One of the themes I often use in my talks is the ‘filling out’ of our picture of the Solar System. In addition to the asteroid belt, we’ve added the icy bodies of the Kuiper Belt and the vast expanse of the Oort Cloud into what once seemed a relatively simple, nine-planet solar system. I could easily add to the ranks the population of so-called ‘Centaurs,’ small bodies that populate the space between the giant planets and show characteristics of both comets and asteroids.

10199 Chariklo is the largest Centaur yet discovered (260 kilometers in diameter), and Saturn’s moon Phoebe may be a captured Centaur, in which case images of it from the Cassini orbiter offer us our first detailed view of such an object. Both Chiron (discovered in 1977) and 60558 Echeclus show signs of a cometary coma; both are classified as asteroids and comets (as is 166P/NEAT). Although differences in definition exist, most agree that Centaurs orbit the Sun between Neptune and Jupiter and eventually cross the orbits of one or more of the gas giants.

Image: Saturn’s moon Phoebe, possibly a captured Centaur. Credit: NASA.

Now we have news from Universidad Complutense of Madrid (UCM) that Crantor, a large asteroid with a diameter of 70 kilometers orbits the Sun in exactly the same time period as Uranus. The asteroid’s orbit, says Carlos de la Fuente Marcos, one of the study’s authors, “…is controlled by the Sun and Uranus but is unstable due to disturbances from nearby Saturn.” Two other asteroids — 2010 EU65 and 2011 QF99 — are also associated with Uranus. The latter is in a stable Trojan orbit, moving 60 degrees in front of Uranus, while Crantor and 2010 EU65 show ‘horseshoe’ orbits that result periodically in close encounters with the planet.

About Crantor itself we have much to learn:

(83982) Crantor is remarkable in several respects: it is the first known minor body to be trapped in a 1:1 mean motion resonance with Uranus; it currently moves in a complex, horseshoelike orbit when viewed in a frame of reference co-rotating with Uranus; and it could be the “Rosetta Stone” for understanding why the overall number of Uranus co-orbitals appears to be significantly below that of Jupiter or Neptune. The object is placed and removed from its horseshoe orbit by the mechanism of the precession of the nodes. This precession is accelerated by the perturbative effects of Saturn. The chaotic nature of the orbit of this object constraints the degree of predictability of its dynamical evolution on timescales longer than a few 10 kyr.

The horseshoe shape of a ‘horseshoe orbit’ appears when we map the movement of the asteroid in relation to the Sun and, in this case, Uranus. While the asteroid always orbits the Sun in the same direction, it periodically catches up with the planet and falls behind again, tracing out the horseshoe outline in relation to Sun and planet. 3753 Cruithne was the first object confirmed to follow a horseshoe orbit, in this case near the Earth, while multiple objects in such orbits have been found around Jupiter, with others identified in association with Venus and Mars.


Image: A NASA illustration of a horseshoe orbit, in this case showing the orbit in relation to the Earth.

I don’t want to leave the topic of Centaurs behind without mentioning an interesting 2010 paper, although its emphasis is on Neptune rather than Uranus. Jonathan Horner (University of Durham) and Patryk Sofia Lykawka (Kinki University, Japan) make the case that a significant fraction, if not more, of the Centaur population comes from the planetary Trojan clouds, considered here as ‘stable reservoirs of objects moving in 1:1 mean-motion resonance with the giant planets…’ The researchers’ simulations show that there should be an ongoing movement of objects into the Centaur population. They go on to suggest that the Neptune Trojans could be the main source of new Centaurs. And I thought this was interesting (from their paper):

We suggest that further observational work is needed to constrain the contribution made by the Neptune Trojans to the ongoing flux of material to the inner Solar system, and believe that future studies of the habitability of exoplanetary systems should take care not to neglect the contribution of resonant objects (such as planetary Trojans) to the impact flux that could be experienced by potentially habitable worlds.

The de la Fuente Marcos paper is “Crantor, a short-lived horseshoe companion to Uranus,” Astronomy & Astrophysics 551: A114, March 2013 (abstract). The Horner/Lykawka paper is “Planetary Trojans – the main source of short period comets?,” International Journal of Astrobiology 9, 227-234 (2010). Preprint online.