DDA 2015 – Irregular Structure in Saturn’s Huygens Ringlet

This is one of a series of notes taken during the 2015 meeting of the AAS Division on Dynamical Astronomy, 3-7 May, at CalTech. An index to this series (all the papers presented at the meeting) is here.

Session: Ring Dynamics

Joseph Spitale (PSI)

Abstract

Saturn’s Huygens ringlet is a narrow eccentric ringlet located ~250 km exterior to the outer edge of Saturn’s B ring. Based on about 5 years of Cassini observations, the ringlet contains multiple wavenumber-2 patterns superimposed on its edges (Spitale et al., in prep). Additional higher-order modes may be present, but a few km of radial variation on the edge of the ringlet likely cannot be explained by normal modes with pattern speeds appropriate for those modes. Instead, there is an irregular component to the ringlet’s shape that moves at a speed near the local Keplerian rate and is recognizable for multiple years. The pattern sometimes appears inverted, suggesting that the shape arises from a perturbation in eccentricity rather than semimajor axis. The synodic period between the inner and outer edges of the ring is ~5 years, so a significant evolution of the pattern would be expected if the shape were driven by multiple embedded perturbers distributed across the ring. The relatively static shape of the pattern may indicate that only perturbers with semimajor axes in a narrow region close to the edges of the ringlet play a role. A better understanding of the effect of embedded bodies on ring edges is needed.

Notes

  • Broad trend: $m=1$
    • Other normal modes present (Spitale & Hahn 2015)
    • $r(\theta,t) = a\{\sum_{i=0}^n e_i \cos m_i \left[\theta\, – \varpi_0^i – \Omega_p^i (t-t_0)\right]\}$
    • width-radius relation: $W(r) = \delta a \left[1\, – \left(e + \frac{q}{e}\right)\left(1-\frac{r}{a}\right)\right]$
  • Features track embedded massive objects
    • Persist for at least 3.5 yr
    • Synogic periods much longer than 3.5 yr
    • Wakes?
      • Wake-like structures originate at two points on the inner edge
      • $\rightarrow$ two dominant masses
      • eccentricity perturbations clues to dynamiics
    • Occupy narrow band near inner edge

DDA 2015 – The Fate of Debris from a Giant Impact on Mars

This is one of a series of notes taken during the 2015 meeting of the AAS Division on Dynamical Astronomy, 3-7 May, at CalTech. An index to this series (all the papers presented at the meeting) is here.

David Minton (Arizona State)

Abstract

We use published models for the formation of the $\sim1 \times 10$ km Borealis Basin on Mars from a ~2000 km impactor to investigate the fate of ejected debris. We use an n-body integrator to show that debris from this event could have been an important contributor to the cratering history of the Earth, Moon, and Mars well aOer the basin formed. We investigate whether this event could have been responsible for the Late Heavy Bombardment (LHB) on these planets. We show that the giant impact debris model has a number of features that are more favorable for explaining the LHB compared with giant planet instability models, such as the Nice model.

Notes

  • Craters
    • fossil record of small bodies
    • previously thought:
      • Strom et al. 2005: Heavily cratered terrains of Moon, Mars, Mercurywere dominated byMBAs ejected in a size-dependent way.
        • resonant sweeping of asteroid belt
      • Gomes et al. 2005: classic Nice model
      • Kring & Cohen 2002: impactors had asteroidal geochemistry
    • But…
      • Nice model only works if Jupiter jumps
        • Only ~1-5 percent of simulations produce required jump.
    • Cratered terrain evolution model
      • Input impactor size & velocity distributions.
      • Constraints:
        • must reach observed crater density in Lunar highlands
        • cannot make more Lunar basins than seen
    • Results:
      • MBA is not a good model for the Lunar highlands
    • So, what was the highlands impactorSFD?
      • Size distribution primordial “bump” around ~100 km is missing in the model
      • SPH codes: not very good at these scales
      • N-body sims:
        • Mars sucks as a scatterer.
        • Collisional evolution then produces the bump.
        • Gets about the right number of basins on Moon and Mars.
        • Bodies collect in theHungarias, kind of no matter what.
          • Thus, we can’t use Hungarias as a constraint.

DDA 2015 – Implications of Resonant and Near-Resonant Planetary Systems for Planet Formation

This is one of a series of notes taken during the 2015 meeting of the AAS Division on Dynamical Astronomy, 3-7 May, at CalTech. An index to this series (all the papers presented at the meeting) is here.

Eric Ford (Penn State)

Abstract

Observations of strongly interacting planetary systems in or near a mean motion resonance are unusually sensitive to planet masses and orbital properties, including dynamical properties that can help illuminate planet formation. Having developed a powerful toolbox for translating Doppler and/or transit timing observations into physics parameters, now we are able to characterize the resonant and secular behaviour of several strongly interacting planetary systems. I will present recent results for selected resonant and near-resonant planetary systems and discuss implications for planet formation. In particular, I will address implications for the nature and extent of orbital migration for giant and low-mass planets.

Notes

  • How didSTIPs form?
    • Three strawman models:
      • In situ formation: wrong
      • Large-scale disk formation: wrong
      • Nearly in situ formation plus modest early radial drift
  • STIP examples:
    • GJ 876: 4 planets
  • 55 Cnc: 5 planets, MMR
  • … Meh.

DDA 2015 – The Öpik Approximation and Giant Planet Shielding of the Inner Solar System

This is one of a series of notes taken during the 2015 meeting of the AAS Division on Dynamical Astronomy, 3-7 May, at CalTech. An index to this series (all the papers presented at the meeting) is here.

William Newman (UCLA)

Abstract

Öpik (1976) proposed that close-range gravitational interactions between planetesimal material and planets could be approximated by a two-step integration scheme: (1) while the planetesimal was outside the gravitational sphere of influence of the planet, its orbit would be described by a heliocentric Keplerian orbit; and (2) once its orbit entered the sphere of influence of the planet, its trajectory would then become a planetocentric Keplerian orbit until it exited the sphere of influence and resumed a heliocentric path. This approximation, however, was also limited by the requirement noted by Öpik that the perihelion or aphelion distance of the planetesimal differ from the orbital distance of the planet from the sun. This approximation proved to be a useful tool during early solar system dynamical investigations but this process was often employed as a numerical integration method without checking Öpik’s requirements, as well as establishing whether the orbital passage through the sphere of influence was sufficiently accurate. Öpik’s scheme was used to establish many features of solar system evolution, including the commonly-held belief that the giant planets serve as a shield preventing substantial numbers of planetesimals from entering the inner solar system. Wetherill (1994) in a pioneering work that exploited the Öpik approximation as an integration scheme estimated that present-day Jupiter could prevent 99.9% of planetesimals from entering the inner solar system. Here, we employ high precision first principles calculations of the orbits of swarms of planetesimals emerging from the Jupiter-Saturn, Saturn-Uranus, and Uranus- Neptune zones and have shown (1) the conditions necessary for Öpik’s approximation to be valid fail for a substantial fraction of the planetesimal population during their lifetimes, and (2) approximately 44% of the planetesimal swarm originating in the Jupiter-Saturn zone alone are injected into the inner Solar System while 18% ultimately become Earth-crossers.

Notes

  • Does Jupiter shield the inner solar system?
    • Impact history
    • Öpik:
      • novel scheme for solar system integrations
      • but identified a useful criterion for valid (numerical) results
      • exploits near-Keplerian orbits of inner SS
    • Öpik’s method:
      • Keplerian time step
    • Criterion: aphelion or perihelion must be different from mean distance. [um…duh]
    • Öpik’s result: 99.95% of outer solar system planetesimals could not have entered inner solar system!
  • Newman: let’s check, using an extremelyaccurate numerical integrator.
    • counted fraction of particles in each planet-planet outer zone fail Öpik’s criterion
    • found: ~20% get through
    • thus: Öpik’s criterion gets violated a lot

DDA 2015 – Tidal Effects in Late Stage Accretion

This is one of a series of notes taken during the 2015 meeting of the AAS Division on Dynamical Astronomy, 3-7 May, at CalTech. An index to this series (all the papers presented at the meeting) is here.

Session: Planet Formation II

Kevin Graves (Purdue)

Abstract

We model the effects of tidal dissipation in the late stages of planetary accretion. We investigate the tidal dissipation during close encounters between embryos and nearly-formed planets using a modified version of the N-body integrator SyMBA. We calculate a total energy lost due to tides per close encounter and estimate the change in velocities of the bodies at each encounter. We measure the effects on the dynamics, evolution, and final outcome of the planets. Our initial results show a clear separation between the tidal and non-tidal case for a relatively strong tidal dissipation factor. We compare these results to traditional late stage simulations both with and without fragmentation.

Notes

  • Overview of late-stage terrestrial planet accretion
    • a few dozen embryos
    • a few thousand planetesimals
    • Morby 2012
    • giant plant migration?
      • increases AMD of inner solar system
      • must therefore start with a lower deficit
    • AMD: Jacobson & Morbidelli 2014
  • Tidal effects on planetary embryos
    • Lots of heat generation from various processes $\rightarrow$ magma oceans
    • Simple model for energy loss during a close encounter (Kaula & Harris 1973): tides
      • free parameters: tidal Love numbers, dissipation param
    • combine to a “tidal parameter”: $\frac{h_2 (k_2 + 1)}{Q}$
    • Tidal effects in an n-body integrator
      • no tides vs. strong tides:
        • plot: mass concentration (Chambers 2013) vs AMD
        • strong tides: higher mass concentration with AMD
        • weak tides: inverse

DDA 2015 – The Formation of Terrestrial Planets from the Direct Accretion of Pebbles

This is one of a series of notes taken during the 2015 meeting of the AAS Division on Dynamical Astronomy, 3-7 May, at CalTech. An index to this series (all the papers presented at the meeting) is here.

Hal Levison (SwRI)

Abstract

Building the terrestrial planets has been a challenge for planeVormation models. In particular, classical theories have been unable to reproduce the small mass of Mars and instead predict that a planet near 1.5 AU should roughly be the same mass as the Earth (Chambers 2001, icarus 152,205). Recently, a new model, known as ‘slow pebble accretion’, has been developed that can explain the formation of the gas giants (Levison+ 2015, Nature submitted). This model envisions that the cores of the giant planets formed from 100 to 1000 km bodies that directly accreted a population of pebbles (Lambrechts & Johansen 2012, A&A 544, A32) – centimeter- to meter-sized objects that slowly grew in the protoplanetary disk. Here we apply this model to the terrestrial planet region and find that it can reproduce the basic structure of the inner Solar System, including a small Mars and a low-mass asteroid belt. In particular, our models show that for an initial population of planetesimals with sizes similar to those of the main belt asteroids, slow pebble accretion becomes inefficient beyond ~1.5 AU. As a result, Mars’s growth is stunted and nothing large in the asteroid belt can accumulate.

Notes

  • Standard view:
    • disk forms, dust settles to midplanet
    • dust accumulates, ~1-10 km
    • runaway growth
    • oligarchic growth
    • late-stage
      • violent endgame for terrestrial planets
    • main problem: Mars is way to small
  • possible solution: pebble accretion
    • dust
    • settling dust creates turbulence
    • ~10 mm – 1 m pebbles
    • large planetesimals can accrete pebbles very effectively
      • strong gas drag $\rightarrow$ huge collision cross section (~Hill sphere)
  • Can this explain the low mass of Mars?
    • low-pebble-mass exponential cutoff
      • encounter time too short
    • A Ceres can grow if $r \lt \sim 1$ AU, but it can’t grow if $r \gt \sim 1$ AU.
    • $\rightarrow$ leaves ~20 planets inside of ~1 AU
    • subsequently very unstable and < 1 AU largely clears out
    • leaves behind essentially the Solar System architecture

DDA 2015 – Did our Solar System once have a STIP?

This is one of a series of notes taken during the 2015 meeting of the AAS Division on Dynamical Astronomy, 3-7 May, at CalTech. An index to this series (all the papers presented at the meeting) is here.

Brett Gladman (CITA)

Abstract

Continuing the established tradition in the field of speculative “fairy tales”, we postulate that our Solar System once had a set of several additional Earth-scale planets interior to the orbit of Venus. This would resolve a known issue that the energy and angular momentum of our inner-planet system is best explained by accreting the current terrestrial planets from a disk limited to 0.7-1.1 AU; in our picture the disk material closer to the Sun also formed planets, but they have since been destroyed. By studying the orbital stability of systems like the known Kepler systems, Volk and Gladman (companion abstract) demonstrate that orbital excitation and collisional destruction could be confined to just the inner parts of the system. In this scenario, our Mercury is the final remnant of the inner system’s destruction via a violent multi-collision (and/or hit-and-run disruption) process.This would provide a natural explanation for Mercury’s unusually high eccentricity and orbital inclination; it also fits into the general picture of long-timescale secular orbital instability, with Mercury’s current orbit being unstable on 5 Gyr time scales. The common decade spacing of instability time scales raises the intriguing possibility that this destruction occurred roughly 0.6 Gyr after the formation of our Solar System and that the lunar cataclysm is a preserved record of this apocalyptic event that began when slow secular chaos generated orbital instability in our former super-Earth system.

Notes

  • Motivation
    • inner edge of terrestrial planet zone
    • Mercury is weird.
    • Why don’t we have a STIP (system of tightly-packed inner planets)?
  • Mercury:
    • surfing the edge of secular chaos
    • not clear how it got to $e^2 + i^2 \sim (0.25)^2$
    • tough to strip mantle without it quickly falling right back
    • Ausphaug & Reiner (2014): Mercury is the end state of a sequence of collisions.
  • Why is there an inner edge?
    • Wetherill 1978 (Protostars & Planets): E and L of terrestrial planets requires an inner edge ~0.6 AU.
    • Historical way out: it’s too hot.
      • But modern studies indicate $T < 1500$K until much later.
  • If there is (collision) debris, where does it go?
    • radiation pressure: days
    • PR drag: kyr
    • meteoritic transfer: kyr-Myr
    • planetary interactions: ~10 Myr
    • $\rightarrow$ disappears quickly
    • if self-collisional, it will still disappear quickly
  • Secular architecture rearrangement
    • pump up to large $e$
    • fast collisions (~50 km/s)
      • vapor production
      • “bullet factory” — erosion of remnants
  • Meng et al. 2014 (Science)
    • spike of hot dust around young star
    • decay ~1 yr

DDA 2015 – On the Robust Production of Super Earths and Suppressed Emergence of Gas Giants in Dynamically Evolving Protostellar Disks

This is one of a series of notes taken during the 2015 meeting of the AAS Division on Dynamical Astronomy, 3-7 May, at CalTech. An index to this series (all the papers presented at the meeting) is here.

Session: Planet Formation I

Doug Lin (UC Santa Cruz) (Brouwer award winner)

Abstract

Radial velocity and transit surveys indicate the presence of super Earth around half of the main sequence stars regardless of their mass and metallicity. In contrast, the frequency of gas giants is much lower and increases with stellar mass and metallicity. I will show how the emergence of super-Earth is a robust process whereas the formation of gas giant planets is a threshold phenomena. The topics to be discussed include physical barriers in the planet building process, the role of migration in their evolving natal disks, planets’ interaction with each other and with their host stars. I will also discuss some key observations which may provide quantitative tests for planet formation theories.

Notes

  • Observed properties of exoplanets: Howard 2013 (Science)
  • Showstoppers:
    • disk formation
    • grain growth: the “meter barrier”
      • Trapping of refractory grains beyond the magnetospheric cavity
      • Tends to pile up at boundary
    • grain growth: the “kilometer barrier”
      • collisional fragmentation vs. grav.
      • oligarchic barrier: isolation mass
        • typically very small
    • embryo retention barrier — Type I migration
      • planet-disk tidal interaction
      • get to high mass $\rightarrow$ migrate outward
      • resonant sweeping $\rightarrow 2^{nd}$ generation
    • core barrier: embryo resonant trapping
      • bypass the resonant barrier
        • inner scattered outward, outer scattered inward $\rightarrow$ collisions $\rightarrow$ impacts of super Earths
    • gas accretion barrier
      • Is there a threshold mass for gas accretion?
      • runaway accretion
        • Why didn’t this happen for observed super Earths?
      • plenty of material left over: why didn’t they turn into gas giants?
      • Measured disk accretion rate…?
      • metal rich stars: no observed dependence, despite theory
        • But metallicity of star and disk need not be the same.
    • Rapid growth of proto gas giants
    • grand design barrier: dynamical instability
      • How did gas giants acquire their eccentricities?
      • Type II migration
        • provides constraint on growth process
      • Why did hot Jupiters stop their inward migration?
  • Close in planets
    • e.g. Kepler-78
      • 8-hour period
      • Star is magnetic
        • ~15 g
        • analogous to Jupiter-Io
        • induced EMF (unipolar induction) $\rightarrow$ energy dissipation at expense of planet’s orbit
        • Planet surface cannot be iron; must be silicates.
        • Flux tube footprints on star should move at period of planetary orbit, not stellar rotation.
  • Many other issues!

DDA 2015 – Inclination Excitation in Compact Extrasolar Planetary Systems

This is one of a series of notes taken during the 2015 meeting of the AAS Division on Dynamical Astronomy, 3-7 May, at CalTech. An index to this series (all the papers presented at the meeting) is here.

Juliette Becker (U. Michigan) (Duncombe prize winner)

Abstract

The Kepler Mission has detected dozens of compact planetary systems with more than four transiting planets. This sample provides a collection of close-packed planetary systems with relatively liRle spread in the inclination angles of the inferred orbits. We have explored the effectiveness of dynamical mechanisms in exciting orbital inclination in this class of solar systems. The two mechanisms we discuss are self-excitation of orbital inclination in initially (nearly) coplanar planetary systems and perturbations by additional unseen larger bodies in the outer regions of the solar systems. For both of these scenarios, we determine the regimes of parameter space for which orbital inclination can be effectively excited. For compact planetary systems with the observed architectures, we find that the orbital inclination angles are not spread out appreciably through self-excitation, resulting in a negligible scaRer in impact parameter and a subsequently stable transiting system. In contrast, companions in the outer solar system can be effective in driving variations of the inclination angles of the inner planetary orbits, leading to significant scatter in impact parameter and resultantly non-transiting systems. We present the results of our study, the regimes in which each excitation method – self-excitation of inclination and excitation by a perturbing secondary – are relevant, and the magnitude of the effects.

Notes

  • Why so many multi-planet transiting system?
  • Ballard & Johnson 2014, Ballard 2014, Morton 2014, Morton & Winn 2014
  • Seems to be a “Kepler dichotomy”
  • $\rightarrow$ inclination excitation important
  • $2^{nd}$ order secular Laplace-Lagrange theory (Murray &Dermott)
    • inc. & ecc. decoupled
    • Inclination as function of time (analytical)
  • Use Kepler 4+ planets as model systems
  • Conclusions:
    • Self-excitation in compact solar system planets does not appear to be a significant mechanism
    • Current Kepler systems with non-transiting planets could have started out transiting but driven out of transit by self-excitation
    • Excitation by compact solar system planets themselves (smear their mass into a disk) does notappearto be a significant mechanism
      • It might be possible to see multi-transiting systems with Jovian masses (if they exist)
    • Dynamical transit duration variations due to secular interactions will be small ($10^{-4}$ to $10^{-7}$ sec) but potentially observable (via statistics on long time series)

DDA 2015 – Secular Star-Disk Coupling and the Origin of Exoplanetary Spin-Orbit Misalignments

This is one of a series of notes taken during the 2015 meeting of the AAS Division on Dynamical Astronomy, 3-7 May, at CalTech. An index to this series (all the papers presented at the meeting) is here.

Christopher Spalding (CalTech) (Duncombe prize winner)

Abstract

A recent paradigm shift in exoplanetary astronomy has come with the detection of a substantial number of planets possessing orbits that are misaligned with respect to the spin axes of their host stars. Moreover, observations of misalignments now include coplanar, multi-transiting systems, suggesting that these planets inherited their orbital planes from a protoplanetary disk which was once itself inclined with respect to the star. It has been proposed that mutual star-disk inclination may arise as a consequence of turbulence within the collapsing molecular cloud core, out of which both the star and its disk form. Alternatively, misalignments may be aRained later on, through secular interactions between the disk and companion stars. In this work, we examine the secular dynamics of the stellar spin axis arising in response to the gravitational and accretional torques communicated between the star and its disk throughout the epoch of star and planet formation. Our analysis shows that even though the disk forms from turbulent material, and is thus expected to exhibit a stochastic variation in its orientation with time during the star formation process, gravitational disk-star coupling adiabatically suppresses the excitation of mutual star-disk inclination under all reasonable parameter regimes. As such, the excitation of mutual star-protoplanetary disk inclination must occur later on in the disk’s lifetime, by way of an encounter with a secular resonance between stellar precession and the gravitational perturbations arising from an external potential, such as a binary companion.

Notes

  • Motivation: our solar system, Laplace 1796
    • Ecliptic disk oriented approx perp to Sun’s spin axis
    • Goldreich & Tremaine 1980:
      • disk-driven migration
      • Jupiters eaten by stars
        • Why aren’t observed hot Jupiters eaten?
    • $\rightarrow$ hot Jupiters should be aligned with their disks
    • But significant fraction is seriously misaligned!
      • Tends to be more massive planets
  • How to getmisalignments?
    • Disk-driven migration doesn’t work
    • High-eccentricity + tidal?
      • Cannot explain multi-transiting misaligned systems (Huber et al. 2013)
  • $\rightarrow$ Are disks really aligned with their stars?
  • Hypothesis 1: misalignment during formation
    • Spalding et al. 2014 (ApJ)
    • Cores are turbulent
    • Spin dir varies randomly by $\approx30^{\circ}$ every ~0.01 pc
    • Shell infall time $\approx 10^4$ yr
    • Disk adopts plane of whatever shell falls last (Bate et al. 2010)
    • Star-disk system forms misaligned
    • BUT: disk-star coupling?
      • Young stars spin rapidly $\rightarrow$ oblate
        • Dynamically equivalent to massive wire around point mass
        • $\rightarrow$ disk precession
      • Use Laplace-Lagrange secular theory
        • Disk annuli act as outer perturbers upon stellar irientation
        • $\rightarrow$ precession period ~100 years(!)
    • Numerical simulation — will star spin axis follow motion of disk?
      • Star trails disk, even though motion stochastic
  • Hypothesis 2: binary companion in orbit around star+disk — disktorquing
    • Companion causes $\gg 10^4$ yr precession
    • Star-disk coupling weakens with time
      • mass loss
      • stellar contraction
    • Spalding & Batygin 2014 (ApJ)
    • Eventually, disk-binary precession ~ star-disk precession
      • hits secular resonance, catapulting disk/star into retrograde orbits
    • Final inclination only depends upon initial binary inclination
  • Summary:
    • Gravitational star-disk coupling prevents misalignment early on.
    • Neighboring stars excite misalignments by was of a secular resonance.
    • Misalignments are consistent with disk-driven migration.

DDA 2015 – MMRs and the Origins of Extrasolar Orbital Architectures

This is one of a series of notes taken during the 2015 meeting of the AAS Division on Dynamical Astronomy, 3-7 May, at CalTech. An index to this series (all the papers presented at the meeting) is here.

Konstantin Batygin (CalTech)

Abstract

The early stages of dynamical evolution of planetary systems are often shaped by dissipative processes that drive orbital migration. In multi-planet systems, convergent amassing of orbits inevitably leads to encounters with rational period ratios, which may result in establishment of mean motion resonances. The success or failure of resonant capture yields exceedingly different subsequent evolutions, and thus plays a central role in determining the ensuing orbital architecture of planetary systems. In this talk, we will show how an integrable Hamiltonian formalism for planetary resonances that allows both secondary bodies to have finite masses and eccentricities, can be used to construct a comprehensive theory for resonant capture. Employing the developed analytical model, we shall examine the origins of the dominantly non-resonant orbital distribution of sub-Jovian extrasolar planets, and demonstrate that the commonly observed extrasolar orbital structure can be understood if planet pairs encounter mean motion commensurabilities on slightly eccentric (e ~ 0.02) orbits. Accordingly, we speculate that resonant capture among low-mass planets is typically rendered unsuccessful due to subtle axial asymmetries inherent to the global structure of protoplanetary disks.

Notes

  • SeeMécaniqueCéleste, Laplace 1805!
    • But origins not really understood until Roy & Ovenden 1954, Goldreich 1964 (MNRAS)
  • Disk-satellite interactions (Goldreich & Tremaine)
  • But what about more than one planet?
  • All tend to migrateinward then lock intoMMRs (Pierens 2013 A&A)
    • $\Rightarrow$ numerical models predict MMR lock
    • BUT only ~15% of observed planet pairs are in resonance
  • The real Hamiltonian (planet-planet interactions) is actually probably a mess.
    • See Poincare’s book, vol. 2(!)
    • Define a canonical rotation that gives an integral of the motion (“generalized reducing transformation” –Poincare)
      • Basically, a generalized Tisserand parameter
    • Batygin & Morbidelli 2013(?)
  • An analytical theory for resonant capture: unrestricted ETB problem.
    • Batygin 2015 (MNRAS, submitted)
    • Capture prob. only depends on total mass of the planets, NOT the mass ratio
      • phase space area occupied by planet is small
    • Kepler sample: critical eccentricity is ~0.02 — very small!
      • Larger than this, capture fails
      • Matches observed value!
    • Explains Jupiter-Saturn MMR lock
    • Perhaps slight deviations from axial symmetry in protoplanetary disks are responsible for the orbital architecture we observe today.

DDA 2015 – Consolidating and Crushing Exoplanet Systems

This is one of a series of notes taken during the 2015 meeting of the AAS Division on Dynamical Astronomy, 3-7 May, at CalTech. An index to this series (all the papers presented at the meeting) is here.

Kathryn Volk (U. British Columbia)

Abstract

Kepler revealed the common existence of tightly-packed planetary systems around solar-type stars, existing entirely on orbits with periods shorter than ~200 days. Those systems must have survived for the ages of their host stars (~5 Gyr), so their formation mechanism must provide inter-planet spacings that permit long-term stability. If one postulates that most planetary systems form with tightly-packed inner planets, their current absence in some systems could be explained by the collisional destruction of the inner system after a period of meta-stability. The signatures of such intense collisional environments may have been observed around stars in the form of rapidly varying debris disks; in these observed disks, collisional products are being disposed of via drag down onto the star or grinding to the nearly instantaneous dust blow-out limit. We use the orbital spacings and planet masses of the observed Kepler multi-planet systems to investigate the stability and long-term behavior of the systems. We find that many of our Kepler system analogs are unstable on 100 Myr timescales, even for initially small eccentricities (0-0.05); the instability timescales in these systems are distributed such that equal fractions of the systems experience planetary collisions in each decade in time. We discuss the likely outcomes of collisions in these systems based on the typical collision speeds from our numerical integrations and what implications this has for interpreting the observed Kepler multi-planet systems. The possible implications for our Solar System are discussed in a companion abstract (Gladman and Volk).

Notes

  • Architectures of close-in (closely packed) planetary systems (from Kepler)
  • Fabrycky 2014
  • ~5-10% ofFGK field stars
    • These systems must be stable on Gyr timescales
  • Are all stars formed tightly packed?
  • Modeled 13 such Kepler systems
    • Preserved $a$ and masses, orbital angles randomized
    • Allowed $e_0$ to vary $0 < e_0 < 0.05$
    • Sudden onset of instability in 11 of these 13 after tens to ~100 Myr
      • [why is she surprised?]
    • These eccentricities are in range of observed values
    • Decay rates consistent with e.g. Holman & Wisdom (1992 AJ)
  • Why sudden onset?
    • History is very sensitive to ICs [duh]
    • Consolidation (low-speed collisions) vs. Destruction (high-speed collisions)
    • First collision is often near the accretion/erosion boundary — i.e., low-speed
    • Masses in 4-5 planet systems tend to be lower, while individual masses in ~3-planet systems are higher: mergers?
    • Tracked collision speeds during integrations.
    • Second collision often goes into erosion regime (i.e., high-speed)
      • Observing debris should be rare (but see Meng et al. 2012)
    • Ergodicity allows large variety of outcomes
  • $\Rightarrow$ tightly packed systems could be ubiquitous initially
    • Young stars should show higher fraction
  • The remaining ~95% should be 0-2 planet systems

DDA 2015 – Capture into Mean-Motion Resonances for Exoplanetary Systems

This is one of a series of notes taken during the 2015 meeting of the AAS Division on Dynamical Astronomy, 3-7 May, at CalTech. An index to this series (all the papers presented at the meeting) is here.

Session: Exoplanet Theory II

Maryame El Moutamid (Cornell)

Abstract

Many bodies in the Solar System and some exo-planets are close to or captured in Mean Motion Resonances (MMR). Capture into such resonances has been investigated by many authors. Indeed, the Hamiltonian equations of motion in presence of migration are given by Sicardy and Dubois Cel. Mech. & Dyn. Astron., 86, 321-350 (2003). Fleming and Hamilton, Icarus 148, 479-493 (2000), studied the problem in a less generic context. In these two papers, the authors studied the problem of 1:1 corotation (Lagrange points L4 and L5), rather than m+1:m corotations (El Moutamid et al, Cel. Mech. & Dyn. Astron, 118, 235-252 (2014)). We will present a generic way to analyze details of a successful (or not) capture in the case of an oblate (or not) central body in the context of Restricted Three Body Problem (RTBP) and a more General Three Body Problem in the context of known statistics for captured exoplanets (candidates) observed by Kepler.

Notes

  • Captures partial near MMR (Fabrycky et al. 2012)
  • No generic study on coupling between associated resonances (ERTB vs. general TB)
  • 1) simple model,2DoF — $(m+1) n’ \approx m n$
    • splitting the corotation and Lindblad resonances (by $J_2 \neq 0$)
    • Lindblad: vary $e$
    • corotation: pendular motion (conserves $e$)
    • plot: $J_c – J_L$ vs. $\phi_C$
  • 2) general case
    • can define a constant of motion: $J_{c,relat} = \frac{A^2 \xi}{m} – \frac{A’^2 e’}{m+1} – ?? = const.$
  • Add dissipationforMMR capture
    • ratio: potential barrier of one vs. other body
    • plot: potential energy vs. critical angle of corotation
    • probability of capture: very very small

DDA 2015 – Obliquity Evolution of Earth-Like Exoplanets in Systems with Large Inclinations

This is one of a series of notes taken during the 2015 meeting of the AAS Division on Dynamical Astronomy, 3-7 May, at CalTech. An index to this series (all the papers presented at the meeting) is here.

Russell Deitrick (U. Washington)

Abstract

In order to properly assess the potential for habitability and prioritize target selection for the characterization of exoplanets, we need to understand the limits of orbital and rotational dynamics. Large satellites may be rare and very difficult to detect. Consequently, it is necessary to quantify the likelihood of a planet’s having extreme obliquity cycles in the absence of a moon and to model the potential impact on the planet’s climate. We explore the obliquity evolution of (1) known exoplanet systems that could contain Earth-like planets in the habitable zone and (2) hypothetical planets in mutually inclined, chaotic resonant configurations that experience some of the most extreme orbital evolution possible. We use a secular obliquity model coupled to either an N-body models or a 4 order secular orbital model. We find that in some known systems, planets’ obliquity variations are small and unlikely to have a major effect on climate, unless undetected planets are present. Systems with three or more planets are significantly more dynamically rich, with planets that undergo obliquity changes of ~10° over 50,000 years and >30° over a few million years. In resonant configurations, Earth-like exoplanets can undergo dramatic and chaotic evolution in eccentricity and inclination while remaining stable for over 10 Gyr. In configurations in which eccentricities and inclinations stay below ~0.1 and~10°, respectively, obliquities oscillate quasi-periodically with amplitudes similar to the non-resonant, three-planet configurations. In more dynamically active configurations, in which eccentricities and inclinations evolve to e > 0.3 and i > 15°, obliquities can extend from ~0° to well past 90°. In extreme cases eccentricities can reach >0.9999 and inclinations >179.9 degrees, driving precession rates in excess of degrees per year. However, these planets can graze or impact the stellar surface and are probably not habitable.

Notes

  • $\upsilon$Andromedae c and d
    • obliquity oscillations
  • Model description
    • Barnes, Deitrick et al. 2015
    • Using the secular disturbing function (Murray & Dermott) and a secular obliquity model (Kinoshita 1975, 1977)
    • HD190360
      • obliquity varies w large amplitude in a “strip” in $\Delta i_0$ – $e_0$ plane — WTH?
      • two planets interacting (an Earth and a super-Jupiter) … somehow
      • Inside the “strip”, a commensurabilitylibrates
        • $(\varpi’ – \varpi) – (\Omega + p_A)$
        • outside the “strip”: no libration
      • Analogous to a compound pendulum
  • Summary
    • Non-coplanar systems in MMR exhibit long-lived chaos.
    • These systems can be formed by scattering.
    • Possible way to form misaligned hot Jupiters.
    • Earth-like planets in these systems can also have chaotic obliquity variations.

DDA 2015 – Loners, Groupies, and Long-term Eccentricity Behavior – Insights from Secular Theory

This is one of a series of notes taken during the 2015 meeting of the AAS Division on Dynamical Astronomy, 3-7 May, at CalTech. An index to this series (all the papers presented at the meeting) is here.

Christa Van Laerhoven (CITA)

Abstract

Considering the secular dynamics of multi-planet systems provides substantial insight into the interactions between planets in those systems. Secular interactions are those that don’t involve knowing where a planet is along its orbit, and they dominate when planets are not involved in mean motion resonances. These interactions exchange angular momentum among the planets, evolving their eccentricities and inclinations. To second order in the planets’ eccentricities and inclinations, the eccentricity and inclination perturbations are decoupled. Given the right variable choice, the relevant differential equations are linear and thus the eccentricity and inclination behaviors can be described as a sum of eigenmodes. Since the underlying structure of the secular eigenmodes can be calculated using only the planets’ masses and semi-major axes, one can elucidate the eccentricity and inclination behavior of planets in exoplanet systems even without knowing the planets’ current eccentricities and inclinations. I have calculated both the eccentricity and inclination secular eigenmodes for the population of known multi-planet systems whose planets have well determined masses and periods. Using this catalog of secular character, I will discuss the prevalence of dynamically grouped planets (‘groupies’) versus dynamically uncoupled planets (‘loners’) and how this relates to the exoplanets ‘long-term eccentricity and inclination behavior. I will also touch on the distribution of the secular eigenfreqiencies.

Notes

  • Secular character of multi-planet system
  • planet-planet interactions
  • only need masses and semimajor axes (not eccentricity, not inclination) to set secular structure
  • two-planet system: two eccentricityeigenmodes
    • $h = e \sin \varpi$, $k = e \cos \varpi$ plot: $e$ is a vector
    • each $e$ vector is the sum of two eigenvectors
  • 3-planet system: “groupie”-ness and loners
    • groupies:
      • $e$ highly variable
      • $\varpi$ precession not uniform
    • loners:
      • $e$ does not vary by much
      • $\varpi$ precesses steadily
  • Kepler-11
    • outer planet is a loner — does not interact with others
    • 5 inner planets are groupies — interact strongly with each other
  • Summary: most planets are “groupies”, “loners” are rare.