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What is the elusive Planet Nine?

Astronomers have recently discovered the solar system's most distant object, thus far, which they creatively nicknamed "FarFarOut." It is 3.5 times more distant than Pluto (140 times farther away than the Earth from the sun). The proposed distance for Planet 9, according to Greg Laughlin, an astronomer at Yale University is, "[the] best estimate for its [planet 9's] current position and brightness put it about 950 times farther than Earth from the Sun." For now, Planet 9 remains an elusive possibility as we continue to explore what lies on the fringes of our own solar system and galaxy.

If someone mentions the “Ninth Planet” various Pluto enthusiasts may have a singular tear drip down their faces. However, Pluto, a dwarf planet, is not actually categorized as a main planet and astronomers now use this term to define an elusive planet on the outskirts of our solar system. There is a current belief in the theorized presence of a  planet in the outer region of our solar system- specifically, in an area known as the Kuiper belt. Many scientists, in fact, deem the term “Planet 9” insensitive  to the discovery and to the discoverer of Pluto and there is a petition to change the name. This can be seen in the July 29 issue of the Planetary Exploration Newsletter. Naming rights typically go to the discoverer and so it is still possible that the nickname “Planet 9” will not persist when the planet is possibly discovered.

At the Carnegie Institute for Science, Scott Sheppard and Chadwick Trujillo suggested the existence of a ninth planet in 2014. California Institute of Technology scientists Konstantin Batygin and Michael E. Brown suggest “Planet 9 could be the core of a giant planet that was ejected from its original orbit by Jupiter during the genesis of the Solar system.” Current estimates place it about 500 AU (astronomical units) away and puts it on about a 10,000 year orbit with an orbital pattern unlike other planets in the solar system. There are several theories circulating on just how the planet was formed. For example, it may have been captured by another star or a rogue planet that was later drawn in.

wubx3wqmmitex9tlv8xl.jpg
Simulated composition of Planet Nine as created by Christoph Mordasini from the University of Bern and his PhD Student Esther Linder 

Some even hypothesize in situ formation, meaning that the planet was made “on site” and not drawn in or ejected by one of the known planets. If it does exist Planet 9 would  be able to explain various odd phenomena occurring at the fringes of our solar system. For example, it would explain the tilts of the Kuiper Belt Objects (KBOs). As stated by Malhotra from the University of Arizona, “The amount of warp we see is crazy…to me, it is the most intriguing evidence for Planet Nine I’ve seen so far.”

The suspected presence of another planet beyond Pluto was posited due to unusual orbits of KBOs that could be impacted by the gravity of a possible distant planet. There are trans-Neptunian objects (TNOs) in and beyond the Kuiper belt (the area outside the orbit of Neptune) that have also been observed to be compelling evidence for the presence of Planet 9 due to the cluster of their orbits among other features.

kuiperbelt.jpg
Image from: http://astronomy.swin.edu.au/cosmos/K/Kuiper+Belt

 

There are several pieces of evidence that have led to the belief in the presence of Planet 9 including:

*clustered orbits for eTNOs (extreme Trans-Neptunian objects)

*high perihelia (a perihelion is the point in the orbit of a planet, asteroid, or comet at which it is closest to the sun) of objects such as 90377 Sedna which are detached from the influence of Neptune.

*high inclinations of the eTNOs with orbits approximately perpendicular to the orbits of the eight known planets of our solar system.

*high inclinations of the TNOs with semi-major axis less than 100 au.

Trujillo and Sheppard showed that 12 extreme TNOs discovered have orbits that are almost aligned rather than spread randomly throughout the belt. The closeness of the clustering leads astronomers to believe in the presence of a planet. However, this is currently only conjecture.

p9_kbo_orbits_labeled
Image from: Caltech, R.Hurt/ IPAC, Worldwide Telescope

In all scientific pursuits it is important to consider alternative explanations for events. There are various other explanations that have been put forth in order to explain some of the strange tendencies surrounding this area of our solar system:

*Shankman et al. concluded that the existence of Planet Nine is unlikely and that the currently observed alignment of the existing eTNOs is a temporary phenomenon that will disappear as more objects are detected. In essence, there is not a planet 9, but rather many objects we just haven’t detected yet.

*Madigan and McCourt posited inclination instability as an explanation for the high inclinations of the eTNOs however after testing the simulation it was determined that most of the objects were ejected too quickly on a timescale for inclination instability to have a chance of occurring.

*Trujillo and Sheppard propose a single large planet shepherding the other objects.  Raùl and Marcos think that there are two planets in resonance (a binary system). This is similar to Bouvard’s observation of Uranus’ motion due to unknown gravitational forces that eventually led to the discovery of Neptune. Both, however, suggest the role of the Kozai Mechanism. Brown and Batygin state that if this mechanism was in fact in place, the ratio of the semi-major axes would be nearly equal to one which indicates multiple planets with orbits in tune with the data set would be necessary.

What is the Kozai Mechanism?: It is one of the most studied phenomena currently. It is a phenomena that affects the orbit of a binary system due to the impacts of a distant third object. The distant object causes the orbit of the binary system that may have, initially, been near circular to become highly eccentric. It can also make an object in prograde motion (motion in the same direction of the central object it orbits) exhibit retrograde motion (motion in the opposite direction of the central object it orbits) .

binorbit.gif
Image from: https://www.atnf.csiro.au/outreach/education/senior/astrophysics/binary_intro.html

New discoveries are always occurring as we realize the vastness of the universe contains so much that we do not already know. For instance, astronomers have recently discovered the solar system’s most distant object, thus far, which they creatively nicknamed “FarFarOut” which is 3.5 times more distant than Pluto (140 times farther away than the Earth from the Sun).

In comparison, the proposed distance for Planet 9, according to Greg Laughlin, an astronomer at Yale University  is, “about 950 times farther than Earth from the Sun.” For now, Planet 9 remains an elusive possibility as we continue to explore what lies on the fringes of our own solar system and galaxy.


References:

(1) https://www.sciencemag.org/news/2019/02/astronomers-discover-solar-system-s-most-distant-object-nicknamed-farfarout

(2) Does Planet Nine Exist? Featuring Dr. Konstantin Batygin

(3) https://iopscience.iop.org/article/10.3847/0004-6256/151/2/22

(4) https://arxiv.org/abs/1706.06609

(5) Kozai mechanism

(6)Planet Nine

(7) https://iopscience.iop.org/article/10.3847/2041-8205/824/2/L23/meta

(8) https://www.popsci.com/could-there-really-be-9th-planet-that-we-didnt-know-about#page-2

(9) https://www.scientificamerican.com/article/closing-in-on-a-giant-ghost-planet/

(10) https://gizmodo.com/the-best-guess-at-what-planet-nine-looks-like-1769847614

I'm a rising sophomore at the LSA Honors College at the University of Michigan-Ann Arbor. Currently, I'm on a pre-medical track. In my free time, I write articles, volunteer at a chronic pain outpatient facility with UM Medicine, do research, compete in HOSA, and, of course, enjoy photography and singing. In my spare time I manage my own travel and lifestyle blog: @journeythedestiantion on instagram and journeythedestination.weebly.com.

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