Trans-Neptunian Dwarf Planets
Authors: Holler Bryan J. 
Published: March 2021

The International Astronomical Union (IAU) officially recognizes five objects as dwarf planets: Ceres in the main asteroid belt between Mars and Jupiter, and Pluto, Eris, Haumea, and Makemake in the trans-Neptunian region beyond the orbit of Neptune. However, the definition used by the IAU applies to many other trans-Neptunian objects (TNOs) and can be summarized as follows: Any non-satellite large enough to be rounded by its own gravity. Practically speaking, this means any non-satellite with a diameter larger than 400 km. In the trans-Neptunian region, there are more than 150 objects that satisfy this definition, based on published results and diameter estimates.

The dynamical structure of the trans-Neptunian region records the history of the migration of the giant planets in the early days of the solar system. The semi-major axes, eccentricities, and orbital inclinations of TNOs across various dynamical classes provide constraints on different aspects of planetary migration. For many TNOs, the orbital parameters are all that is known about them, due to their large distances, small sizes, and low albedos. The TNO dwarf planets are a different story. These objects are large enough to be studied in more detail from ground- and space-based observatories. Imaging observations can be used to detect satellites and measure surface colors, while spectroscopy can be used to constrain surface composition. In this way, TNO dwarf planets not only help provide context for the dynamical evolution of the outer solar system, but also reveal the composition of the primordial solar nebula as well as the physical and chemical processes at work at very cold temperatures.

The largest TNO dwarf planets, those officially recognized by the IAU, plus others like Sedna, Quaoar, and Gonggong, are large enough to support volatile ices on their surfaces in the present day. These ices are able to exist as solids and gases on some TNOs, due to their sizes and surface temperatures (similar to water on Earth) and include N2 (nitrogen), CH4 (methane), and CO (carbon monoxide). A global atmosphere composed of these three species has been detected around Pluto, the largest TNO dwarf planet, with the possibility of local atmospheres or global atmospheres at perihelion for Eris and Makemake. The presence of non-volatile species, such as H2O (water), NH3 (ammonia), and complex hydrocarbons, provides valuable information on objects that may be too small to retain volatile ices over the age of the solar system. In particular, large quantities of H2O mixed with NH3 point to ancient cryovolcanism caused by internal differentiation of ice from rock. Complex hydrocarbons, formed through radiation processing of surface ices, such as CH4, record the radiation histories of these objects and provide clues to their primordial surface compositions.

The dynamical, physical, and chemical diversity of the more than 150 TNO dwarf planets are key to understanding the formation of the solar system and its subsequent evolution to its current state. Most of our knowledge comes from a small handful of objects, but we are continually expanding our horizons as additional objects are studied in more detail.