Delving into the Kuiper Belt

Laura Buchanan is a PhD student at Queen’s University Belfast studying the compositions of Kuiper Belt Objects and the colours of cats.

Far out in our solar system, beyond the planet Neptune, lies a sea of small icy bodies known as the Kuiper Belt. These are all that remains of small planet-forming bodies, or planetesimals, from the early solar system that failed to form into a planet beyond Neptune. In these early years, the Kuiper Belt looked very different from how we now see it, containing many objects which made up a massive disk far closer to the Sun than in the present day.

Artist rendition of the Kuiper Belt and the outer planets (NASA)

A popular theory for the formation of the Solar System, known as the Nice Model, suggests that the early Solar System consisted of this massive disk of planetesimals, along with five giant planets (rather than the four we have now). This far out in the solar system smaller planets, such as Earth or Mars, have little effect, so we disregard them. 

Over time, the orbits of these planets interacted with each other, leading to instabilities resulting in one of the giant planets being entirely ejected from our solar system (hence four, not five!). These instabilities also caused Neptune’s orbit to become highly eccentric (non-circular), damped over time to its current orbit by its interactions with the massive planetesimal disk. These interactions caused the planetesimals to be scattered onto either their current orbits (much further from the Sun) or out of the solar system entirely.

As a result of all of these interactions, these objects were left dynamically excited or ‘dynamically hot’, with elongated orbits at various inclinations to the plane of the original planetesimal disk, and so were named the hot population. In contrast, there is a smaller population that were originally formed at these much further distances from the Sun, called the cold population since they are ‘dynamically cold’, with more circular orbits that were less disturbed, and so left at low inclinations. Therefore, the difference in the inclinations of their orbits can distinguish these populations.

As the majority of Kuiper Belt Objects (KBOs) originated at approximately same distance from the Sun, the areas they formed in would have been at roughly equal temperatures. This means that the KBOs were expected so have very similar compositions of molecular ices, as well as similar dust to ice ratios. This was tested by observing the KBOs under different colour filters, where their composition could be inferred from their surface colour. It was found that they have a range of colours, varying from very red, to even more red (often referred to as ‘blue’ and ‘red’). The cold population (which formed at its current position) is observed to show redder surface colours than the other populations. 

It is believed that the colours/compositions of KBOs are a result of where in the disk they formed. As a part of my PhD I will be testing the initial layout of colours in the disk by inserting colour distributions into a model of the Kuiper belt before and after Neptune’s migration. This can then be compared with observations to hopefully give some idea of the primordial distribution of each colour of Kuiper belt objects.

This should enable us to see where in the planetesimal disk they originated, which will help us to understand how the solar system formed initially.