A couple weeks ago, I wrote about Ultima Thule, a Kuiper Belt Object that will be explored on New Year’s Day next year. One of the weirdest things about Ultima Thule is its normal-looking orbit. Now I’d like to touch on why the properties of this object’s orbit are so strange in contrast to other, much larger, objects, or even dwarf planets, located in the Kuiper Belt. Most things in the Kuiper Belt have highly elliptical and/or eccentric orbits; however, the problem is scientists are not too sure about the reason for such orbits. There are many theories though, and most focus on the existence of a large undiscovered planet beyond Neptune that may have caused objects in the Kuiper Belt to skew their orbits due to the gravitational influences of this mystery planet.
Recently, NASA and the American Astronomical Society have been discussing the orbits of Kuiper Belt objects and planets. A common theory to explain these orbits involves a giant planet with a “mass of 10 times that of Earth… circling 20 times farther out than Neptune” (1). However, the problem with this theory is that gravitationally, the theory might work, but on a visual level, there is simply no evidence to support it. This is because it is extremely difficult to observe planets on the outer rim of our solar system as there is no good background to observe for changes. A planet that is 20 times farther than Neptune will have a very poor apparent magnitude (2), and thus will be almost impossible to observe.
One planet that could fit the characteristics is Sedna, a dwarf planet on the very fringes of our solar system. Sedna orbits the Sun roughly every 10,000 years and we just happened to observe it in time! This dwarf planet is 76 AU away from the Sun at its closest and 936 AU away at its farthest (3). When Sedna begins its journey to its farthest point we won’t be able to see it anymore as it will become too dim. However, since the difference between the distances is so drastic, Sedna cannot be the mystery planet that influences orbits. Sedna’s orbit is simply to eccentric to be considered as this great influencer. There are other objects that have an even longer orbital period, but they are simply not planet-sized.
Now, what about Ultima Thule? Why is the orbit so circular and not as inclined or tilted as other objects? One explanation proposed by the American Astronomical Society is the presence of large clusters of asteroids instead of a giant planet. Asteroids could have an effect on specific regions of space rather than the whole Kuiper Belt. Scientists ran simulations on a model of the Kuiper Belt, and such clusters of asteroids had a significant effect on the eccentricity of the orbits. This could explain why some objects in the Kuiper Belt have eccentric orbits while others are nearly circular. Unfortunately, another mystery remains: the inclination of the Kuiper Belt objects’ orbits. It seems like the orbits tilt every which way in this region of space, which might mean some objects were gravitationally ejected from the inner or outer solar system but remained in obit. However, more research is required to determine what really causes the heavy inclinations of these orbits. (1)
(2) Apparent Magnitude: the brightness of an object in space as seen from Earth.