We are at an exciting time in the study of the vast, mostly unexplored region of the Solar System beyond Neptune. Over the last two decades, less than two thousand small bodies have been discovered in the Kuiper Belt, ranging in size from a few tens of kilometers to several thousand kilometers across (for example, Pluto and Eris).
Many of these objects were only observed a few times and then lost; for most of them only the basic orbital parameters and a few photometric (color and intensity of light) measurements have been recorded. With Rubin Observatory in the coming decade, we will detect about 40,000 objects beyond Neptune, exquisitely measuring their orbital parameters and also obtaining hundreds of photometric measurements for each object, allowing the measurement of colors and lightcurves, or how their brightness changes over time, for a large population of these trans-Neptunian objects (TNOs).
With new information into this population, we can gain a new understanding of planetary formation. The formation and evolution of the giant planets of our Solar System has left a deep mark on the TNO population, making these distant objects tracers of the planetary formation process. As small planetesimals, the building blocks of planets, they also show signatures of planetary formation directly in their size distribution. We can also learn about the birth environment of our Sun, by studying the orbital parameters of a subset of these most distant objects, called scattered disk objects (SDOs).
There is also the potential for totally unexpected discoveries in the outer solar system. Not only may we find new dwarf planets the size of Pluto or larger, we very likely will find objects with physical characteristics quite unlike those of the currently better-known classes of solar system bodies. These may have extremely dark surfaces or particularly unusual orbits. It's even likely that we will detect small bodies captured from other Solar Systems; the challenge here will be identifying those outliers among the more common objects.
A few outstanding questions about the Kuiper Belt and TNOs:
To address these questions Rubin Observatory will search the entire southern sky for objects of all types beyond Neptune, identifying and characterizing on the order of 40,000 TNOs and SDOs.