Plutinos and Kuiper Belt Objects

Kuiper belt objects, which are a specific belt of asteroids having orbits beyond that of Neptune, can be split up into a few groups, including Classical Kuiper Belt Objects (CKBOs) and objects labeled Plutinos. Plutinos are KBOs having a 3:2 orbital resonance with Neptune, meaning that for every three complete orbits that Neptune makes around the Sun, a plutino completes nearly two. Plutinos also differ from CKBOs in that they are closer to the sun. Additionally, Plutinos have inclinations less than twenty degrees, as can be seen in Graph 1, while inclinations of CKBOs can reach up to about twice that. Graph 2 displays the distribution of aphelion distances for the known plutinos, with the average value at about 48-50 AU from the Sun. In Graph 3, the perihelion distribution is plotted, and it can be seen that they occupy nearly the same range of values as the aphelion distances, but in this case the variance is larger.

Pluto is also in a 3:2 orbital resonance with Neptune, leading to speculation that it is actually a large asteroid and shouldn't be considered a planet. Pluto differs from other Kuiper Belt Objects, though. First, it is about twice as large as the largest known KBO. Also, because of this, its slight atmosphere makes it much brighter than the rest. Most likely, Pluto is actually a member of the Kuiper Belt and originated in the same way as other asteroids in the belt. As a result, Pluto probably shouldn't be considered a planet. However, since Pluto has historically been considered a planet, one could argue that it should remain a planet, rather than be "demoted."

There are several ideas about how Plutinos and other KBOs got into their orbits. One possible explanation for the large inclinations of CKBOs is that Neptune scattered a few large planetismals into the Kuiper Belt. However, if this occurred, the orbital resonances of the KBOs would also be disturbed. Since nearly ¼ of the known KBOs are Plutinos (having a 3:2 orbital resonance with Neptune), this explanation doesn't seem likely. Others say that a passing star may have excited their inclinations. This would only seem possible if the Sun had formed within a cluster of stars, as stars rarely come that close to the Sun. As for the large number of KBOs in a 3:2 orbital resonance with Neptune, it has been theorized that angular momentum exchange with comets by Neptune during the early stages of the solar system caused Neptune to move outward. As it passed through the surrounding accretion disk, its orbital resonance was forced onto planetismals within the disk. However, it is uncertain how much Neptune actually moved, and in what direction, leading to doubts as to whether that is what actually occurred.