaged?
If a lunar industry could manufacture the bulk of a phased laser array, power generation and storage, and emplace it on the lunar farside (obviating the concerns of a space laser being used as a weapon against terrestrial targets (or starting forest fires ;-) ) this might be a useful utility that could be harnessed for a number of tasks, including planetary protection, and powering spacecraft for both operation and propulsion. The lunar location would avoid the issues of atmospheric effects on the beam, although it would only address all 360 degrees of azimuth in the sky every month rather than every day. This might be mitigated somewhat by having multiple arrays placed across the farside equator.
MITIGATION of timing gap to cover 360 degrees azimuth.
Assume the phased arrays are located near each limb of the Moon. Located on a flat plain they can be directed through 180 degrees from horizon to horizon. To prevent any possible targeting of Earth their location requires that the minimum direction of their beams must be offset from Earth by 10 degrees. Therefore their total coverage at any moment is 340 degrees of azimuth. As the Moon makes its monthly orbit, those 20 degrees can be covered by one of the arrays in just 1.7 days. Therefore the whole sky coverage can be complete with no more than 1.7 days of gap for any given point in the 20-degree “blindspot”.
This is probably an extreme case, that allows the arrays to target across 180 degrees. If we use a more conservative 60 degrees of targeting, 6 laser arrays could accomplish the same coverage. While not as parsimonious as Clarke’s 3 geosynchronous satellites to cover the Earth in radio reception and transmission, it does suggest a way to deliver beamed energy in a way that protects the Earth from any harm. If the laser wavelength was short, say in the short UV to X-ray range, then even malicious targeting with popup mirrors would mostly fail due to atmospheric absorption, making this a safe means of using such powerful lasers for peaceful use without much fear of their use as a weapon against terrestrial targets on the ground. 10 degrees offset would even protect satellites and space stations out beyond geosynchronous orbit. Only satellites and space stations in lunar orbit would be vulnerable to the use of these laser arrays as weapons.
Reusable, heavy-lift rockets could deliver robots and manufacturing plants to the Moon to churn out the needed lasers, solar arrays to harvest the sun’s energy and batteries to store the energy for the short periods of beaming. Only the high technology, precision components need to be shipped from Earth, and minerals unavailable in the regolith.
While the capital cost would be very high, it would offer a capability that would allow us to send small packages across the solar system in days, and into interstellar space within feasibly short times. This would mean that conventional batteries could supplant RTGs and other nuclear power sources for many space probes.
If the arrays were also eventually built at various destinations, then a rapid 2-way delivery service for small and perishable goods could be set up, with the lasers at the target used to decelerate the packages for local delivery and vice versa. Package size would determine the delivery time, while the cost would remain a fixed fee irrespective of distance, just like postal services for envelopes and parcels of up to a certain weight, as the energy used would be fixed.
I emphasize that with this sort of laser array power, only small packages could be sent. A 1000 kg package, e.g. a human in a hibernaculum, would travel very slowly and this would not be feasible at all. Conventional rocketry would be used instead. But small packages of biological material, advanced microchips, and other hardware, could be shipped within short times to facilities both human and robotic in locations across the system, facilitating commerce not just in information, but in high value products as well.
Hi Paul
With everything going on, I was wondering when things were at with the search in the outer Solar system, The Vera Rubin should provide some amazing results.
It sounds like our proposed “Planet 9” had gone from a mini Neptune to a super Earth type planet on a closer orbit. I’ll have to read and digest the paper
Thanks Edwin
Suppose you had a probe far from the Sun, which counts individual molecules of parahydrogen (I’m thinking H2 with opposing nuclear spins might ignore magnetic fields?). As each H2 struck a detector, the probe would determine its velocity vector. Then could you extrapolate some of those molecules back to when and where they escaped from Planet Nine’s exosphere as the planet moved along?
Glad to see this topic crop up here. About every 6 weeks or so, the thought crosses my mind to check whether there is anything new to report at Konstantin Batygin and Mike Brown’s blog, The Search for Planet Nine, at http://findplanetnine.blogspot.com. But it hasn’t been updated since 2021. Does anyone know if there has been any commentary on ruling out more parts of the search space since then?
Off topic: Paul, might readers expect any report from the interstellar conference in Montreal this summer? I live in Montreal, but was away when it happened (plus it would have been pretty expensive to attend the talks if they were enforcing conference registration…)
I couldn’t make Montreal either because of travel problems (I can’t travel at all right now), but I do plan to be writing up some of the papers presented there. Andrew Higgins and the whole IRG team did a great job. Check here as the videos have now begun to appear:
https://www.youtube.com/@InterstellarResearchGroup/videos
Colin Warn wrote up the event for i4IS and we plan to run his report on the next IRG newsletter as well. I plan to offer an advance look at it on Centauri Dreams next week.
“If a lunar industry could manufacture the bulk of a phased laser array, power generation and storage, and emplace it on the lunar farside (obviating the concerns of a space laser being used as a weapon against terrestrial targets (or starting forest fires ;-) ) this might be a useful utility that could be harnessed for a number of tasks, including planetary protection, and powering spacecraft for both operation and propulsion. ”
If this was possible to build there, would it not be “just” as easy to build a larger probe in space – with what ever propulsion you want?
While ideally a much larger probe would be better, especially for a telescope, and there are current limitations on miniaturization, all other envisaged propulsion systems assume around a 50-year flight to the start of the SGL (starting ~550 AU), so reduce that accordingly for a 400 AU orbit to a hypothetical for planet 9.
The only approach we have to reduce that flight time is using lasers to push a sail and payload to much higher velocities. But a “reasonable” laser array on Earth can only achieve these velocities with small payloads. [The Windrider magsail concept might work well if it proves out and could propel a larger payload.]
Why the Moon? If a laser array pushing a sail is needed, the energies generated could be used as a weapon. This has been stated a number of times. Placing it in space would therefore allow it to be used to target Earth. The second reason to build on the Moon is that the resources needed do not need to be lifted against even the low lunar gravity. (c.f. O’Neill’s idea of magnetic launch of regolith to build his space colonies in Earth’s orbit.) Lastly, the Moon provides a massive platform to support the laser array.
Why is a reduced mission time desirable? For all the talk about multi-generational cathedral building, only institutions are multigenerational. A scientist is not going to spend a lifetime on a project without seeing any results. In addition, the technology will be obsolete long before the probe reaches its target. Consider how old the Voyager probes are after 40+ years of development. [Obsolete technology was part of the plot of “Space Cowboys”.] We were dragging out Cobol programmers to fix the Y2K problem less than 30 years ago. So ideally we want the best technology that can be used to arrive while it is still reasonably new. It would also help if the probe were not faced with the space environment slowly degrading the probe while on its long journey.
In summary, the idea is to be able to reach distant targets with much shorter mission times. At this time, only laser-driven sails seem capable of doing this, but at the cost of only delivering very small payloads. However, once the lasers are built, they can be reused for a variety of tasks that can be amortized to reduce mission costs.
I am open to any idea that avoids the need to build a laser array to launch only small payloads that would reduce flight times. I am just not aware of any approach that will do this without resorting to exotic, speculative technology.
Have a look at Phil Lubin’s talk at the IRG in Montreal. Just watched it and the numbers are very interesting. He makes some of the same points as I do, so I may not be asking for the Moon.
Large Scale Directed Energy for Relativistic Flight
Collectively, learning “that little Pluto really didn’t explain everything” says a lot about the responsibilities imposed on numbered planets and the consequences of falling to fulfill those expectations. While I have to admit that Pluto does “fall short” in many respects, consider that it has six moons. More like an outer planet in miniature. Also, somehow in its isolation it exhibits a heck of a lot of evidence of tectonic activity. Compare with Mercury. Now which planet should be pushed off the cliff?
In addition, considering some of Pluto’s sins or emissions against the backdrop of many exoplanets’ behaviors, I wonder if Pluto has really done that much harm. Or, shall we say, failed to do that much of merit.
Our definition of a planet might not be entirely arbitrary, but it is rooted in what the ancients observed without telescopes as wandering stars. Most exoplanets do not give a blank about Bode’s Law. And that is probably just the start of their disregard for all this.
Now if a planet could not be observed to wander by the astronomers of antiquity – and we know who those planets are – then we could argue that anything observed beyond Saturn now is not necessarily a classic instance of a planet either.
My point is that much about planet definition is arbitrary, based on historical lines drawn, amended with subclassifications later.
Should planets have genealogy or genes, something akin to heredity, would it make more sense? Well, the best my imagination allows is mythology, where they got their names in the first place or early on. Repeat: mythology.
Stars, by comparison, are much easier to categorize.
To the above, an edit: “sins or emissions” to “sins or omissions”.
Wish I could blame an AI for anticipating my next entry, but it probably decided not to intervene.
You have to admit, it’s a most amusing typo!
Could the WST be used to try to directly image this body? It has very high sensitivity in the infrared and also very high angular resolution, so perhaps it could monitor a large area for infrared points that move slightly relative to the background starfield over the year due to the parallax of the telescope’s motion in orbit around the Sun.
Can the Gravitational Effect of Planet X be Detected in Current-era Tracking of the Known Major and Minor Planets?
https://iopscience.iop.org/article/10.3847/PSJ/acc7a2
Ive just found this one online today and its next on my reading list manged to read the paper last night Paul,
Cheers Edwin
Good. Let us know your thoughts on it when you’re done. Thanks.
In my neighborhood, a university campus sprang up decades ago with a social science faculty devoted to the fashionable study of futurism. Some of the faculty would explain the importance of futurism as a race to the goal of defining or anticipating it. Looking back into the past, one could see many ways in which this exercise has been done before, but we (in the “present”) tend to ignore it.
It is still tempting to pursue this Planet 9 topic from the contrarian point of view. For when we consider the introduction of the notion of “planet” in antiquity ( say in the Ptolemy’s Almagest or earlier records), it precedes the heliocentric or Copernican concept of planetary motion. Planets distinguished themselves by visibly moving in the celestial sphere vs. the fixed stars.
And what’s more, they were observable.
Whether they reflected light or had an internal sources, to the ancients this was not clear. But on the other hand, since their early “international astronomical union conventions” were summarized in Ptolemy’s work, one could infer that a planet had to be seen, if not necessarily heard (Kepler, Music of the spheres perhaps). A planet as described above is defined to fulfill the “needs” of 21st century dynamic astronomy: a body that would perturb outer planets enough to explain discrepancies in their motion and changing orbital elements.
Nineteenth century concerns, to be sure, but not necessarily the concern of earlier observers, which if we go back far enough, deliberated as to whether a morning and evening star that moved in the celestial sphere was the same entity we call Venus.
What I am suggesting, is that the exercise of revoking Pluto’s credentials as a planet in a formal astronomical union procedure opens the door to much more arbitrary assertions. If a planet does not reflect enough light to be observed, then does its gravitational effect make up for this shortcoming? Or if it does not shift its position in the celestial sphere by a certain decrement, should it be considered a planet either? If a trans-plutonian planet is found with terrestrial or more mass, should that aspect of its parameters outweigh its other planetary shortcomings, such as for all practical purposes being invisible?
Pitch that to Ptolemy and his colleagues.