Retrograde-rotating Exoplanets Experience Obliquity Excitations in an Eccentricity-enabled Resonance

Kreyche, Steven M.; Barnes, Jason W.; Quarles, Billy; Lissauer, Jack J.; Chambers, John E.; Hedman, M. M.

doi:10.3847/psj/ab8198

Cited by 6 publications

(5 citation statements)

References 43 publications

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“…Murray & Dermott 1999or Laskar et al 2012). This is not the case in more realistic situations, as recalled for instance by Kreyche et al (2020) in the context of obliquity dynamics. In planetary systems featuring mean-motion resonances, the spin-axis of a planet can be affected by shifted orbital precession frequencies (Millholland & Laughlin 2019) or by secondary resonances (Quillen et al 2017(Quillen et al , 2018.…”

Section: Orbital Solutionmentioning

confidence: 95%

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The future large obliquity of Jupiter

Saillenfest

Lari

Courtot

2020

A&A

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Aims. We aim to determine whether Jupiter’s obliquity is bound to remain exceptionally small in the Solar System, or if it could grow in the future and reach values comparable to those of the other giant planets. Methods. The spin-axis of Jupiter is subject to the gravitational torques from its regular satellites and from the Sun. These torques evolve over time due to the long-term variations of its orbit and to the migration of its satellites. With numerical simulations, we explore the future evolution of Jupiter’s spin axis for different values of its moment of inertia and for different migration rates of its satellites. Analytical formulas show the location and properties of all relevant resonances. Results. Because of the migration of the Galilean satellites, Jupiter’s obliquity is currently increasing, as it adiabatically follows the drift of a secular spin-orbit resonance with the nodal precession mode of Uranus. Using the current estimates of the migration rate of the satellites, the obliquity of Jupiter can reach values ranging from 6° to 37° after 5 Gyr from now, according to the precise value of its polar moment of inertia. A faster migration for the satellites would produce a larger increase in obliquity, as long as the drift remains adiabatic. Conclusions. Despite its peculiarly small current value, the obliquity of Jupiter is no different from other obliquities in the Solar System: It is equally sensitive to secular spin-orbit resonances and it will probably reach comparable values in the future.

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Section: Orbital Solutionmentioning

confidence: 95%

“…More generally, secular spin-orbit resonances are thought to strongly affect the obliquity of exoplanets (see e.g. Atobe et al 2004;Brasser et al 2014;Deitrick et al 2018b,a;Shan & Li 2018;Millholland & Laughlin 2018Quarles et al 2019;Saillenfest et al 2019;Kreyche et al 2020).…”

Section: Introductionmentioning

confidence: 99%

The future large obliquity of Jupiter

Saillenfest

Lari

Courtot

2020

A&A

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“…The number of strong harmonics in the precession spectrum of a planet is larger for planetary systems featuring substantial eccentricities and inclinations. The various orbital architectures observed in exoplanetary systems should therefore allow for much more diverse secular spin-orbit resonances than for the giant planets of the solar system, which have today rather cold orbits [22]. The mechanism presented in this letter could therefore greatly affect the obliquity distribution of exoplanets.…”

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confidence: 97%

The large obliquity of Saturn explained by the fast migration of Titan

Saillenfest¹,

Lari²,

Boué³

2021

Nat Astron

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The obliquity of a planet is the tilt between its equator and its orbital plane. Giant planets are expected to form with near-zero obliquities [1,2]. After its formation, some dynamical mechanism must therefore have tilted Saturn up to its current obliquity of 26.7 • . This event is traditionally thought to have happened more than 4 Gyrs ago during the late planetary migration [3,4,5] because of the crossing of a resonance between the spin-axis precession of Saturn and the nodal orbital precession mode of Neptune [6]. Here, we show that the fast tidal migration of Titan measured by [7] is incompatible with this scenario, and that it offers a new explanation for Saturn's current obliquity. A significant migration of Titan would prevent any early resonance, invalidating previous constraints on the late planetary migration set by the tilting of Saturn [8,9,10]. We propose instead that the resonance was encountered recently, about 1 Gyr ago, forcing Saturn's obliquity to increase from a small value (possibly less than 3 • ), up to its current state. This scenario suggests that Saturn's normalised polar moment of inertia lies between 0.224 and 0.237. Our findings bring out a new paradigm for the spin-axis evolution of Saturn, Jupiter [11], and possibly giant exoplanets in multiple systems, whereby obliquities are not settled once for all, but continuously evolve as a result of the migration of their satellites.We investigate whether the spin-axis dynamics of Saturn could have been influenced by the migration of its satellites. The torque applied by the sun on Saturn's equatorial bulge produces a precession of its spin axis with a mean frequency ψ = α (1−e 2 ) −3/2 cos ε, where e is the eccentricity of Saturn, ε is its obliquity, and α is called its precession constant [12]. The value of α incorporates the extra torques produced by Saturn's satellites [1,13]. It also depends on Saturn's orbital and physical parameters, among which all are well known except Saturn's normalised polar moment of inertia λ. Estimates found in the literature broadly range in λ ∈ [0.200, 0.240] but they are model-dependent and a proper uncertainty range is hard to define (see Methods). Exploring this whole interval gives a current precession constant α 0 ranging from 0.747 to 0.894 /yr.We take into account the migration of Saturn's satellites by changing their contributions to α. The measurements of [7] unveiled a fast migration for Titan, and a similar tidal timescale t tide = a/ ȧ for all six satellites of Saturn studied. Both these findings support the "resonance locking" tidal 1

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“…If ever the system manages to go through the singular point S 1 in some way, one could even imagine a scenario where the planet then picks a retrograde resonance (see e.g. Kreyche et al 2020) and goes on tilting up to 180 • . Spin-axis precession period of a planet orbited by a single satellite.…”

Section: Effect Of a Satellitementioning

confidence: 99%

Future destabilisation of Titan as a result of Saturn's tilting

Saillenfest,

Lari

2021

Preprint

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Context. As a result of Titan's migration and Saturn's probable capture in secular spin-orbit resonance, recent works show that Saturn's obliquity could be steadily increasing today and may reach large values in the next billions of years. Satellites around high-obliquity planets are known to be unstable in the vicinity of their Laplace radius, but the approximations used so far for Saturn's spin axis are invalidated in this regime. Aims. We aim to investigate the behaviour of a planet and its satellite when the satellite crosses its Laplace radius while the planet is locked in secular spin-orbit resonance. Methods. We expand on previous works and revisit the concept of Laplace surface. We use it to build an averaged analytical model that couples the planetary spin-axis and satellite dynamics. Results. We show that the dynamics is organised around a critical point, S 1 , at which the phase-space structure is singular, located at 90 • obliquity and near the Laplace radius. If the spin-axis precession rate of the planet is maintained fixed by a resonance while the satellite migrates outwards or inwards, then S 1 acts as an attractor towards which the system is forced to evolve. When it reaches the vicinity of S 1 , the entire system breaks down, either because the planet is expelled from the secular spin-orbit resonance or because the satellite is ejected or collides into the planet. Conclusions. Provided that Titan's migration is not halted in the future, Titan and Saturn may reach instability between a few gigayears and several tens of gigayears from now, depending on Titan's migration rate. The evolution would destabilise Titan and drive Saturn towards an obliquity of 90 • . Our findings may have important consequences for Uranus. They also provide a straightforward mechanism for producing transiting exoplanets with a face-on massive ring, a configuration that is often put forward to explain some super-puff exoplanets.

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Retrograde-rotating Exoplanets Experience Obliquity Excitations in an Eccentricity-enabled Resonance

Cited by 6 publications

References 43 publications

The future large obliquity of Jupiter

The future large obliquity of Jupiter

The large obliquity of Saturn explained by the fast migration of Titan

Future destabilisation of Titan as a result of Saturn's tilting

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