Until 2013, the only rings which were known were those around the giant planets, of which the most spectacular was of course those surrounding Saturn. Astronomers were surprised to discover, firstly, dense rings around Chariko, a Centaure class asterooïd with a diameter of about 250 km in an orbit situated between Saturn and Uranus. And again, in 2017 : this time around Haumea, a dwarf planet, currently situated a over 50 astronomical units from the Sun, and famous as being one of the largest transneptunian bodies and having a cigar shaped form whose axis is about 2 300 km long.
Over and above their scientific oddity, these rings furnish astronomers with a completely fresh field of study to explore in more detail the celestial dynamics operating in the Solar System. Directed by Bruno Sicardy, who discovered the two mini rings, the scientific team has as a consequence launched a theoretical study, and their conclusions diverge considerably from what we knew thus far about the giant planets characterised by a very regular morphology.
The irregularities of these small bodies – as for example a topography dominated by craters or mountains or an extremely elongated shape, as in the case of Haumea – impact significantly on the evolution of their rings. These deformations lead to an important interaction between the celestial bodies and their rings via the so called « resonance » phenomena. The disc is thereby the seat of a particle migration process.
The migration follows various scenarios, depending on the positions of the particles : whether they are inside or outside of the synchronous orbit1 of the object. The study thus shows that on Chariklo for example, a mountain with an altitude of hardly 5km can lead to the fall of particles on the body if they are initially within the synchronous orbit ; on the other hand, it can push them away towards the external regions, if they are beyond this orbit [1].
Another surprise : this process operates on time scale of a million years, i.e. on a time scale which is short compared to the age of the solar system, about 4 500 million years. Taking into account the elongated shapes of Haumea or Chariklo, these time scales turn out to be still shorter : only several years, which is just en “eye blink” on the cosmic time scale.
« Lthis mechanism, as revealed by the study, opens the door to a new field of hypotheses to help understand other situations in the Solar System » emphasizes Bruno Sicardy, the leading author of the paper. Applied elsewhere, this could help explain, for example the formation of statellites around small bodies. Thus, the orbit of an asteroïd or a transneptunian body, following an impact, could have been pushed out beyond a region (referred to as the « Roche limit ») where the tidal effects of the body have become small enough as to enable the disc to transform itself into satellites. There are other a applications : for example, to understand the mountain chain which trace out the equator of Iapetus, one of Saturn’s satellites . It could have been caused by the fall of an ancient ring which accumulated itself around this body.
Be that as it may, scientists, in their quest to understand how rings in general evolve how have a new terrain to study, furnished by Nature, and which is very different to that of the giant planets,.
Après 3 mois (image du haut) la plupart des particules à l’intérieur de l’orbite synchrone (à 190 km du centre de Chariklo) sont tombées sur le corps.
Après une année (image du milieu), toute la zone interne a été vidée,
Après douze ans (image du bas), les particules continuent leur migration vers les zones externes.
Référence
This research will be published under the title « Ring dynamics around non-axisymmetric bodies with applications to Chariklo and Haumea », by B. Sicardy et.al., in the 19th November 2018 letters issue of Nature Astronomy.
This research was partially financed by the Conseil européen de la recherche du projet ’Lucky Star’, directed by Bruno Sicardy (ERC Advanced Grant n°669416). Ils sont le fruit d’une collaboration internationale comprenant quatre chercheurs français : B. Sicardy (Professeur Sorbonne Université, chercheur à l’Observatoire de Paris – PSL), S. Renner (Maître de Conférences Lille-I, chercheur associé à l’Observatoire de Prais - PSL), F. Roques (astronome de l’Observatoire de Paris – PSL), J. Desmars (post-doc Lucky Star, Observatoire de Paris – PSL), ainsi que trois chercheurs étrangers : R. Leiva, M. El Moutamid et P. Santos-Sanz
[1] This orbit – referred to as geostationary in the case of the Earth - corresponds to the fact that the time for the particle to turn around the body is the same as that for the latter ti turn around its axis.
