How to design a planetary system for different scattering outcomes: giant impact sweet spot, maximizing exocomets, scattered discs

How to design a planetary system for different scattering outcomes: giant impact sweet spot, maximizing exocomets, scattered discs

Authors:

Wyatt et al

Abstract:

This paper considers the dynamics of the scattering of planetesimals or planetary embryos by a planet on a circumstellar orbit. We classify six regions in the planet's mass versus semimajor axis parameter space according to the dominant outcome for scattered objects: ejected, accreted, remaining, escaping, Oort Cloud, and depleted Oort Cloud. We use these outcomes to consider which planetary system architectures maximize the observability of specific signatures, given that signatures should be detected first around systems with optimal architectures (if such systems exist in nature). Giant impact debris is most readily detectable for 0.1–10 M⊕ planets at 1–5 au, depending on the detection method and spectral type. While A stars have putative giant impact debris at 4–6 au consistent with this sweet spot, that of FGK stars is typically ≪1 au contrary to expectations; an absence of 1–3 au giant impact debris could indicate a low frequency of terrestrial planets there. Three principles maximize the cometary influx from exo-Kuiper belts: a chain of closely separated planets interior to the belt, none of which is a Jupiter-like ejector; planet masses not increasing strongly with distance (for a net inward torque on comets); and ongoing replenishment of comets, possibly by embedded low-mass planets. A high Oort Cloud comet influx requires no ejectors and architectures that maximize the Oort Cloud population. Cold debris discs are usually considered classical Kuiper belt analogues. Here we consider the possibility of detecting scattered disc analogues, which could be betrayed by a broad radial profile and lack of small grains, as well as spherical 100–1000 au mini-Oort Clouds. Some implications for escaping planets around young stars, detached planets akin to Sedna, and the formation of super-Earths are also discussed.

Are a Chain of Nemesis Class Exoplanets Scattering Comets Inwards in the η Corvi System?

ALMA observations of the η Corvi debris disc: inward scattering of CO-rich exocomets by a chain of 3-30 M⊕ planets?

Authors:

Marino et al

Abstract:

While most of the known debris discs present cold dust at tens of AU, a few young systems exhibit hot dust analogous to the Zodiacal dust. η Corvi is particularly interesting as it is old and it has both, with its hot dust significantly exceeding the maximum luminosity of an in-situ collisional cascade. Previous work suggested that this system could be undergoing an event similar to the Late Heavy Bombardment (LHB) soon after or during a dynamical instability. Here we present ALMA observations of η Corvi with a resolution of 1."2 (~22au) to study its outer belt. The continuum emission is consistent with an axisymmetric belt, with a mean radius of 152au and radial FWHM of 46au, which is too narrow compared to models of inward scattering of an LHB-like scenario. Instead, the hot dust could be explained as material passed inwards in a rather stable planetary configuration. We also report a 4sigma detection of CO at ~ 20au. CO could be released in situ from icy planetesimals being passed in when crossing the H2O or CO2 ice lines. Finally, we place constraints on hidden planets in the disc. If a planet is sculpting the disc's inner edge, this should be orbiting at 75-100au, with a mass of 3-30 M⊕ and an eccentricity < 0.08. Such a planet would be able to clear its chaotic zone on a timescale shorter than the age of the system and scatter material inwards from the outer belt to the inner regions, thus feeding the hot dust.

Scattered light mapping of protoplanetary disks

Scattered light mapping of protoplanetary disks

Authors:

Stolker et al

Abstract:

High-contrast scattered light observations have revealed the surface morphology of several dozens of protoplanetary disks at optical and near-infrared wavelengths. Inclined disks offer the opportunity to measure part of the phase function of the dust grains that reside in the disk surface which is essential for our understanding of protoplanetary dust properties and the early stages of planet formation. We aim to construct a method which takes into account how the flaring shape of the scattering surface of an (optically thick) protoplanetary disk projects onto the image plane of the observer. This allows us to map physical quantities (scattering radius and scattering angle) onto scattered light images and retrieve stellar irradiation corrected (r^2-scaled) images and dust phase functions. We apply the method on archival polarized intensity images of the protoplanetary disk around HD 100546 that were obtained with VLT/SPHERE in R'-band and VLT/NACO in H- and Ks-band. The brightest side of the r^2-scaled R'-band polarized intensity image of HD 100546 changes from the far to the near side of the disk when a flaring instead of a geometrically flat disk surface is used for the r^2-scaling. The decrease in polarized surface brightness in the scattering angle range of ~40-70 deg is likely a result of the dust phase function and degree of polarization which peak in different scattering angle regimes. The derived phase functions show part of a forward scattering peak which indicates that large, aggregate dust grains dominate the scattering opacity in the disk surface. Projection effects of a protoplanetary disk surface need to be taken into account to correctly interpret scattered light images. Applying the correct scaling for the correction of stellar irradiation is crucial for the interpretation of the images and the derivation of the dust properties in the disk surface layer.

Dynamically hot Super-Earths from outer giant planet scattering

Dynamically hot Super-Earths from outer giant planet scattering

Authors:

Huang et al

Abstract:

The hundreds of multiple planetary systems discovered by the Kepler mission are typically observed to reside in close-in (≲0.5 AU), low-eccentricity, and low-inclination orbits. We run N-body experiments to study the effect that unstable outer (≳1 AU) giant planets, whose end orbital configurations resemble those in the Radial Velocity population, have on these close-in multiple Super-Earth systems. Our experiments show that the giant planets greatly reduce the multiplicity of the inner Super-Earths and the surviving population can have large eccentricities (e≳0.3) and inclinations (i≳20∘) at levels that anti-correlate with multiplicity. Consequently, this model predicts the existence of a population of dynamically hot single-transiting planets with typical eccentricities and inclinations in the ranges of ∼0.2−0.5 and ∼10∘−40∘. We show that these results can explain the following observations: (i) the recent eccentricity measurements of Kepler super-Earths from transit durations, (ii) the tentative observation that single-transiting systems have a wider distribution of stellar obliquity angles compared to the multiple-transiting systems; (iii) the architecture of some eccentric super-Earths discovered by Radial Velocity surveys such as HD125612c. Future observations from TESS will reveal many more dynamically hot single transiting planets, for which follow up Radial Velocity studies will be able to test our models and see whether they have outer giant planets.

Kepler-108: A Mutually Inclined Giant Planet System

Kepler-108: A Mutually Inclined Giant Planet System

Authors:

Mills et al

Abstract:

The vast majority of well studied giant-planet systems, including the Solar System, are nearly coplanar which implies dissipation within a primordial gas disk. however, intrinsic instability may lead to planet-planet scattering, which often produces non-coplanar, eccentric orbits. Planet scattering theories have been developed to explain observed high eccentricity systems and also hot Jupiters; thus far their predictions for mutual inclination (I) have barely been tested. Here we characterize a highly mutually-inclined (I ~ 15-60 degrees), moderately eccentric (e >~ 0.1) giant planet system: Kepler-108. This system consists of two approximately Saturn-mass planets with periods of ~49 and ~190 days around a star with a wide (~300AU) binary companion in an orbital configuration inconsistent with a purely disk migration origin.

Circumbinary Exoplanets get Scattered, not Collide

Planet Scattering Around Binaries: Ejections, Not Collisions

Authors:

Smullen et al

Abstract:

Transiting circumbinary planets discovered by Kepler provide unique insight into binary and planet formation. Several features of this new found population, for example the apparent pile-up of planets near the innermost stable orbit, may distinguish between formation theories. In this work, we determine how planet-planet scattering shapes planetary systems around binaries as compared to single stars. In particular, we look for signatures that arise due to differences in dynamical evolution in binary systems. We carry out a parameter study of N-body scattering simulations for four distinct planet populations around both binary and single stars. While binarity has little influence on the final system multiplicity or orbital distribution, the presence of a binary dramatically effects the means by which planets are lost from the system. Most circumbinary planets are lost due to ejections rather than planet-planet or planet-star collisions. The most massive planet in the system tends to control the evolution. Aside from the absence of very close-in planets, which are unstable around a binary, dynamical evolution alone does not introduce systematic differences in the final planet populations. Thus, we suggest that any intrinsic differences in the populations are imprinted by formation.

Period Ratio Distribution of Near-Resonant Planets Indicate Planetesimal Scattering

Period Ratio Distribution of Near-Resonant Planets Indicate Planetesimal Scattering

Authors:

Chatterjee et al

Abstract:

An intriguing trend among Kepler's multi-planet systems is an overabundance of planet pairs with period ratios just wide of mean motion resonances (MMR) and a dearth of systems just narrow of them. In a recently published paper Chatterjee & Ford (2015; henceforth CF15) has proposed that gas-disk migration traps planets in a MMR. After gas dispersal, orbits of these trapped planets are altered through interaction with a residual planetesimal disk. They found that for massive enough disks planet-planetesimal disk interactions can break resonances and naturally create moderate to large positive offsets from the initial period ratio for large ranges of planetesimal disk and planet properties. Divergence from resonance only happens if the mass of planetesimals that interact with the planets is at least a few percent of the total planet mass. This threshold, above which resonances are broken and the offset from resonances can grow, naturally explains why the asymmetric large offsets were not seen in more massive planet pairs found via past radial velocity surveys. In this article we will highlight some of the key findings of CF15. In addition, we report preliminary results from an extension of this study, that investigates the effects of planet-planetesimal disk interactions on initially non-resonant planet pairs. We find that planetesimal scattering typically increases period ratios of non-resonant planets. If the initial period ratios are below and in proximity of a resonance, under certain conditions, this increment in period ratios can create a deficit of systems with period ratios just below the exact integer corresponding to the MMR and an excess just above. From an initially uniform distribution of period ratios just below a 2:1 MMR, planetesimal interactions can create an asymmetric distribution across this MMR similar to what is observed for the Kepler planet pairs.

Cross-sections for ExoPlanetary Systems Interacting With Passing Stars and Binaries

Cross-sections for planetary systems interacting with passing stars and binaries

Authors:

Li et al

Abstract:

Most planetary systems are formed within stellar clusters, and these environments can shape their properties. This paper considers scattering encounters between solar systems and passing cluster members, and calculates the corresponding interaction cross-sections. The target solar systems are generally assumed to have four giant planets, with a variety of starting states, including circular orbits with the semimajor axes of our planets, a more compact configuration, an ultracompact state with multiple mean motion resonances, and systems with massive planets. We then consider the effects of varying the cluster velocity dispersion, the relative importance of binaries versus single stars, different stellar host masses, and finite starting eccentricities of the planetary orbits. For each state of the initial system, we perform an ensemble of numerical scattering experiments and determine the cross-sections for eccentricity increase, inclination angle increase, planet ejection, and capture. This paper reports results from over 2 million individual scattering simulations. Using supporting analytic considerations, and fitting functions to the numerical results, we find a universal formula that gives the cross-sections as a function of stellar host mass, cluster velocity dispersion, starting planetary orbital radius, and final eccentricity. The resulting cross-sections can be used in a wide variety of applications. As one example, we revisit constraints on the birth aggregate of our Solar system due to dynamical scattering and find N ≲ 104 (consistent with previous estimates).