A Ninth Planet In Our Solar System: Why Is Modern Astronomy Advocating For A Giant Body In The Kuiper Belt? (Giuseppe Liuzzi)

This website post is characterized by a scientific nature, and will scrutinize one of astronomy’s latest, most challenging research enterprises that has spawned an enormous amount of peer-reviewed papers and news article in the past three years, that is the probable existence of a giant, icy, remote planet located in the outer region of the solar system, named Planet 9.

Readers will be presented with a comprehensive, detailed, and up-to-date analysis of the data that has been accumulated to validate the hypothesis of a ninth planet, ever since the latter was announced in 2016. Furthermore, it will also lay out the methodologies that have been utilized to capture data, and how the latter has been evaluated and interpreted by astronomers who are actively engaged in finding direct evidence. The essence of the analysis will revolve around the following research question:

 

What is the role of Planet 9 within our solar system, what has scientific inquiry discovered about the ways in which it exerts an influences on it, and what could it mean for the past, present, and future of our solar neighborhood?

 

After an introduction providing a general overview of the history of the solar system, attention will be redirected to a specific outer region where Planet 9 is thought to be located in, called Kuiper Belt. A thorough review of the astronomical past of this outer zone will be offered, whereby a number of discoveries will be linked together until they ideologically formed a theoretical framework upon which considerations of a giant planet in that area began to be justified. The subsequent section will present a vast series of data available advocating for the existence of Planet 9, including its influences on the region hosting it and on the whole solar system. Successively, contrasting opinions about the research procedures utilized to gather evidence about it will be scrutinized, and alternative interpretations advocating for the non-existence of the newly-proposed celestial body will be discussed. Finally, a conceptualized analysis of the ways in which the presence of a ninth planet would hold beneficial connotations will conclude the essay.

INTRODUCTION: Space activist and entrepreneur Rick Tumlinson is a prominent figure in the cosmological community, on top of being a well-known advocate for the exploration and colonization of our solar system. One of his most notorious aphorisms states that “discovering new things that will tell us about the history of our solar system, help reveal the secrets of life, and continue blazing the trail that may someday be traveled by the rest of us”.¹

Jamaican journalist and political activist Marcus Garvey, the founder of garveyism, an ideological movement which advocated for the empowerment and consolidation of African-American individuals in the early 20th century, declared on several occasions that “a people without the knowledge of their past history, origin and culture is like a tree without roots”.² Civil rights movement leader and Christian minister Martin Luther King Jr. has often quoted the past and its pertinence in the moulding of the human condition, with apothegms such as “we are not makers of history. We are made by history”.³

It is not relevant whether or not one’s background is comprised of astronomical interests, aspirations for equal rights regardless of skin colour, or proneness toward a societal standardization whereby constitutional and legal rights are justly enforced for everybody. The common denominator uniting the vast majority of the influential characters which have galvanized millions of individuals around the globe, regardless of their ideological drives, is represented by the awareness that the more efficient, virtuous way to understand ourselves is to inquire about our history. When projecting this mindset onto the bigger, cosmogenetic picture, it is then imperative to scrutinize our solar system’s past in order to discern and eventually decipher the dilemmas comprising our existence.

 

OUR STELLAR HOME: A BRIEF OVERLOOK ON THE HISTORY OF THE SOLAR SYSTEM: Scientific inquiry on the topic of how the solar system was formed has been going on for centuries, and specifically saw its intellectual boost as the result of the Copernican Hypothesis, which challenged the centennial Ptolemaic, geocentric model by describing Earth as a wandering planet orbiting the Sun. ⁴

The most widely accepted theory is the Nebular Hypothesis. ⁵ The theorem revolutionized cosmogony and was initially proposed in the 1700s as the result of the conjunct works of Kant, Swedenborg, and Laplace. This perspective advocates for the birth of planets in relation to the evolution of nebulous material. ⁶

According to the nebular hypothesis, our solar system originated from a Nebula, that is an interstellar cloud mainly comprised of gas and stellar dust, roughly 4.5 billion years ago. As millions of years went by, gravity eventually stimulated such nebula which began to condensate whilst forming different sections, or regions, presenting dissimilar degrees of density. The portions of the nebula characterized by denser material subsequently started amassing into more solid matter. This phase represents the birth of our solar system, which later led to the spawning of what we now experience as our Sun, planets, and moons.

As gravity gradually applied its force to make the gas in those regions denser, the rotation of the gas itself started to increase. The positive correlation between these two factors provoked a phenomenon called Accretion, whereby the diffusion of the gas cloud moulded the aforementioned zones into a rotating disk which in turn started to gravitationally attract even more nearby matter of gaseous nature. Conservation of momentum, meanwhile, made the disk rotate at increasingly higher rates, a process which resulted in both increased internal pressure and heat. Accretion caused our solar system to be shaped like a plane, but most importantly is the first pivotal seed upon which our Sun was cosmologically conceived.

In fact, the Sun was formed in the most active phase of accretion where the majority of the condensed material was redirected toward the highly-spinning center of its disk, until a ball in its center began to take form. While this celestial object was accumulating most of the stellar material in its vicinity, and kept on heating up, the rest of the objects were still affected by the gravitational pull of the latter, hence they remained orbiting about it, though were able to escape its grip and avoided being absorbed. These stellar bodies flattened out into the vast zone surrounding the accretion disk, and originated the second pivotal seed of our solar system, that is a Protoplanetary Disk.

The protoplanetary disk is a vast formation of gas, surrounded by sparse and tiny amounts of dust, which orbits a newly-born star. Within the disk, matter is slowly directed inwards, and dust particles are progressively enlarged and solidified until they grow into small rocks. Protoplanetary disks are thought to be the precursors of what will later become larger and more massive planetesimals, a primitive versions of planets which diameters can reach hundreds of kilometers.

The Planetesimal Hypothesis, ⁷ therefore, explains how the material of the protoplanetary disk is gradually pulled together by the gravitational pull exerted by the main object at its center, in such a way that the gas and dust composing the disk is eventually stiffen into small chunks of pebbles which get larger and larger, until they transform into planets and moons. The newly-formed larger objects closer to the main star presented silicates and metals which originally came from the nebula itself, and were characterized by high boiling points that can solidify, making up the telluric composition of what we today know are the Sun’s closest sisters, that is Mercury, Venus, Earth, and Mars. Conversely, planetesimals that formed in the outer region of the accretion disk, which have been named Jupiter, Saturn, Uranus, and Neptune, were not as exposed to the solar radiation emanating from the center of the disk itself, hence presented more icy compounds than silicates and metal. As a consequence, they grew way larger than the inner planets because they also developed atmospheres capable of capturing and storing vast amounts of helium and hydrogen.

However, not all the gaseous and dusty material comprising the accretion disk eventually turned into planetesimals. Especially in the outer region farther away from the star, plenty of fragments and detritus accumulated in a number of zones. One of them is called Kuiper Belt, and represents the area which is supposedly hosting the object upon which the focal analysis of this paper will revolve around, that is Planet 9.

The final piece of the puzzle in the nebular hypothesis is how the Sun became the potent energy source with which planets have evolved, ending up in the differentiated terrestrial and atmospheric conformations they are characterized by. As accretion took place along millions of years, the levels of pressure and density of hydrogen at the center of its disk, where the Sun was formed, eventually became so enormous that the process of thermonuclear fusion was achieved. The Sun, which was called a protostar back then, saw a constant and gradual increase in pressure, density, and temperature, until an equilibrium between the outward thermal pressure and the weight of the material located above it and pressing downward, hence inward the object, had been reached. At that point, the star was balanced and thermonuclear reaction fully active. This process is called the Hydrostatic Balancing Of Stars. ⁸ This mechanisms represents the final phase in the formation of a solar system. Now that the Sun is stabilized and planets and moons have established their elliptical behaviours within the system, nature does its course; these celestial bodies evolve and mutate along billion of years, in reference to their proximity to the main star, to the configurations of the gaseous envelope surrounding their outer structure, and to the terrestrial composition of the principal materials they are comprised of.

This video, narrated by renowned professor Stephen Hawking, features a spectacular 3d reconstruction of the formation of the solar system through the utilization of CGI techniques (WordPress refused to embed it, so a direct link has been provided for the readers).

FAR AWAY, MYSTERIOUS, AND FASCINATING: THE KUIPER BELT MAY HOST ONE OF 21th CENTURY’S BIGGEST MYSTERIES: As discussed above, not all of the gas and dust within an accretion disk is pressurized enough to be remodeled into planetesimals, and if all goes well, into moons or planets afterwards. A portion of it is likely to be pushed farther from the center of the disk, and may later on assemble into outer, colder regions. In these areas, it is common to find asteroids and massive, icy bodies which still do not escape the Sun’s bending of the spacetime, slowly revolving about it.

An example of such space territories is the Kuiper Belt, a vast ring of frozen celestial bodies located outside our solar system’s farthest planet’s orbit, that is Neptune. ⁹ Relatively close to it we find Pluto, reclassified in 2006 to a Dwarf Planet, ¹⁰ a new typology of stellar objects which do not fully fit within the physical characteristics of a regular planet, especially in terms of dimensions and gravitational dominance. ¹¹ Pluto was once called the ninth planet of our solar system, until it lost that prestigious title. The Kuiper Belt is comprised of a number of different objects, including dwarf planets Pluto, Eris, and Haumea, on top of several comets, ice, and floating rocks. As mentioned above, these objects are so far away from their maternal star that it may take them a very long time to complete a full orbit about the latter. For instance, Eris completes its orbit in 557 years. ¹²

Planetologist Alan Stern declared that “going to the Kuiper Belt is like an archaeological dig into the history of the solar system”. ¹³ Scrutinizing such a mysterious place, in fact, may lead cosmology to a more comprehensive understanding of the history of the solar system, and such enterprise is the main focal objective of the New Horizons spacecraft, which in 2015 reached past Pluto’s orbit, in an attempt to further investigate the Kuiper Belt area.

This video discusses 10 things to know about the Kuiper Belt area (WordPress refused to embed it, so a direct link has been provided for the readers).

PLANET 9 MAY BE OUT THERE: HISTORICAL LINKAGES ABOUT HOW IT WAS “DISCOVERED”: One year after New Horizons was launched, an astounding announcement was made by a group of astronomers: a giant, undiscovered planet may be orbiting in the Kuiper Belt zone, in the vicinity of Neptune. If we took a look at the history books, however, we would realize that speculations on massive body in such area have actually started roughly 200 years ago.

The first scientist to notice something was going on was Paris resident Bouvard, ¹⁴ who in 1820 was studying Uranus and noticed discrepancies between old recordings and new data of the planet’s position while engaged in its orbit. He came to the conclusion that another planet may be pulling it from its path, even if just slightly. Ever since such discoveries, the orbital behaviour of Uranus has appeared unconventional. Few decades later it was French mathematician Le Verrier who calculated the possibility of another planet past Uranus, until astronomer Galle sighted it, and was later named Neptune. The newly added eighth planet stimulated further curiosity in the field, and scientists were gradually more interested in what may be lying past it, predicting that a ninth planet could possibly exist in that area.

However, as more data was gathered by new telescopic technologies and spacecraft, a better understanding of the mathematical models and physics regulating the movements of the planets, moon, and objects in our system was achieved. According to these readings, the shifting and positioning of them was not atypical, and in 1993, after decades of speculations, the search for a giant ninth planet was abandoned due to the fact that no dissimilarities in the orbital and elliptical behaviours of stellar bodies in the outer region could be detected.

As analyses of the Kuiper Belt area kept accumulating data about that region, further scrutiny of Pluto made astronomers realize how vast and diversified such region was. This realization is what maintained scientists inspired about the possibility of other relevant bodies in the belt, and in 2003 a significant discovery was made, that is a massive body, later named 2003 VB12, which represented the most distant object ever seen by science orbiting our star. However, it was not a giant planet, but rather a small body later called Sedna with an extremely elongated orbit that made it complete a full revolution in about 10,000 years. Calculations about Sedna’s peculiar orbital behaviour is what continued whispering in astronomer’s ears that something massive was shaping the orbits of a number of objects in the Kuiper Belt. This was ulterior confirmation that something was exerting relevant gravitational pull, in fact three years earlier orbital analysis of object 2000 CR105 had already suggested that a Mars-sized planet may have slightly pulled it away from its Neptunian orbit. Interestingly enough, this discovery was classified as a miscalculation until Sedna was found, and scientists noted that both entities had been redirected in the same direction. ¹⁵

As more objects were being identified in the Kuiper Belt zone, and their elliptical movements were analyzed and compared, it became apparent that some of them exhibited orbital alignments that were tugged by something massive in the same direction of the aforementioned Sedna and 2000 CR105. After several computer simulations, four researchers announced in 2016 that a massive planet 10 times the size of Earth, located at about 600 astronomical units from the Sun, was likely to exist in the Kuiper Belt area. ¹⁶ They later named it Planet 9. ¹⁷

HOW CAN WE PROVE IT? PLANET 9’s ACCUMULATED EVIDENCE AND ITS INFLUENTIAL BEHAVIOURS ON THE KUIPER BELT, AND POSSIBLY, ON THE WHOLE SOLAR SYSTEM: Hypothesizing about the existence of a ninth planet in our solar system is therefore, in sum, justified by observational and computational data pointing toward the presence of a giant body in the Kuiper Belt influencing the elliptical behaviour of some of the latter’s objects. Specifically, it is its gravitational influence on those objects, and surprisingly its potential side effects on the solar system, that are motivating astronomic research to maintain its interest focused on the outer section of our stellar backyard.

First and foremost, it appears that several of those objects are both pushed and pulled by the bending of spacetime caused by something comprised of huge mass, which has not been detected by modern telescopes yet. Planet 9’s influence is thought to mainly dictate the angular orbital demeanor of some trans-Neptunian objects (TNOs), and data points toward lower perihelion distances for the potential new planet that may cause angular restrictions for celestial objects in the Kuiper Belt zone denoted by perihelion distances longer than 40 astronomical units, and semi-major axes greater than 250 astronomical units. ¹⁷

Furthermore, the speculated characteristics of Planet 9 and its gravitational pull on nearby objects that have been computed during the last few years eventually led to the formulation of a mathematical model, which has resulted both in the gradual discovery of TNOs, ¹⁹ and in substantiated predictions about the latter’s orbital behaviours in the cold region hosting them. ²⁰

Another piece of evidence which is interconnected to the previously discussed analysis of TNOs orbital demeanors results once again from the observation of some icy objects and dwarf planets, which orbits demonstrate a tendency of aggregating, or clustering together, due to the gravitational influence of something of massive proportions that must be located in their vicinity. ²¹

Observations of TNO’s featuring high inclination and semi-major axes being less than 100 astronomical units have also been put in the mix, as a possible result of Planet 9’s influence. Since these objects enter perpendicular orbits, they show low perihelia so that their orbits converge with those of Neptune. An encounter with a giant celestial body such as Planet 9 is predicted to be, may explain the lowering of such TNO’s semi-major axes to less than 100 astronomical units, where the object’s orbits is no longer influenced by the massive body. This would justify the orbits of some TNOs such as, for example, 2008 KV42. ²²

Planet 9’s manipulations may not limit themselves to the Kuiper Belt region. According to recent scientific data and simulations, it may have a greater impact on the entire solar system itself, in the form of a six-degree tilt of the latter between the proportional plane of the system’s bigger planets and the solar equator. ²³ This perspective emphasizes an evolutionary model of the inclination of the inner big planets’ plane in relation to Planet 9ʼs deducted mass and orbital components. The longitude of the ninth planet’s ascending node is supposedly connected to the longitude of the ascending node of the other inner planets’ plane, while also inhibiting the former’s longitude of the node on the ecliptic, that is the imaginary line indicating the path of the Sun as it appears to move over the course of one year. Conversely, some interpretations on the matter of the tilting of the solar system are more conservative and less invasive on the current body of knowledge, advocating for a mere one-degree tilt on the latter operated by Planet 9. ¹⁶

This video features a 16-minute interview with the two scientists who, in 2016, published a series of peer-reviewed journal articles advocating for the existence of Planet 9. This audio-visual document offers an interesting perspective on the ways in which evidence from the Kuiper Belt area has been processes and interpreted, coming directly from the main sources of what could be one of the 21th century’s biggest discoveries in the astronomy field (WordPress refused to embed it, so a direct link has been provided for the readers).

THE (SPECULATED) COMPOSITION AND BIOGRAPHY OF THE NINTH PLANET: Not much is known about the characteristics of Planet 9 besides its potential mass of five to ten times that of Earth, and the time that it should take it to perform a revolution about the Sun, that is roughly 10,000 to 20,000 years. The dearth of information is due to the absence of physical observations, and to the difficulties in properly locating it in the celestial sphere because of its supposedly extremely dim luminosity.

As far as its origins are concerned, yet again there is no certainty and most of the proposed hypotheses are essentially of speculative nature. While astronomers around the world are waiting for the launch of the James Webb Telescope in 2022, which will provide photographic evidence of the Kuiper Belt area thus definitely confirming or disproving the existence of Planet 9, along with analytical data on how and why it was formed, what is now at the public’s disposal is a number of theories on the factors which may have contributed to originate it.

A prominent theory has been recently advanced by one of the researchers who initially announced the probable existence of such giant body in the outer region, Michael Brown. The scientist affirms that Planet 9 may have formed alongside Neptune and should present an overall conformation very similar to the latter, though as the solar system evolved along million of years, it eventually got pushed farther away by gravitational forces, ²⁴ to the remote and sunlight-free region known as Kuiper Belt.

PLANET 9 MAY NOT BE AS FASCINATING, BUT RATHER… DANGEROUS: Alternative views are less optimistic. A study published in 2019 advocates for the possibility that the variations of TNOs’ orbital behaviours may not be the result of a big planet’s influence on spacetime, but rather the effects of a primordial black hole of very small diametrical extension. ²⁵

An episode of popular YouTube channel SciShow Space tackles the issue of Planet 9 possibly being a black hole in this riveting video (WordPress refused to embed it, so a direct link has been provided for the readers).

Moreover, other conjectures align themselves on the same frequency of pessimism. When the Sun will become a white dwarf in about seven billion years, our current physics model, Einstein’s general relativity, predicts that the inner planets will be sucked into its mass, while the bigger, more distant planets such as Uranus and Neptune will instead be pushed farther away at a safe distance from the galactic blast. Planet 9’s existence would change the solar system’s fate, leading to its entire demise. According to some researchers, the giant body would instead be attracted by the Sun’s explosion, heading inwards toward it, and thus exerting its gravitational pull on those outer planets which would follow it into collision with the star, hence to their death. ²⁶ This process would result in the destruction of our entire solar system, or in the best case scenario, in its significant reduction to a mere one or two planets.

PLANET NINE OR PLANET NONE?: Contrasting opinions about the ninth planet are also directed toward perspectives promoting its non-existence. Some have proposed that the scientists that identified Planet 9 may simply have miscalculated the data related to unconventional TNOs by utilizing wrong detection techniques, and showed only a limited number of objects supposedly being affected by the giant body whereas according to mathematical models, more should be impacted by such massive entity. Others declared that they may even be the victims of detection bias, where the effects of a phenomenon are over- or under-estimated in order to fit a preordained model. ²⁷

In addition, there is debate among the astronomical community about the orbits of some NTOs, as they represent one of the main arguments for the existence of Planet 9. The discoverers of the ninth planet advocate for such stellar body to cause the orbits of these NTOs to be clustered, but parallel surveys have not been able to detect these clustered orbits, and have instead shown that they are randomly distributed throughout the region. ²⁸

PLANET 9 AND US: WHY ITS EXISTENCE IS RELEVANT TO OUR IDENTITY AS CITIZENS OF THE UNIVERSE: After having presented a thorough and meticulous examination of the evidence advocating for the existence of a ninth, giant, icy planet unhurriedly revolving our star without ever been physically spotted, readers may ask themselves whether or not the presence of another massive stellar body in human’s solar system may be momentous at all, and if it is, why would that be?

At the beginning of this online journey, I suggested that it is important to know our history as a mean to better understand ourselves. As a consequence, I advocated for the notion that inquiring about Planet 9 is essential to further establish the history of our solar system, hence of our astronomical past. Furthermore, I referred to a number of peer-reviewed, scientific material declaring that Planet 9’s influential attributes may extend not only to the region hosting it, but to the entire system itself, and possibly, even to the other giant planets comprising it when the latter will suffer the consequences of the Sun’s mutation into a deadly white dwarf. Apart from the repercussions of a ninth planet on the morphology of our solar neighborhood, there are a number of reasons why, should it be one day confirmed, it represents a relevant leap for mankind.

Firstly, its scientific corroboration would directly position itself into the history books, which will have to declare how another important planet in our solar system was discovered after taciturnity on the subject which has lasted for more than 170 years (or 85 if some readers still want to call Pluto a real planet).

Secondly, it would further demonstrate that our detection techniques to investigate the galaxy, such as telescopic imaging and spacecraft enterprises, represent a valid and efficient way to scrutinize even the most remote regions surrounding us, and that even if scientific consensus is often difficult to achieve, synchronized collaboration between parties from all over the world can produce enthralling revelations, regardless of the physical separation between us and the objects we want to investigate.

Thirdly, Planet 9’s confirmation would make our solar system a little more similar to other distant star systems astronomers have been scrutinizing for the last couple of decades. Some of these systems include planets orbiting distant stars (exoplanets) with masses that are at least ten times more massive than Earth. Until 2016, when the possibility of the existence of a ninth planet was announced, scientists were not granted with the chance to directly research them, due to the incredible distances separating us from them that our current technology cannot handle. Now that evidence points towards the presence of an exoplanet, or super-Earth in our solar neighborhood, direct observations of such mysterious entities would be possible, and in the near future, robots may even be sent on the surface of Planet 9, and astounding data would be relayed back to us, just like it is happening nowadays thanks to the rovers sent on Mars. The prospects of a confirmation and direct observation of Planet 9 would mould our universal identity as citizens of the universe, by possibly infusing a sense of humility and demureness, as we realize that our solar system may not be so special after all when compared to others, and that super Earths like the ninth planet are not peculiar to our neighbors only.

 

References:

1 Exploration: Process vs. Product, https://spacenews.com/exploration-process-vs-product/
2 Goodreads, https://www.goodreads.com/quotes/280448-a-people-without-the-knowledge-of-their-past-history-origin
3 Mayo, A., Mayo, A. J., & Nohria, N. In their time: The greatest business leaders of the twentieth century. Harvard Business Press. (2005).
4 Westman, R. S. The comet and the cosmos: Kepler, mästlin and the copernican hypothesis. In The Reception of Copernicus’ Heliocentric Theory (pp. 7-30). Springer, Dordrecht. (1973).
5 Moulton, F. R. On the evolution of the solar system. The Astrophysical Journal, 22, 165. (1905).
6 How Was the Solar System Formed? – The Nebular Hypothesis, https://www.universetoday.com/38118/how-was-the-solar-system-formed/
7 Brush, S. G. The origin of the planetesimal theory. Origins of Life, 8(1), 3-6. (1977).
8 Schwarzschild, K. On the equilibrium of the sun’s atmosphere. Nachrichten von der Königlichen Gesellschaft der Wissenschaften zu Göttingen. Math.-phys. Klasse, 195, p. 41-53, 195, 41-53. (1906).
9 Levison, H. F., & Morbidelli, A. The formation of the Kuiper belt by the outward transport of bodies during neptune’s migration. Nature, 426 (6965), 419. (2003).
10 Why Isn’t Pluto a Planet Anymore?, https://www.space.com/why-pluto-is-not-a-planet.html
11 What Is A Dwarf Planet?, https://www.universetoday.com/72717/what-is-a-dwarf-planet/
12 NASA Science – Solar System Exploration, https://solarsystem.nasa.gov/planets/dwarf-planets/eris/in-depth/
13 On Pluto Time: Q&A with New Horizons Leader Alan Stern, https://www.space.com/30934-pluto-new-horizons-alan-stern-interview.html
14 Holman, M. J., & Payne, M. J. Observational constraints on planet nine: Astrometry of pluto and other trans-neptunian objects. The Astronomical Journal, 152(4), 80. (2016).
15 Morbidelli, A., & Levison, H. F. Scenarios for the origin of the orbits of the trans-neptunian objects 2000 CR105 and 2003 VB12 (sedna). The Astronomical Journal, 128(5), 2564. (2004).
16 Batygin, K., Adams, F. C., Brown, M. E., & Becker, J. C. The planet nine hypothesis. Physics Reports. (2019).
17 How we discovered Planet Nine, http://www.astronomy.com/magazine/2018/10/how-we-discovered-planet-nine
18 Cáceres, J., & Gomes, R. The influence of planet nine on the orbits of distant tnos: The case for a low-perihelion planet. The Astronomical Journal, 156(4), 157. (2018).
19 The Evidence for Planet Nine’s Existence I NOVA I PBS (YOUTUBE VIDEO), https://www.youtube.com/watch?v=hiJxgfBVGr8
20 New evidence in support of the Planet Nine hypothesis, https://www.eurekalert.org/pub_releases/2017-07/f-sf-nei071217.php
21 Brown, M. E., & Batygin, K. Observational constraints on the orbit and location of planet nine in the outer solar system. The Astrophysical Journal Letters, 824(2), L23. (2016).
22 Batygin, K., & Brown, M. E. Generation of highly inclined trans-neptunian objects by planet nine. The Astrophysical Journal Letters, 833(1), L3. (2016).
23 Gomes, R., Deienno, R., & Morbidelli, A. The inclination of the planetary system relative to the solar equator may be explained by the presence of Planet 9. The Astronomical Journal, 153(1), 27. (2016).
24 Planet Nine SHOCK: Astronomer reveals how Planet 9 was EJECTED from the solar system, https://www.express.co.uk/news/science/1130087/Planet-Nine-news-Planet-9-orbit-ejected-solar-system-Mike-Brown-Caltech
25 Scholtz, J., & Unwin, J. What if planet 9 is a primordial black hole?. arXiv preprint arXiv:1909.11090. (2019)
26 Was Planet Nine Found? Mysterious World Could Destroy Solar System (In Seven Billion Years), https://www.ibtimes.com/was-planet-nine-found-mysterious-world-could-destroy-solar-system-seven-billion-years-2409849
27 This Is Why Most Scientists Think Planet Nine Doesn’t Exist, https://www.forbes.com/sites/startswithabang/2018/09/14/this-is-why-scientists-think-planet-nine-doesnt-exist/#751176fb6e77
28 Sokol, J. New haul of distant worlds casts doubt on planet nine. (2017).