OC405: Chasing Planet Nine
Ben Franklin Centre for Theoretical Research
PO Box 27, Subiaco, WA 6008, Australia.
This is Number 5 in a suite of web articles about the Oort Cloud, the volume of space immediately outside our Solar System.
Looking at the Oort Cloud with telescopes and space probes
So far in this series we have mostly looked at the Oort Cloud from the outside, using evidence and logic to deduce what lies within it. But increasingly improved telescopic instruments and methods, and numerous space probe missions, have allowed us to actually locate some of the many bodies which lurk on the inner boundary of the Oort Cloud and the outer areas of the Heliosphere.
It will be remembered that the Heliosphere which contains the Solar System is about 100 AU in radius (1 AU is the Earth-Sun distance). The planets proper, Mercury to Neptune, all lie within the closest 30 AU of this sphere. Beyond 30 AU and out to 100 AU, the precinct is called the Kuiper Belt.
Fig. OC405-F1. Plane of the planets, the Kuiper Belt, and the Oort Cloud. From [E6].
All the planets essentially orbit the Sun in the Sun's Equatorial Plane, only varying by a few degrees from this. Bodies beyond Neptune, in the Kuiper Belt, have orbits which tilt more and more from this plane, the further out you go. Objects with orbits which take them beyond the Heliosphere, into the Oort Cloud (mostly small stuff like comets) may have any inclination to the Equatorial Plane. The red orbit shown in the Kuiper Belt is that of Pluto, tilted at 17 degrees. Pluto's orbit is also much more elliptical than those of the planets.
In the early days of the Solar System, the objects which we recognize as planets had much more random orbits. They have been regularized into their current plane by a gravitational force called Equatorial Forcing. This has resulted in a Heliosphere in which almost all the mass is in the Equatorial Plane -- above and below this plane is mostly vacuum.
Since Pluto lost its status as a planet, it has been placed in the category of a dwarf planet. Dwarf Planets are objects large enough to have gravity pull them into a more or less spherical shape, and are usually over 200 km in diameter.
In [E2] it says "The "dwarf planets" are all of those objects which are not one of the eight dominant bodies (Mercury through Neptune) yet still, at least in one way, resemble a planet. In other words, a dwarf planet is something that looks like a planet, but is not a planet. Specifically this means that dwarf planets are bodies in the solar system which are large enough to become round due to their own gravitational attraction". Figure F2 shows ten of the largest known TNOs (Trans-Neptunian Objects), which include Dwarf Planets as well as much smaller stuff.
Fig. OC405-F2. Largest known Trans-Neptunian Objects (TNOs). From [E1].
Continuing work on TNOs has had its focus at the California Institute of Technology (Caltech), with Mike Brown, Professor of Planetary Astronomy, and his colleagues. Caltech was also formerly the main work environment of the brilliant Swiss-American astrophysicist Fritz Zwicky, mentioned in OC402: The Oort Cloud and Mass in the Galaxy for his identification of Dark Matter.
Fig. OC405-F3. Mike Brown. From [E3].
There are now a huge number of TNOs known. In [E2], Mike Brown lists 2247 of these, of which 741 are believed to have diameters of 200 km or more, and could turn out to be be Dwarf Planets.
However, an important point to remember is that most of these TNOs have only ever been identified as points of light on photographic plates (or their modern equivalents), rather than as disc images. Even for Pluto, relatively large and close as TNOs go, the most powerful of today's telescopes can barely show the object as a disc. So detailed photo images of TNOs have so far only been obtained for the few objects visited by space probes, which includes Pluto.
So most of the images in Figure F2 of TNOs are only notional, artist's impressions, based on best estimates of size, colour, reflectivity and so on. Notice that several of the TNOs have moons of their own. We can tell this because as long as the moon is a reasonable distance from its primary, it will have its own point of light.
Fig. OC405-F4. Haumea and its satellites, from the Hubble Space Telescope.
Haumea is the second-closest to the Sun of the TNO Dwarf Planets, at about 43 AU. Pluto is the closest of this group, at about 39 AU (although its elliptical orbit takes it closer than Neptune at one point). Eris, which may actually be bigger than Pluto, is the furthest at about 68 AU, while Makemake is a little further out than than Haumea at at about 46 AU.
Dwarf Planets, TNOs, and Moons
One Dwarf Planet which is not a TNO is Ceres, the largest of the asteroids in the Asteroid Belt lying between Mars and Jupiter, only 2.76 AU out. There are also a number of moons of the outer planets which would be classified as Dwarf Planets if they were not tied to one of the planets -- 15 moons with diameters over 400 km, and 3 of diameter 200-400 km.
Fig. OC405-F5. Proteus, Neptune's second-largest moon. From [E4].
Among these is Proteus, Neptune's second-largest moon. It was discovered only in 1989 by the Voyager 2 spacecraft [E4]. Proteus is about 200 km across, and not quite spherical. Its surface is very dark, and it orbits quite close to Neptune.
The point to be made is that all these dwarf planets and moons are not necessarily divided into separate classes -- whether they are TNOs, asteroids, or planetary moons is probably a matter of chance. Individual dwarf planets will, however, have their own histories, which determine whether they are mostly of rocky material, or contain a good proportion of water ice or voids (leading to lower densities).
In OC407: Chaos in Oort, we will assert that all the dwarf planets are merely random captures from the Oort Worlds, the larger of the bodies in the Oort Soup filling interstellar space. And the same will be true of planet-sized bodies, right up to Brown Dwarf or sub-star size.
The most distant body from the Sun for which we have detailed photographs is called Ultima Thule. It orbits the Sun about 44 AU out, and was imaged in 2019 by the passing New Horizons space probe.
Fig. OC405-F6. Ultima Thule. From [E5].
The image was taken on January 1, 2019, when the spacecraft was 16,694 km from Ultima Thule and 6.6 billion km from Earth [E5]. The object clearly consists of two vaguely spherical lobes which have stuck together. The larger lobe, nicknamed Ultima, is measured at about 21.6 km across its longest axis while the smaller lobe, Thule, is measured at 15.4 km across its longest axis.
A similar feature, that of two dissimilar parts stuck together, was noted in the core of Comet 67P/Churyumov-Gerasimenko, which was visited by the Rosetta spacecraft in 2014/5. With space objects travelling around at very high speeds, it is tempting to think that great impact collisions may be common, but in fact many such meetings may be slow and gentle.
Orbits of TNOs
With each passing year, more and TNOs, objects in the solar system that have an orbit beyond Neptune, are discovered in great numbers. For the belt from 30 to 50 AU out, from Neptune to halfway across the Heliosphere, there are estimated to be perhaps 70,000 TNOs, each at least 100 km across.
The volume beyond 100 AU, which on our working definition is the inner boundary of the Oort Cloud, is of particular interest to us. Inhabitants of this volume are sometimes called eTNOs, extreme Trans-Neptunian Objects. All eTNOs spend much of their lives in the Oort Cloud proper.
Figure F7 following shows the orbits of some of the known eTNOs. It will be remembered that, at this distance from the Sun, orbits no longer lie in the planetary plane, but are at fairly random inclinations. And although the Sun is still a major gravitational influence at these distances, it is no longer the sole arbiter of orbits -- bodies further out in Oort Space will have increasing influence.
Fig. OC405-F7. Orbits of some extreme TNOs. From [E7].
In the figure, all the orbits except that of the large green oval have been calculated, calculated from observations when the objects were in sections of their orbits close to the Sun, within the 100 AU radius Heliosphere. The furthest out from the Sun has any object actually been detected (as a dot of light) is about 81 AU, for "Biden", one of the three currently known objects classed as a "Sednoid" (meaning similar to Sedna, another of the three).
Fig. OC405-F8. Orbits of the Sednoids. From [E13].
Here is an extract from what [E13] says about the Sednoids. "A sednoid is a trans-Neptunian object with a perihelion greater than 50 AU and a semi-major axis greater than 150 AU. Only three objects are known from this population: 90377 Sedna, 2012 VP113, and 2015 TG387, but it is suspected that there are many more. All of them have perihelia greater than 64 AU. Some astronomers, such as Scott Sheppard, consider the sednoids to be inner Oort cloud objects".
To explain some of the technical terms, its perihelion is the closest an object comes to the Sun, its aphelion or semi-major axis is its furthest distance away. Sedna was the first discovered, in 2003, and has been officially named. The other two were discovered later, and have official names starting with the year of discovery -- these also have unofficial nicknames (in brackets in the following list).
Sedna: Perihelion 76 AU, Aphelion 936 AU, Diameter 1000 km. Orbit 11,400 years.
2012 VP113 (Biden): Perihelion 81 AU, Aphelion 441 AU, Diameter 600 km. Orbit 4,300 years.
2015 TG387 (The Goblin): Perihelion 65 AU, Aphelion 2123 AU, Diameter 400 km. Orbit 32,000 years.
One of the assertions I've made previously about the Oort Cloud is that it is far more heavily populated than previously assumed. Earlier Segments have shown why theory supports this, and a little application of logic to the above list shows how it is also supported by observation of real objects.
The thing is, objects like the Sednoids have only been detectable while within the Heliosphere, closer to the Sun. They have also only been detectable because of their relatively large sizes, over about 400 km diameter. So if the known Sednoids are typical of what lies out there, all the others in their class are currently in parts of their orbit beyond 100 AU, and not currently detectable -- and as their orbital periods are measured in thousands of years, their total number must be very great.
And this applies to objects with orbits which actually venture within the Heliosphere. We will go on now to look at the evidence for objects which never come close enough to the Sun to be detectable by current techniques.
The search for Planet Nine
Figure F9 gives another view of the orbits of eTNOs, large Trans-Neptunian Objects in extreme orbits which are mostly within the Oort Cloud. It will be noticed that in the orientation shown, most of the orbits lie to one side, the left in the picture.
Fig. OC405-F9. Orbits of extreme TNOs and Planet Nine. From [E8].
In January 2015, Caltech astronomers Konstantin Batygin and Mike Brown announced new research that provides evidence of a giant planet tracing an unusual, elongated orbit in the outer solar system [E14]. The prediction is based on detailed mathematical modelling and computer simulations, not direct observation. The gravitational influence of this large object could explain the unique orbits of at least five of the eTNOs. The researchers nicknamed this object "Planet Nine".
If it exists, Planet Nine is estimated to have a mass 8-15 times that of the Earth (similar to the mass of Uranus or Neptune) and orbit the Sun in an orbit which takes it from 400 to 800 AU out -- entirely in Oort Space. It would take 10,000 to 20,000 years to complete one orbit.
So, knowing the probable orbit of a distant body, why not point a powerful telescope in the right direction and image it there? There are a number of difficulties.
Fig. OC405-F10. Planet Nine and the Solar System. From [E7].
First, it really is long way away. To image an object by by the light it receives from the Sun and reflects back to an Earth telescope, it means searching for a really tiny amount of light, as the light is reduced by the inverse-square law for each unit of distance travelled. Even at the minimum distance, it would mean identifying a planet-sized object 400 AU away -- five times the current record of about 80 AU.
To give a visual feel for the problem, Figure F10 is an artist's rendering of what it might look like if you were situated behind Planet Nine and looking towards the main plane of the Milky Way. The little bright oval on the top right is the entire Solar System. Planet Nine is a long way out.
Another problem is, there doesn't seem to be any consensus as to exactly where in its orbit is Planet Nine's current position. Searching for an object which might lie anywhere in an ellipse stretching right round the sky and 400 AU away is never going to be easy. Further computation might narrow down its likely location.
One plus is that Planet Nine, if the size of Uranus or Neptune, would be easier to find than a smaller body. Astronomers reckon that success in the search is on the edge of possible, with current techniques. But is there an easier way to find Planet Nine?
Looking in the Mid-Infra-Red
There may be an easier way to find Planet Nine than by looking for the tiny amount of light it reflects from the Sun. That is, to look for its own radiation.
We have already seen how the inverse-square law means that general bodies out in the depths of the Oort Cloud receive very little radiation from the Sun, and so have equilibrium temperatures below 3 K. Their normal thermal (black-body) radiation is consequently in the microwave/ far-infra-red bands, which we class as CMBR.
But this does not apply to larger bodies of planetary size, with masses greater than that of Mars, because these bodies are likely to be generating their own internal heat, by the Concore Model described earlier in "XT807: The Concore Model of planet and star interiors" [E16].
There is a more detailed account of how internally-generated heat figures in the energy budget for Earth in "Temperatures of the Earth -- a Globe in Space" [E17]. Briefly, if you put the average amount of energy that the Earth receives each day from the Sun at one million "BLU units", the amount generated internally is relatively tiny -- about 213 BLUs comes up to the surface from below each day, and the average daily yield from earthquakes is about 7110 BLUs [E19], with a total of about 7300 BLUs.
Even these amounts are rather greater than human-usage figures, with Man's total energy use about 77 BLUs per day, of which 62 BLUs comes from fossil fuels. So it's fair to say that human energy use is trivial in size compared to the 1,000,000 BLUs which the Sun sends us. Of course, every day the Earth also sheds the exact input equivalent of about 1,007,400 BLUs into space as infra-red radiation.
But as you move out further from the Sun, the internally-generated fraction of a planetary heat budget becomes more and more significant. Jupiter generates more heat from its core than it receives from the Sun. This is partly because Jupiter is much more distant from the Sun than Earth, and partly because it has a much more massive core. Beyond Jupiter, core-generated heat becomes significantly more important than solar radiation received.
Calculations based on how the Sun's energy falls off with distance gives us theoretical temperatures for each of the planets, according to their distances from the Sun. Actual average surface temperatures can be worked out for each from spectroscopic studies, and these actual temperatures are considerably higher than theoretical ones for the outer four planets.
The Concore Model tells us that, the more massive the body, the greater the amount of core heat which it is producing. And this is borne out by the figures. Jupiter has an average temperature of 128 K, Saturn is at 105 K, Uranus at 49 K, and Neptune at 55 K [E18]. The source notes that Jupiter has a very hot core, while the Uranus core produces only one-fifth the heat that Jupiter does, and less than half of that from Saturn. It also points out that Neptune has a hotter core than Uranus.
Dependence on mass does explain why Neptune is hotter than Uranus, even though it receives less radiation from the Sun. According to NASA planetary data, the relative masses of Jupiter, Saturn, Uranus, and Neptune are 317.8, 95.2, 14.5, and 17.1. So their temperatures follow their masses, and presumed core activity, quite closely.
Fig. OC405-F11. Black-body peak radiation at lower temperatures. From [E10].
We already know that the temperature of a surface determines the peak wavelength of its black-body radiation. Figure F11 shows these peak wavelengths plotted for various temperatures -- we are most interested in the lower black curve, for bodies at 100 K, which radiate at wavelengths between 10 and 100 um (1 um is 1 micron, a millionth of a metre).
Reference [E9] gives the link to an on-line calculating tool giving the peak radiation wavelength for a body at a given temperature.
The average temperatures for Jupiter, Saturn, Uranus, and Neptune were mentioned above. Slotting these temperatures into the calculator gave the following peak wavelengths (in um):
So if Planet Nine is of similar size to Uranus or Neptune, it should be radiating at a similar wavelength, about 50-60 nm, and it should be detectable using telescopes designed for this infra-red band, and set up to distinguish individual objects. Although infra-red carries less power than visible light, the objects looked for will be very much closer than any star.
If this technique can be successfully used, it should give a whole new picture of the "Oort Worlds" like Planet Nine, perhaps not only individual worlds and how they are distributed, but also their moons and their groupings -- sub-solar systems.
A final thought on Planet Nine
A recent suggestion has been that Planet Nine, if it exists, may be small black hole [E15] -- a Vortex Planet. A Vortex Planet would be like a Vortex Star, only lower in mass. The lowest-mass White Dwarf currently known has a mass about 17% of the Sun's [E20], so a Vortex Planet would have less than about 0.15 of one solar mass.
It could be argued that "Vortex Planet" would be an inaccurate name for a rapidly-rotating vortex body, of unknown shape, just because it had the mass of a planet. If such objects are ever found, a better name may be devised. But the inaccuracy is no greater than that in calling a Neutron Star a star.
Although at present unknown, there seems no reason why Vortex Planets could not exist. As they would emit in the near infra-red, in highly directional Vortex Beams, they would be hard to find off-axis. One distinguishing feature of their infra-red radiation is that it should be polarized..
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References and Links
[E1]. Larry McNish. RASC Calgary Centre -- Dwarf Planet Update. https://calgary.rasc.ca/dwarfplanets.htm .
[E2]. Mike Brown. How many dwarf planets are there in the outer solar system?. http://web.gps.caltech.edu/~mbrown/dps.html .
[E3]. Caltech Astronomy - Mike Brown. http://www.astro.caltech.edu/people/faculty/Mike_Brown.html .
[E4]. List of Solar System objects by size. https://en.wikipedia.org/wiki/List_of_Solar_System_objects_by_size .
[E5]. New Horizons Spacecraft Returns Its Sharpest Views of Ultima Thule. https://www.nasa.gov/feature/new-horizons-spacecraft-returns-its-sharpest-views-of-ultima-thule .
[E6]. File:F oort cloud.jpg. https://commons.wikimedia.org/wiki/File:F_oort_cloud.jpg/ .
[E7]. Planet Nine. https://en.wikipedia.org/wiki/Planet_Nine .
[E8]. Planet Nine may be responsible for tilting the Sun. http://www.astronomy.com/news/2016/10/planet-nine-tilting-the-sun .
[E9] David Noel. CM603: How to Locate Planet Nine. http://www.aoi.com.au/Cameos/CM603/index.htm .
[E10] Black-body spectrum. http://www.sun.org/uploads/images/BlackbodySpectrum_2.png .
[E11] List of Solar System objects by greatest aphelion. https://en.wikipedia.org/wiki/List_of_Solar_System_objects_by_greatest_aphelion .
[E12]. 2017 MB7. https://en.wikipedia.org/wiki/2017_MB7 .
[E13]. Sednoid. https://en.wikipedia.org/wiki/Sednoid .
[E14]. Hypothetical Planet X. https://solarsystem.nasa.gov/planets/hypothetical-planet-x/in-depth/ .
[E15]. Sid Perkins. ‘Planet Nine’ may actually be a black hole. https://www.nasa.gov/feature/new-horizons-spacecraft-returns-its-sharpest-views-of-ultima-thule .
[E16]. David Noel. XT807: The Concore Model of planet and star interiors. http://www.aoi.com.au/Extracts/XT807.htm/ .
[E17]. David Noel. Temperatures of the Earth -- a Globe in Space (a re-analysis with some surprising results). http://www.aoi.com.au/bcw/EarthTemp/index.htm .
[E18]. Matt Williams. What is the average surface temperature of the planets in our solar system?. http://https://phys.org/news/2014-12-average-surface-temperature-planets-solar.html .
[E19] David Noel. Finally, the True Origin of Earthquakes?. http://www.aoi.com.au/bcw1/Finally/index.htm .
[E20] Mukremin Kilic et al. The Lowest Mass White Dwarf. https://arxiv.org/abs/astro-ph/0611498 .
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Version 1.0 published November 2019 as Segment E of the book "The Oort Cloud: Almost all the Universe". AOI Press, ISBN 9798614884314.
Version 2.0 placed on web at "AOI.com.au", 2022 Jun XX.