Solar System Evolution
The locations of the known Centaurs in the outer Solar system on 1st January, 2000, from Horner et al. 2020.
The Centaurs - parents of the Jupiter Family Comets
The first step on my journey as a professional astronomer came with studies of the Centaurs - a population of icy bodies between the orbits of Jupiter and Neptune.
When I started my doctoral studies, in 2000, only a handful of these enigmatic objects were known. Where do they come from? Where do they go? And how long lived are they? Through the course of my doctoral work, I investigated these questions using an n-body dynamics computer package called 'Mercury', carrying out what was, at the time, the most detailed study of the Centaur population ever attempted.
I simulated the evolution of multiple 'clones' of each of 32 of
the Centaurs that had well constrained orbits at the time,
looking at how those clusters of clones would evolve in the future, and where they had been in the past. I found that even the most stable of the Centaurs are relatively short-lived objects, with lifetimes measured in hundreds of thousands or millions of years - far shorter than the age of the Solar system.
My simulations allowed me to compare the different Trojans known at the time, working out are most likely to become short period comets at some point in the future. They also revealed the possibility that Centaurs can be captured by the giant planets as 'temporary Trojan companions', predicting that such captured objects likely lurk amongst the stable Trojans of Jupiter and Neptune. In the years since, a number of temporarily captured Trojans have been discovered.
The sizes, orbital radii, and orbital inclinations of selected Neptunian Trojans (in green) and the Plutinos (in red) - objects trapped in mean-motion resonance with the giant planet Neptune. Image source: Wikimedia/Eurocommuter; CC-SA 3.0
The Origin of the Neptune Trojans
In the Solar system's youth, the giant planets migrated over large distances, both inwards and outwards, before finally settling on their current orbits. Four billion years later, how can we study that migration, to get a feel for how far, how fast, and how smoothly the giant planets travelled in their migration?
The answer comes in the form of populations of 'resonant objects', captured by the giant planets during their migration, and carried along with those planets as they moved. The Neptune Trojans are one such population - two groups of objects that share Neptune's orbit, clustering sixty degrees ahead and behind the planet as it moves around the Sun.
The Neptune Trojans did not form where they are today. Instead, they were captured by Neptune as it migrated outwards, moving away from the Sun. Once captured, they were pushed along with the planet, their orbits being 'excited' as they did so - becoming ever more eccentric and inclined.
Along with my colleague and friend A/Prof Patryk Sofia Lykawka, I have carried out a number of studies of the Neptune Trojan population - revealing that Neptune migrated outwards by at least a billion kilometres to reach its current orbit, and finding that some members of the population (such as 2001 QR322 and 2008 LC18) are actually somewhat unstable, one day to escape and roam the Solar system once more.
Our results therefore suggest that the Neptune Trojans likely contribute to the Solar system's Centaur population - and, combined with the Jovian Trojans, could well be a significant source of new short period comets.
Artist's impression of Centaur 10199 Chariklo, along with its narrow, dense, ring system - which was studied by Dr Jeremy Wood, during his doctoral studies with me at USQ, as detailed in Wood et al., 2017. Image credit: ESO/L. Calçada/M. Kornmesser/Nick Risinger (skysurvey.org).
Centaurs With Rings
In the 2010s, observations revealed narrow, dense rings of debris orbiting the Centaurs 10199 Chariklo and 2060 Chiron. This was, to say the least, something of a surprise.
Compared to the other objects around which rings were known (the giant planets), the two Centaurs are tiny - both around three hundred kilometres in diameter. Not only that, but, like all Centaurs, they move on highly chaotic orbits, experiencing frequent encounters with the giant planets - encounters that might have the potential to strip or disrupt the rings entirely!
This posed an obvious question - were the rings primordial, having formed with the Centaurs before accompanying them through their voyage through the Solar system? Could we rule this out by exploring the history of the Centaurs in question?
Finding the answer to this question formed the core of Dr Jeremy Wood's doctoral studies with me at USQ. Jeremy carried out large suites of simulations following the evolution of the orbits of the two Centaurs overtime, finding that, despite their chaotic past and future journeys, it is highly unlikely that either 10199 Chariklo or 2060 Chiron ever approached one of the giant planets sufficiently closely for their rings to be disrupted.
In other words - its remains entirely possible that Chiron and Chariklo could have hosted those rings since the birth of the Solar system, more than four billion years ago!
View of the inner Solar system, with the Jovian Trojans shown in purple, clustered in two groups centred sixty degrees ahead and behind the location of Jupiter in its orbit. Figure taken from Horner et al. 2020 (figure 2 of that work).
Astrocladistics and the Jupiter Trojans
Jupiter hosts a vast population of Trojan companions, trapped sharing the giant planet's orbit in two clouds, sixty degrees ahead and behind the planet in its orbit. Those Trojans were captured during the final stages of planet formation, as Jupiter migrated through the Solar system to its current orbit.
It is likely that the Jovian Trojan population contains objects that formed throughout the Solar system, which just happened to be moving through the Jovian region when the Trojans were captured. There will be Trojans whose origins lie in the asteroid belt, in the inner Solar system, and also Trojans that formed farther from the Sun, beyond Neptune's orbit.
As a result, it is likely that the Jovian Trojan population is amongst the most diverse in the Solar system. It seems likely, too, that there are families of object trapped within the Trojan clouds that share an even more intimate origin - the collisional fragments of larger Trojans, shattered in the distant past.
How can we disentangle the history of the Jovian Trojans? How can we identify groups of objects with a similar history? These questions formed the core of Dr Tim Holt's doctoral studies with me here at USQ. Tim introduced a technique called 'cladistics', commonly used in the biological sciences, to try to answer these questions - taking a technique developed to build the 'tree of life' and applying it to the Solar system's past.
Tim's work expanded our knowledge of the stability of the Jovian Trojans, and the collisional families in the population, led to the identification of the first known 'Trojan pair', and produced a detailed cladistical analysis of the Jovian Trojans, identifying 48 distinct groups of objects that likely share a common origin amongst the larger population.