Tilting Uranus via the migration of an ancient satellite

Context. The 98{\deg}-obliquity of Uranus is commonly attributed to giant impacts that occurred at the end of the planetary formation. This picture, however, is not devoid of weaknesses. Aims. On a billion-year timescale, the tidal migration of the satellites of Jupiter and Saturn has been shown to strongly affect their spin-axis dynamics. We aim to revisit the scenario of tilting Uranus in light of this mechanism. Methods. We analyse the precession spectrum of Uranus and identify the candidate secular spin-orbit resonances that could be responsible for the tilting. We determine the properties of the hypothetical ancient satellite required for a capture and explore the dynamics numerically. Results. If it migrates over 10 Uranus' radii, a single satellite with minimum mass 4e-4 Uranus' mass is able to tilt Uranus from a small obliquity and make it converge towards 90{\deg}. In order to achieve the tilting in less than the age of the Solar System, the mean drift rate of the satellite must be comparable to the Moon's current orbital expansion. Under these conditions, simulations show that Uranus is readily tilted over 80{\deg}. Beyond this point, the satellite is strongly destabilised and triggers a phase of chaotic motion for the planet's spin axis. The chaotic phase ends when the satellite collides into the planet, ultimately freezing the planet's obliquity in either a prograde, or plainly retrograde state (as Uranus today). Spin states resembling that of Uranus can be obtained with probabilities as large as 80%, but a bigger satellite is favoured, with mass 1.7e-3 Uranus' mass or more. Yet, a smaller ancient satellite is not categorically ruled out, and there is room for improving this basic scenario in future studies. Interactions among several pre-existing satellites is a promising possibility.Comment: Accepted for publication in Astronomy and Astrophysic

Coupling protoplanetary disk formation with early protostellar evolution: influence on planet traps

International audienceProtoplanetary disk structures are known to be shaped by various thermal and compositional effects such as (though not limited to) shadowed regions, sublimation lines, density bumps... The resulting irregularities in the surface mass density and temperature profiles are key elements to determine the location where planetary embryos can be trapped. These traps provide hints of which planets are most likely to survive, at what distance from the star, and potentially with what composition (Baillié, Charnoz, Pantin, 2015, A&A 577, A65; Baillié, Charnoz, Pantin, 2016, A&A 590, A60). These structures are determined by the viscous spreading of the disk, that is initially formed by the collapse of the molecular cloud.Starting from the numerical hydrodynamical model detailed in Baillié & Charnoz., 2014, ApJ 786, 35 which couples the disk thermodynamics, its photosphere geometry, its dynamics and its dust composition in order to follow its long-term evolution, we now consider the early stages of the central star. We model the joint formation of the disk and the star: their mass are directly derived from the collapse of the molecular cloud while the star temperature, radius and brightness are interpolated over pre-calculated stellar evolutions. Therefore, our simulations no longer depend on the initial profile of the "Minimum Mass Solar Nebula", and allow us to model the influence of the forming star on the protoplanetary disk. In particular, we will present the resulting distribution of the sublimation lines of the main dust species, as well as the locations of the planet traps at various disk ages. In the longer term, we intend to investigate the influence of the star properties on the selection of the surviving planets

Coupling protoplanetary disk formation with early protostellar evolution: influence on planet traps

International audienceProtoplanetary disk structures are known to be shaped by various thermal and compositional effects such as (though not limited to) shadowed regions, sublimation lines, density bumps... The resulting irregularities in the surface mass density and temperature profiles are key elements to determine the location where planetary embryos can be trapped. These traps provide hints of which planets are most likely to survive, at what distance from the star, and potentially with what composition (Baillié, Charnoz, Pantin, 2015, A&A 577, A65; Baillié, Charnoz, Pantin, 2016, A&A 590, A60). These structures are determined by the viscous spreading of the disk, that is initially formed by the collapse of the molecular cloud.Starting from the numerical hydrodynamical model detailed in Baillié & Charnoz., 2014, ApJ 786, 35 which couples the disk thermodynamics, its photosphere geometry, its dynamics and its dust composition in order to follow its long-term evolution, we now consider the early stages of the central star. We model the joint formation of the disk and the star: their mass are directly derived from the collapse of the molecular cloud while the star temperature, radius and brightness are interpolated over pre-calculated stellar evolutions. Therefore, our simulations no longer depend on the initial profile of the "Minimum Mass Solar Nebula", and allow us to model the influence of the forming star on the protoplanetary disk. In particular, we will present the resulting distribution of the sublimation lines of the main dust species, as well as the locations of the planet traps at various disk ages. In the longer term, we intend to investigate the influence of the star properties on the selection of the surviving planets

Trapping planets in an evolving protoplanetary disk: preferred time, locations, and planet mass

International audienceContext. Planet traps are necessary to prevent forming planets from falling onto their host star by type I inward migration. Surface mass density and temperature gradient irregularities favor the apparition of traps (planet accumulation region) and deserts (planet depletion zone). These features are found at the dust sublimation lines and heat transition barriers.Aims. We study how planets may remain trapped or escape these traps as they grow and as the disk evolves viscously with time.Methods. We numerically model the temporal viscous evolution of a protoplanetary disk by coupling its dynamics, thermodynamics, geometry, and composition. The resulting midplane density and temperature profiles allow the modeling of the interactions of this type of evolving disk with potential planets, even before the steady state is reached.Results. We follow the viscous evolution of a minimum mass solar nebula and compute the Lindblad and corotation torques that this type of disk would exert on potential planets of various masses that are located within the planetary formation region. We determine the position of planet traps and deserts in relationship with the sublimation lines, shadowed regions, and heat transition barriers. We notice that the planet mass affects the trapping potential of the mentioned structures through the saturation of the corotation torque. Planets that are a few tens of Earth masses can be trapped at the sublimation lines until they reach a certain mass while planets that are more massive than 100 M ⊕ can only be trapped permanently at the heat transition barriers. They may also open gaps beyond 5 au and enter type II migration.Conclusions. Coupling a bimodal planetary migration model with a self-consistent evolved disk, we were able to distinguish several potential planet populations after five million years of evolution: two populations of giant planets that could stay trapped around 5.5 and 9 au and possibly open gaps, some super-Earths trapped around 5 and 7.5 au, and a population of close-in super-Earths, which are trapped inside 1 au. The traps that correspond to the last group could help to validate the in situ formation scenarios of the observed close-in super-Earths

The Eccentricity Of Mimas And The Cassini Division: A Common History

International audienceAstrometric measurements reveal the possibility that the saturnian satellite Mimas could be evolving inward instead of outward (Lainey et al. 2012), as usually thought. Based on this assumption, we studied the behavior of the satellites and the rings over 20 Myr. A numerical integration of the equations of the satellites shows that Mimas has crossed several resonances with Enceladus in the past. Moreover, a recent resonance with Tethys explains its current eccentricity (2e-2). An implementation of Mimas' dynamics in the hydrodynamical code Hydrorings (Charnoz et al. 2010, 2011) shows that we can open the Cassini Division this way, with the right width. We also highlight the mechanism of resonance leaking, able to create ringlets. We show in particular how the recent resonance with Tethys creates the Huygens ringlet. Numerical simulations were made on the local computing ressources (Cluster URBM-SYSDYN) at the University of Namur. This work is supported by EMERGENCE-UPMC grant (contract number: EME0911)

The Eccentricity Of Mimas And The Cassini Division: A Common History

International audienceAstrometric measurements reveal the possibility that the saturnian satellite Mimas could be evolving inward instead of outward (Lainey et al. 2012), as usually thought. Based on this assumption, we studied the behavior of the satellites and the rings over 20 Myr. A numerical integration of the equations of the satellites shows that Mimas has crossed several resonances with Enceladus in the past. Moreover, a recent resonance with Tethys explains its current eccentricity (2e-2). An implementation of Mimas' dynamics in the hydrodynamical code Hydrorings (Charnoz et al. 2010, 2011) shows that we can open the Cassini Division this way, with the right width. We also highlight the mechanism of resonance leaking, able to create ringlets. We show in particular how the recent resonance with Tethys creates the Huygens ringlet. Numerical simulations were made on the local computing ressources (Cluster URBM-SYSDYN) at the University of Namur. This work is supported by EMERGENCE-UPMC grant (contract number: EME0911)

The Eccentricity Of Mimas And The Cassini Division: A Common History

International audienceAstrometric measurements reveal the possibility that the saturnian satellite Mimas could be evolving inward instead of outward (Lainey et al. 2012), as usually thought. Based on this assumption, we studied the behavior of the satellites and the rings over 20 Myr. A numerical integration of the equations of the satellites shows that Mimas has crossed several resonances with Enceladus in the past. Moreover, a recent resonance with Tethys explains its current eccentricity (2e-2). An implementation of Mimas' dynamics in the hydrodynamical code Hydrorings (Charnoz et al. 2010, 2011) shows that we can open the Cassini Division this way, with the right width. We also highlight the mechanism of resonance leaking, able to create ringlets. We show in particular how the recent resonance with Tethys creates the Huygens ringlet. Numerical simulations were made on the local computing ressources (Cluster URBM-SYSDYN) at the University of Namur. This work is supported by EMERGENCE-UPMC grant (contract number: EME0911)

The Cassini Division and Mimas' eccentricity: A Common History

International audiencePossible Mimas' orbital decay has been revealed recently from astrometric measurements of the main Saturnian moons (Lainey et al. 2012). Based on this assumption, we studied Saturn's ring evolution over 20 Myr, taking into account resonancesa ssociated with Mimas, like the 2:1 resonance currently placed at the outer edge of the B-ring. Depending on the typical size of the particules, we show that the Cassini division and its structures could be explained by Mimas orbital decay. Simultaneously, we examined the behavior of Mimas orbit while it shrinked from 190,000 up to its current position. We found its current eccentricity can be explained by a recent resonance crossing with Tethys. This work is supported by Campus Spatial (Paris Diderot) and partly supported by EMERGENCE-UPMC grant (contract number: EME0911)

The Cassini Division and Mimas' eccentricity: A Common History

International audiencePossible Mimas' orbital decay has been revealed recently from astrometric measurements of the main Saturnian moons (Lainey et al. 2012). Based on this assumption, we studied Saturn's ring evolution over 20 Myr, taking into account resonancesa ssociated with Mimas, like the 2:1 resonance currently placed at the outer edge of the B-ring. Depending on the typical size of the particules, we show that the Cassini division and its structures could be explained by Mimas orbital decay. Simultaneously, we examined the behavior of Mimas orbit while it shrinked from 190,000 up to its current position. We found its current eccentricity can be explained by a recent resonance crossing with Tethys. This work is supported by Campus Spatial (Paris Diderot) and partly supported by EMERGENCE-UPMC grant (contract number: EME0911)

Tilting Uranus via the migration of an ancient satellite

International audienceContext. The 98° obliquity of Uranus is commonly attributed to giant impacts that occurred at the end of the planetary formation. This picture, however, is not devoid of weaknesses. Aims. On a billion-year timescale, the tidal migration of the satellites of Jupiter and Saturn has been shown to strongly affect their spin-axis dynamics. We aim to revisit the scenario of tilting Uranus in light of this mechanism. Methods. We analyse the precession spectrum of Uranus and identify the candidate secular spin-orbit resonances that could be responsible for the tilting. We determine the properties of the hypothetical ancient satellite required for a capture and explore the dynamics numerically. Results. If it migrates over 10 Uranus’s radii, a single satellite with minimum mass 4 × 10 −4 Uranus’s mass is able to tilt Uranus from a small obliquity and make it converge towards 90°. In order to achieve the tilting in less than the age of the Solar System, the mean drift rate of the satellite must be comparable to the Moon’s current orbital expansion. Under these conditions, simulations show that Uranus is readily tilted over 80°. Beyond this point, the satellite is strongly destabilised and triggers a phase of chaotic motion for the planet’s spin axis. The chaotic phase ends when the satellite collides into the planet, ultimately freezing the planet’s obliquity in either a prograde or a plainly retrograde state (as Uranus today). Spin states resembling that of Uranus can be obtained with probabilities as large as 80%, but a bigger satellite is favoured, with mass 1.7 × 10 −3 Uranus’s mass or more. Yet, a smaller ancient satellite is not categorically ruled out, and we discuss several ways to improve this basic scenario in future studies. Interactions among several pre-existing satellites are a promising possibility. Conclusions. The conditions required for the tilting seem broadly realistic, but it remains to be determined whether Uranus could have hosted a big primordial satellite subject to substantial tidal migration. The efficiency of tidal energy dissipation within Uranus is required to be much higher than traditionally assumed, more in line with that measured for the migration of Titan. Hints about these issues would be given by a measure of the expansion rate of Uranus’s main satellites