Pluto, a dwarf planet; Triton, a moon of Neptune; and Phoebe, a moon of Saturn; all exhibit unusual characteristics, such as distinct elemental composition and retrograde orbits; as compared to other, similar objects in planetary portion of our solar system. This suggests their genesis is not identical with the origin of the eight planets orbiting the sun.
Through the use of mass spectroscopy, astronomers and physicists are able to discern the elemental building blocks of the various structures in the vastness of space. The universe and the Milky Way are composed, mainly of hydrogen and helium, adding up to about 98% of its total visible mass and energy; followed in relative abundance, by mass, at least in the Milky Way, by oxygen, carbon, neon, iron, nitrogen, silicon, magnesium, and sulfur. Smaller structures have slightly different compositions from the universe as a whole, as shown in the table below.
Hydrogen and helium were produced early in the formation of the universe at the time of the Big Bang. The other elements, listed above, are generally created from nuclear fusion within stars with a mass at least 1.3 times more than our sun, up to the mass of iron, likely billions of years after the Big Bang. Elements greater than the mass of iron require a supernova for creation. Distribution of elements in the various structures of the universe provide clues to how these structures were formed and where. Objects with different elemental compositions versus its neighbors suggests they were assembled in a different area of the cosmos.
Objects with retrograde orbits, as compared to their orbit around its primary object can not form in the same area as objects with prograde orbits. Retrograde orbits imply that the object was captured, gravitationally, by an object of greater mass.
Pluto, once considered the outermost and ninth planet from the sun was humiliatingly, and controversially, reassigned to the an inferior status of dwarf planet in 1992, and is now considered part of the inner disc of the Kuiper Belt. Its orbit is more inclined and elliptical than that of the other planets. Pluto’s surface is composed of approximately 98% frozen nitrogen along with a weak atmosphere composed of nitrogen and carbon dioxide. It has a density greater than all the outer gas giants but less than the inner rocky planets. The planet is spewing nitrogen into space at prodigious rates, but doesn’t seem to run out of that gas, suggesting that its core is solid nitrogen, a chemical makeup that is at odds with the other planets.
Triton, a retrograde orbiting moon of Neptune, the largest retrograde orbiting object in the solar system, also has a surface composed mainly of nitrogen with the added tourist attraction of year-long eruption of nitrogen geysers from its surface, creating a predominately nitrogen atmosphere. It is believed that Triton’s entire composition is similar to that of Pluto’s but slightly more dense, hinting at a larger rocky core.
Phoebe, a small, retrograde orbiting moon in Saturn’s outer rings, has a density slightly less than Pluto’s. Its surface is composed of frozen carbon dioxide and water but also contains iron, silicates and nitrates, the second most compositionally diverse body in the solar system with the Earth being the most varied.
The nitrogen surface and atmosphere of Pluto and Triton suggests that they are not associated with the eight known planets of the solar system. Triton and Phoebe’s retrograde orbit probabilistically confirm that they formed beyond the area of the primaries they orbit. Pluto’s inclined and excessively elliptical orbit, relative to the planets, also implies a different provenance. The simplest, and thus the most likely explanation, Occam’s Razor, of the origin of Pluto, Triton and Phoebe is that they formed within the Kuiper Belt beyond Neptune and are remnants retaining the original celestial mix of matter contained within the dense, cold, interstellar cloud that formed our solar system billions of years ago. The three objects were likely gravitationally nudged by the orbit of Neptune into their current positions. Phoebe’s complex composition also implies that not only did it come from the Kuiper Belt but it may also have collided with other planetary objects on its path to capture by Saturn.