A Search for Planet Nine

by Hazal Kara
Despite our Solar System’s close proximity to Earth relative to the rest of the universe, there is far too much that humans do not know about it yet. One of the biggest unanswered questions is whether or not there are planets beyond Pluto. As with most scientific mysteries, there is a lot of discourse between researchers on this topic. What do the proponents claim? What do the opponents of the Planet Nine hypothesis say? This article will explore both sides of this debate.

While the search for a ninth planet has been ongoing for decades, Konstantin Batygin and Michael Brown made headlines in 2016 with their article “Evidence for a Distant Giant Planet in the Solar System” [1]. In the article, Batygin and Brown reason that there must be a ninth planet due to the “unexpected clustering” of six high perihelion objects in the Kuiper belt. They postulate that another body is affecting the orbits of these objects and only a body the size of a planet can induce such a change.
Batygin and Brown used computer simulations to determine some characteristics of this hypothetical planet. They came to the conclusion that it must have a mass around 10 times that of Earth’s [2]. They also found that it must orbit around the Sun at a distance 20 times greater than Neptune, which comes out to be about 600 astronomical units (AU) or 89.76 billion kilometers.

However, there have been a lot of counter arguments to Batygin and Brown’s case for Planet Nine. One of the most notable comes from Dr. Ann-Marie Madigan from the University of Colorado, Boulder. Taking the collective gravitational forces between all relevant objects into account, Madigan and her team claim that the impacted orbits of the six Kuiper Belt objects (KBOs) could be caused by a disk of icy debris in a way that resembles a giant planet [3]. Other research groups have since conducted investigations regarding the dynamics of objects in and beyond the Kuiper Belt, coming to similar conclusions as Dr. Madigan.
A criticism of Madigan\’s hypothesis comes from Scott Tremaine at the Institute for Advanced Study in Princeton. He states that the disk of icy debris must be on the upper end of the mass astronomers currently predict in our outer Solar System, thus making the possibility of its existence less likely [4].
Jakub Scholtz and James Unwin from Durham University and the University of Illinois at Chicago suggest that Planet Nine may be a primordial black hole (referring to black holes formed in the early universe), and, in fact, one of many in the vicinity. They also state that there must be a halo of dark matter around the black hole in that scenario [5]. Harvard researchers have since asserted that the detection of such a black hole would be possible through the yet to be completed Vera C. Rubin Observatory [6].
In the end, all sides make relatively strong arguments for what may be affecting the orbit of numerous KBOs. Hopefully, in the next few years, with new astronomical observations and data, scientists will gain insight into which case is more probable. Still, that won’t be the end of the journey – there is still much to discover about our Solar System.


[1] Batygin, Konstantin, and Michael E. Brown. “Evidence for a Distant Giant Planet in the Solar System.” arXiv.org, January 20, 2016. https://arxiv.org/abs/1601.05438.
[2] “Hypothetical Planet X.” NASA. NASA. Accessed October 4, 2020. https://solarsystem.nasa.gov/planets/hypothetical-planet-x/in-depth/.
[3] Zderic, Alexander, and Ann-Marie Madigan. “Giant Planet Influence on the Collective Gravity of a Primordial Scattered Disk.” arXiv.org, May 26, 2020. https://arxiv.org/abs/2004.00037.
[4] Skibba, Ramin. “Planet Nine Could Be a Mirage.” Scientific American. Scientific American, May 5, 2020. https://www.scientificamerican.com/article/planet-nine-could-be-a-mirage/.
[5] Scholtz, Jakub, and James Unwin. “What If Planet 9 Is a Primordial Black Hole?” arXiv.org, September 24, 2019. https://arxiv.org/abs/1909.11090.
[6] Overbye, Dennis. “Is There a Black Hole in Our Backyard?” The New York Times. The New York Times, September 11, 2020. https://www.nytimes.com/2020/09/11/science/astronomy-planet-nine-black-hole.html .
Figure References
Fig. 1: “Hypothetical Planet X.” NASA. NASA. Accessed October 4, 2020. https://solarsystem.nasa.gov/planets/hypothetical-planet-x/in-depth/.
Fig. 2: Hand, Eric. “Astronomers Say a Neptune-Sized Planet Lurks beyond Pluto.” Science, December 10, 2017. https://www.sciencemag.org/news/2016/01/astronomers-say-neptune-sized-planet-lurks-beyond-pluto.

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