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Words Matter: What is Planet Nine?

What is Planet Nine?

Lily Amirzadeh


Planet Nine, a subject of constant curiosity, has made headlines once again. After decades of speculation, it is possible that scientists are coming closer to finding a definitive explanation of its existence. Harvard scientists Dr. Avi Loeb and Amir Siraj are developing an approach to find out if Planet Nine is a black hole, and, by extension, use this method to find other black holes in the universe.


Planet Nine, sometimes called Planet X, is questionably an object of our Solar System. It has not been directly observed, but scientists theorize that it might exist. Its existence would serve as an explanation for irregular orbits of objects in the farthest reaches of the Solar System, known as extreme trans-Neptunian objects (eTNOs). The eTNOs orbit in a far more elliptical pattern than they should, so the existence of Planet Nine as a mass causing these orbits would explain this irregularity. Planet Nine disrupts the orbits because its theoretical massive weight changes the gravitational pull that the eTNOs feel. Gravitational attraction to another object is essentially what causes orbits. Gravity causes all matter to be attracted to all other matter


. The heavier something is, the greater the force of gravity it exerts on other objects. For example, the Earth is attracted to the extremely heavy Sun because of its strong gravitational pull, and that gravitational attraction keeps the Earth in its orbit around the Sun. Likewise, the existence of a massive Planet Nine would disrupt the orbits of planets on the outskirts of the Solar System due to their mass, which gives it a very strong gravitational pull.


The Harvard professor and his undergraduate student will use a new method of their own creation to identify possible black holes. Black holes do have a staggering amount of mass, so Planet Nine being a black hole may explain the gravitational effects the eTNOs feel. However, black holes are very hard to detect due to their darkness. In order to determine if the object is a black hole, the team will use a new telescope, the Legacy Survey of Space and Time (LSST), to study the interactions between black holes and the objects near them, such as comets. Siraj explained that “In the vicinity of a black hole, small bodies that approach it will melt as a result of heating from the background accretion of gas from the interstellar medium onto the black hole.” This melting will eventually lead to accretion flares, which light up the area around the black hole, making it more visible.



Discourse surrounds Planet Nine on all sides. Whether it is a black hole, a giant undetectable planet, a collection of millions of smaller objects, or simply nonexistent, is a question with little evidence pointing towards a single answer. Theories on the subject vary greatly with new material being published on it yearly. The lack of a simple explanation regarding Planet Nine serves as a reminder of just how little we know about many of the celestial bodies relatively close to us.



Edited by: Yashaswini Meduri, Katie Last, Nicole Wilkes

Graphic by: Buse Koldaş


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