Melaine Saillenfest

Postdoctoral Research Fellow

ASD Team

Research fields

My work is mainly dedicated to the dynamics of planetary systems and small Solar System bodies. Through the study of their long-term orbital and/or spin-axis evolution, I focus both on the formation and evolution of the Solar System, and on how it can be compared to exoplanetary systems.

My current post-doctoral fellowship deals with the stability of planetary spin axes (work in collaboration with Jacques Laskar et Gwenaël Boué). The orientation of the spin axis of planets is directly linked to seasons and climate stability. Laskar et al. (1993) showed that the Moon stabilises the spin axis of the Earth, which would be otherwise subject to a chaotic motion. The resulting large variability of seasons would possibly not have allowed the development of life. It is thus crucial to study the spin axes dynamics in the general case, in order to link these results to exoplanetary systems and to refine the criteria for the apparition of life. An analytical formulation of the long-term spin-axis motion of exoplanets can be derived, yet giving precise quantitative results if the parameters are well known. This allows to perform quick and straightforward explorations of the spin-axis dynamics. Bounds for the poorly known parameters can be obtained both from physical grounds and dynamical considerations. The maximal extent of the chaotic regions can then be computed only from the mass, the semi-major axis and the eccentricity of the planets present in the system. We can also classify which observed exoplanets are necessarily out of major spin-orbit secular resonances (unless the precession rate is affected by the presence of massive satellites).

My Ph.D. project (supervised by Marc Fouchard and Giacomo Tommei, and in collaboration with Giovanni B. Valsecchi) was focussed mainly on the orbital dynamics of trans-Neptunian objects, in particular those having trajectories very distant from the planetary region. Despite very weak orbital perturbations, many of them have very eccentric orbits, indicating that they did not form in their current orbital state. One-degree-of-freedom secular systems can be obtained in the non-resonant and resonant cases, allowing to represent any trajectory as a level curve of the Hamiltonian function. In the case of a mean-motion resonance with Neptune, these models reveal trajectories leading to very remote perihelia, as well as "capture mechanisms" able to maintain objects on distant orbits for billions of years. The application to known trans-Neptunian objects shows graphically which observed orbits require a complex scenario (as the planetary migration or an external perturber) and which ones can be explained simply by the effects of known planets on their current orbits.

After having characterised the secular dynamics beyond Neptune considering only the known planets, it is interesting to determine which structures persist if we add the perturbations from an hypothetical massive external planet. The secular dynamics obtained is not integrable in general, but it can be studied with the help of Poincaré sections. When we increase the semi-major axis of the small body, chaos appears around the libration islands due to Kozai mechanism, and it spreads all over the range of orbital inclination. This large chaotic region allows "orbital flips" for trans-Neptunian objects, that is, the change between prograde and retrograde orbits. In the middle of the chaotic sea, the most persistent dynamical structures are secular resonances producing aligned and anti-aligned trajectories with respect to the orbit of the distant planet.

I am also involved in the research project conducted by Etienne Behar (Swedish Institute of Space Physics, Kiruna) about the dynamics of solar protons around active comets. A simplified model allows to consider the protons as test-particles evolving in an effective magnetic field. The flux of particles naturally produces an asymmetric density structure, as well as a central cavity devoid of any particle. These results are in good agreement with the observations from the Rosetta probe around comet 67P.

My other ongoing collaborations deal with the dynamics of long-period comets and the initial shape of the Oort Cloud (Marc Fouchard), the origin of the orbital inclination of the Saturnian satellite Japet (William Polycarpe), and the long-term orbital dynamics of the Galilean satellites (Giacomo Lari).

I was also involved in the study conducted by Stéfan Renner about the orbital dynamics of Saturnian satellites. We showed that the superposition of orbital resonances between Atlas and Prometheus is a source of chaos.


PhD manuscript