Exploring the relics of planet formation

: the Kuiper Belt colour/composition connection

Abstract

In the outer Solar System beyond Neptune lies the Kuiper Belt, a sea of icy planetesimals which are the remains of the small planet forming bodies that failed to form into a planet beyond Neptune. Studying these small bodies sheds light on planetary formation, along with revealing the giant planets’ early dynamical history and the compositional structure of the Solar System’s primordial planetesimal disk. The smaller members of the Kuiper belt (with r mag > 22) generally show linear and featureless spectra. Additionally, due to the dimness of these objects observing their spectra can be particularly difficult. Therefore, broadband photometry is often used to characterise their surfaces. This broadband photometry can be used as a proxy for composition, as it provides enough information to characterise spectral slopes of these Trans Neptunian Object (TNO) surfaces.

Throughout this PhD, my aim has been to understand TNO surfaces and their implications on planetary migration. Dynamically excited TNOs show a bimodal distribution in their broadband surface colours made up of very red surfaces, and more neutral coloured surfaces. It is believed that these differing surfaces may be an indicator of where these objects formed in the early Solar System, before the migration of Neptune sculpted the Kuiper Belt we see today. Therefore, this bimodality suggests the presence of a surface change in the massive planetesimal disk from which the Kuiper belt originated, possibly a surface-colour changing ice line. The Colours of the Outer Solar System Origins Survey (Col-OSSOS) probed TNO surface properties via near-simultaneous g-, r- and J-band photometry. Additionally, the Outer Solar System Origins Survey (OSSOS), from which Col-OSSOS selected objects brighter than 23.6 r-band magnitude, has well characterised and quantified biases, allowing a unique and exciting opportunity for comparisons between these colour observations and numerical models of the Kuiper belt.

In our initial investigation, we examined the location of a dominant, surface colour changing ice-line, considering two possible configurations within the primordial disk: (1) inner neutral surfaces and outer red surfaces, and (2) inner red surfaces and outer neutral surfaces. Through simulations with a truncated primordial disk extending up to 30 au, we incrementally shifted the color transition by 0.5 au intervals. Our results demonstrated that both configurations are consistent with the observed color distribution. Furthermore, we expanded our analysis by including an additional Neptune migration model and considering a new surface classification known as brightIR/faintIR (defined based on the TNO's distance from the solar reddening line), in addition to the previously studied red/neutral colors. Once again, our simulations indicated that no specific disk layout could be ruled out, implying that Neptune efficiently scatters TNOs throughout its migration to the extent that much of their history is lost.

Additionally, the preciseness of the colour measurements of Col-OSSOS has allowed the identification of two TNOs with outlying surface colours. These objects separated out from the rest of the neutral cloud in (g - r) versus (r - J) colours, with (g - r) colour near solar colour. For these objects we took follow-up optical photometry in griz, allowing a further investigation of these TNO's surfaces. For 2014 UL225 we explore a potential collisional family origin for its unusual surface, or alternatively a mixture of organic tholins with either water ice or methane. For 2013 JR65 we discuss similarities to other TNOs with potentially aqueously altered hydrated silicates on their surfaces. Finally, as a side effect of Col-OSSOS’s observing technique we have a sample of objects with repeated optical colours, and some repeated near-infrared colours. We also have taken additional optical photometry of a small sample of TNOs with outlying surface colours. This allows us to investigate the possibility of photometric variation across multiple epochs for this sample of objects. We find a total of three TNOs that show greater than 3σ variation between visits, with possible causes including phase effects for one and surface variation for the other two.

Date of Award	Dec 2023
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	Megan Schwamb (Supervisor) & Alan Fitzsimmons (Supervisor)