A field of asteroids, known as the Trojans, are captured in the gas giant’s orbit
The Trojans are a population of asteroids caught within two of Jupiter’s Lagrange points – regions where the gravitational influence of the gas giant and the Sun, as well as the orbital centrifugal force, are all balanced. The L4 and L5 points are 60° ahead and behind Jupiter in its orbit, respectively. They act like gravitational traps and objects captured into them can be stable for the history of the Solar System.
It’s thought that the objects making up the Trojans didn’t actually form near Jupiter’s orbit, but originated much further out in the Solar System. Somehow, they were transported inwards and captured. Spectroscopic observations show them to have the general appearance of dormant comets. But perhaps the most remarkable feature of the Trojans is that many have highly inclined orbits – some as much as 35° out of the ecliptic plane – and so are described as being ‘dynamically hot’. This poses a big challenge for any theory of their origin – not only must these bodies have been moved from the outer Solar System and injected into the Jovian Lagrange points, but also kicked into high inclinations.
The research paper that I’ve picked to look at this month is slightly different from most of those I cover in this column. It’s not a primary research paper, reporting on new results, but is a review paper. It summarises everything we know about the Trojans, what remains mysterious about them, and how planetary scientists hope the upcoming Lucy mission may be able to fill in many of these key gaps in our understanding.
A shifting Solar System
William Bottke, at the Southwest Research Institute in Boulder, Colorado, and his co-authors explain that the leading theory of where the Trojans came from invokes a dramatic period of orbital migration for the giant planets early in the Solar System’s history. Neptune moved outwards through the primordial Kuiper belt and scattered these icy bodies ac
