Research

Projects:

Check out summaries of assorted recent projects:

Slowly-growing planets trigger weaker vortices: Hammer, Kratter, & Lin, 2017

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The presence of a giant planet in a low-viscosity disc can create a gap edge in the disc’s radial density profile sharp enough to excite the Rossby wave instability. This instability may evolve into dust-trapping vortices that might explain the `banana-shaped’ features in recently observed asymmetric transition discs with inner cavities. Previous hydrodynamical simulations of planet-induced vortices have neglected the time-scale of hundreds to thousands of orbits to grow a massive planet to Jupiter size. In this work, we study the effect of a giant planet’s runaway growth time-scale on the lifetime and characteristics of the resulting vortex. For two different planet masses (1 and 5 Jupiter masses) and two different disc viscosities (α = 3 × 10-4 and 3 × 10-5), we compare the vortices induced by planets with several different growth time-scales between 10 and 4000 planet orbits. In general, we find that slowly-growing planets create significantly weaker vortices with lifetimes and surface densities reduced by more than 50 per cent. For the higher disc viscosity, the longest growth time-scales in our study inhibit vortex formation altogether. Additionally, slowly-growing planets produce vortices that are up to twice as elongated, with azimuthal extents well above 180° in some cases. These unique, elongated vortices likely create a distinct signature in the dust observations that differentiates them from the more concentrated vortices that correspond to planets with faster growth time-scales. Lastly, we find that the low viscosities necessary for vortex formation likely prevent planets from growing quickly enough to trigger the instability in self-consistent models.

Debris in the Pluto-Charon System: Smullen & Kratter, 2017

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The Pluto-Charon system has come into sharper focus following the fly by of New Horizons. We use N-body simulations to probe the unique dynamical history of this binary dwarf planet system. We follow the evolution of the debris disc that might have formed during the Charon-forming giant impact. First, we note that in-situ formation of the four circumbinary moons is extremely difficult if Charon undergoes eccentric tidal evolution. We track collisions of disc debris with Charon, estimating that hundreds to hundreds of thousands of visible craters might arise from 0.3-5 km radius bodies. New Horizons data suggesting a dearth of these small craters may place constraints on the disc properties. While tidal heating will erase some of the cratering history, both tidal and radiogenic heating may also make it possible to differentiate disc debris craters from Kuiper belt object craters. We also track the debris ejected from the Pluto-Charon system into the Solar System; while most of this debris is ultimately lost from the Solar System, a few tens of 10-30 km radius bodies could survive as a Pluto-Charon collisional family. Most are plutinos in the 3:2 resonance with Neptune, while a small number populate nearby resonances. We show that migration of the giant planets early in the Solar System’s history would not destroy this collisional family. Finally, we suggest that identification of such a family would likely need to be based on composition as they show minimal clustering in relevant orbital parameters.

Scattering in Circumbinary Systems: Smullen, Kratter, & Shannon, 2016

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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 affects 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. Systems similar to the only observed multiplanet circumbinary system, Kepler-47, can arise from much more tightly packed, unstable systems. Only extreme initial conditions introduce differences in the final planet populations. Thus, we suggest that any intrinsic differences in the populations are imprinted by formation.

Stability of Massive Planetary Systems: Morrison & Kratter, 2016

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In this paper, we explore the stability of  high mass, multi-planet systems detectable by direct imaging surveys. While empirical relationships exist that predict how system stability scales with planet spacing at low masses, we show that extrapolating to super-Jupiter masses can lead to up to an order of magnitude overestimate of stability for massive, tightly packed systems. We show that at both low and high planet masses, overlapping mean-motion resonances trigger chaotic orbital evolution, which leads to system instability. We attribute some of the difference in behavior as a function of mass to the increasing importance of second order resonances at high planet–star mass ratios. We use our tailored high mass planet results to estimate the maximum number of planets that might reside in double component debris disk systems, whose gaps may indicate the presence of massive bodies.

 

Kepler 444 or “Marses” in a Triple Star System: Dupuy, Kratter, et al 2016

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This work uses precision astrometry and radial velocity data to reveal the orbital configuration of one of the most extreme planetary systems: Kepler 444. The primary star in this system hosts 5 transiting sub-earth size planets with orbital periods less than 20 days. This planetary system is orbited by a highly eccentric (e ~0.9) spectroscopic M-dwarf binary that passes within 5 AU of the primary. These stars are likely co-eval with the planetary system suggesting that these tiny planets formed in a truncated disk. In addition, the outer pair has an orbital plane that is well aligned with the orbital plane of the planets! The movie below shows how the planetary orbits *would* evolve if the planetary orbits were misaligned with the outer M-dwarf pair. The rapid precession of nodes induced by the binary corroborates the finding from the best fit orbits that the planes are aligned. Otherwise it would be unlikely to catch all 5 planets in transit.

 

Circumbinary Planet Dynamics: Kratter & Shannon 2014

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circumbin_ttvThe recent discovery of planets orbiting main-sequence binaries will provide crucial constraints for theories of binary and planet formation. The formation pathway for these planets is complicated by uncertainties in the formation mechanism of the host stars. In this paper, we compare the dynamical states of single- and binary-star planetary systems. Specifically, we pose two questions: (1) What does it mean for a circumbinary system to be dynamically packed? (2) How many systems are required to differentiate between a population of packed or sparse planets? We determine when circumbinary systems become dynamically unstable as a function of the separation between the host-stars and the inner planet, and the first and second planets. We show that these represent unique stability constraints compared to single-star systems. We find that although the existing Kepler data is insufficient to distinguish between a population of packed or sparse circumbinary systems, a more thorough study of circumbinary Transit Timing Variations (TTVs) combined with an order of magnitude increase in the number of systems may prove conclusive. Future space missions such as TESS provide the best opportunity for increasing the sample size.
In this figure, we show a simple estimate for TTVs induced by a hypothetical interior planet on Kepler 47c. The magnitude of the expected TTVs from the newly discovered Kepler 47d is labelled as well. The black dashed line divides dynamically packed and sparse regions. The white area indicates dynamical instability.

Star Hoppers: Kratter & Perets 2012

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In this paper, we explore what happens to planets around one star in a binary system as that star evolves off of the main sequence and lose mass. Many planets are observed in stellar binary systems, and their frequency may be comparable to that of planetary systems around single stars. Mass loss can trigger dynamical instabilities that drive planets into chaotic orbits. This instability leads to planet-star collisions, exchange of the planet between the binary stars “star-hoppers”, and ejection of the planet from the system.Because planets often suffer close encounters with the primary on the Asymptotic Giant Branch, captures during collisions with the stellar envelope are also possible. Such capture could populate the habitable zone around white dwarfs.

The movie above shows planets in the rotating reference frame of the binary, and their zero-velocity curves. As the primary (blue) loses mass, the orbits expand. When the zero velocity curves open, transfer is allowed between the two stars, and ultimately their orbits become chaotic and the planets begin bouncing back and forth. As mass loss continues, and the curves pinch off, planets can get trapped around the secondary star.

Triple Evolution Destabilization Instability: Perets & Kratter 2012

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Physical collisions and close approaches between stars play an important role in the formation of exotic stellar systems. Standard theories suggest that collisions are are, occurring only via random encounters between stars in dense clusters. We present a different formation pathway, the triple evolution dynamical instability (TEDI), in which mass loss in an evolving triple star system causes orbital instability. The subsequent chaotic orbital evolution of the stars triggers close encounters, collisions, exchanges between the stellar components, and the dynamical formation of eccentric compact binaries like Sirius. We find that the dominant type of stellar collisions is qualitatively different; most collisions involve asymptotic giant branch stars, rather than main sequence, or slightly evolved stars, which dominate collisions in globular clusters.

In the movie above, we show an example of the chaotic orbital evolution which follows mass loss. Here the blue star has recently finished losing mass to become a white dwarf star.