University of Toronto, Centre for Planetary Sciences
I am an astronomer and planetary scientist. Underlying most of my work is a deep interest in how planets, and the systems they reside in, form and evolve. Within that central theme my work encompasses quite a broad range of investigations. The breadth of this topic also means that it touches on many different fields of expertise, and so I work closely with other researchers. I typically approach questions from a theoretical perspective, whilst always endeavouring to link back to observational or experimental data.
I am a CPS fellow at the Centre for Planetary Sciences of the University of Toronto.
University of Toronto, Centre for Planetary Sciences
Arizona State University, School of Earth and Space Exploration
Ph.D. in Astronomy
University of Cambridge, Institute of Astronomy
Master of Physics
University of Oxford
Image credit: Gemini observatory/AURA, artist's impression of the Moon-forming giant impact
My research interests cover quite a wide range of topics, but the primary theme that unites them is a desire to understand the formation and evolution of planets and the systems in which they reside. I typically approach questions from a theoretical perspective, whilst always endeavouring to link back to observational or experimental data. Recently I have also been becoming more directly involved with observations and experiments.
A major topic that has been part of many aspects of my research is giant impacts, and in particular the debris that they produce. Giant impacts occur in the chaotic final stages of terrestrial planet formation when massive planetary bodies collide with one other to form the final terrestrial planets. These are some of the most violent events to occur during the planet formation process and can strongly influence the final makeup of terrestrial planets, both in terms of their mass and in terms of the proportions of elements and minerals of which they are composed. As a result of the violence of these giant impacts, in addition to producing the final terrestrial planets large quantities of small debris is also released.
The debris released will go into orbit around the host star and will, as I have shown in my work, be much more visible than the parent planetary bodies owing to its much larger area, in much the same way that if you blow even a small amount of flour or chalk dust into the air it can make it hard to see through. This debris can potentially remain visible for tens of millions of years after the originating giant impact and observing it would give us key insights into ongoing planet formation processes.
At the same time the debris released by a giant impact resides in a system of planetary bodies and will go on to collide with them after its initial release. This is an important mechanism to remove the detectable debris signature, and means that in addition to producing bright, dusty debris visible to astronomical observations, it will also result in a large spike the rate of smaller collisions onto the planetary bodies, especially those involved in the original giant impact.
Asteroids and other small bodies in our solar system are relics of this early, violent, period of time and may also record evidence of the processes that led to the formation of the planets. In addition asteroids are now being considered as targets for deep space mining operations, and for both these purposes it is important to understand their internal structure and how this will have evolved over time.
Many of these topics are represented in the projects listed on the projects page, in addition major ambitions for the future include improving our knowledge of the size distribution of debris produced in impacts as this is a significant issue for all debris studies, and reducing our reliance on computationally intensive N-body simulations.
In the past I have also studied the evaporation of planetary atmospheres under the influence of high-energy radiation, and maintain an interest in the physics of planetary atmospheres and their evolution.
While at ASU I was fortunate to meet and work with Viranga Perera and Travis Gabriel, who continue to work with me on several projects after my move to the University of Toronto
Viranga Perera worked with me examining granular mechanics on asteroids, specifically the so-called 'Brazil nut effect' in which larger particles rise to the surface of a mixture as it is shaken, to examine the influence this could have on the internal structure of asteroids. He is also now working with me on the Stop Hitting Yourself project, focussing on the influence re-impacting debris might have on the cooling of the Moon and formation of the lunar crust.
Travis Gabriel works with me on both the Aftermath of Giant Impacts and Stop Hitting Yourself projects. For the Aftermath of Giant impacts he is currently analysing the change in core-mantle fraction during giant impacts, while for the Stop Hitting Yourself project he is assisting with the investigation of debris dynamics.
I encourage you to visit their own websites to learn more, and about their other projects!
Image credit: Me, summit of Mauna Kea looking towards the Keck and Subaru telescopes from the Gemini North telescope.
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.
Discs of dusty debris around main-sequence stars indicate fragmentation of orbiting planetesimals, and for a few A-type stars, a gas component is also seen that may come from collisionally-released volatiles. Here we find the sixth example of a CO-hosting disc, around the ∼30 Myr-old A0-star HD 32997. Two more of these CO-hosting stars, HD 21997 and 49 Cet, have also been imaged in dust with SCUBA-2 within the SONS project. A census of 27 A-type debris hosts within 125 pc now shows 7/16 detections of carbon-bearing gas within the 5-50 Myr epoch, with no detections in 11 older systems. Such a prolonged period of high fragmentation rates corresponds quite well to the epoch when most of the Earth was assembled from planetesimal collisions. Recent models propose that collisional products can be spatially asymmetric if they originate at one location in the disc, with CO particularly exhibiting this behaviour as it can photodissociate in less than an orbital period. Of the six CO-hosting systems, only β Pic is in clear support of this hypothesis. However, radiative transfer modelling with the ProDiMo code shows that the CO is also hard to explain in a proto-planetary disc context.
Many asteroids are likely rubble-piles that are a collection of smaller objects held together by gravity and possibly cohesion. These asteroids are seismically shaken by impacts, which leads to excitation of their constituent particles. As a result it has been suggested that their surfaces and sub-surface interiors may be governed by a size sorting mechanism known as the Brazil Nut Effect. We study the behavior of a model asteroid that is a spherical, self-gravitating aggregate with a binary size-distribution of particles under the action of applied seismic shaking. We find that above a seismic threshold, larger particles rise to the surface when friction is present, in agreement with previous studies that focussed on cylindrical and rectangular box configurations. Unlike previous works we also find that size sorting takes place even with zero friction, though the presence of friction does aid the sorting process above the seismic threshold. Additionally we find that while strong size sorting can take place near the surface, the innermost regions remain unsorted under even the most vigorous shaking.
Giant impacts refer to collisions between two objects each of which is massive enough to be considered at least a planetary embryo. The putative collision suffered by the proto-Earth that created the Moon is a prime example, though most Solar System bodies bear signatures of such collisions. Current planet formation models predict that an epoch of giant impacts may be inevitable, and observations of debris around other stars are providing mounting evidence that giant impacts feature in the evolution of many planetary systems. This chapter reviews giant impacts, focussing on what we can learn about planet formation by studying debris around other stars. Giant impact debris evolves through mutual collisions and dynamical interactions with planets. General aspects of this evolution are outlined, noting the importance of the collision-point geometry. The detectability of the debris is discussed using the example of the Moon-forming impact. Such debris could be detectable around another star up to 10 Myr post-impact, but model uncertainties could reduce detectability to a few 100 yr window. Nevertheless the 3% of young stars with debris at levels expected during terrestrial planet formation provide valuable constraints on formation models; implications for super-Earth formation are also discussed. Variability recently observed in some bright disks promises to illuminate the evolution during the earliest phases when vapour condensates may be optically thick and acutely affected by the collision-point geometry. The outer reaches of planetary systems may also exhibit signatures of giant impacts, such as the clumpy debris structures seen around some stars.
Giant impacts between planetary scale bodies release large quantities of debris into their host systems. This debris, especially vapour condensates, may be extremely bright and optically thick. The variation in the shape of the dust cloud as it orbits the star, and undergoes Keplerian shear, can lead to large variations in the optical thickness, and consequent large variations in the observed flux, producing complex light-curves. By studying the light-curves of these extreme debris disks we can gain a powerful probe into the properties of the forming planets in the system.
In addition to building planets giant impacts also release large quantities of debris. The ultimate fate of this is largely re-accretion, and this debris population could be the dominant source of impactors in the early solar system.
Here we investigate a novel Giant Impact Debris (GID) hypothesis to explain a number of observations regarding the LHB. In the GID hypothesis, the formation of the crustal dichotomy on Mars (Borealis Basin) generates LHB impactors.
The giant impact that formed the Moon ejected several percent of an Earth mass out of cis-lunar space in the form of small debris. Using collisional and dynamical models, we show its return can reproduce the Moon’s pre-Nectarian impact record.
The Oort cloud is usually thought of as a collection of icy comets inhabiting the outer reaches of the Solar system, but this picture is incomplete. We use simulations of the formation of the Oort cloud to show that ~4 per cent of the small bodies in the Oort cloud should have formed within 2.5 au of the Sun, and hence be ice-free rock-iron bodies. If we assume that these Oort cloud asteroids have the same size distribution as their cometary counterparts, the Large Synoptic Survey Telescope should find roughly a dozen Oort cloud asteroids during 10 years of operations. Measurement of the asteroid fraction within the Oort cloud can serve as an excellent test of the Solar system's formation and dynamical history. Oort cloud asteroids could be of particular concern as impact hazards as their high mass density, high impact velocity, and low visibility make them both hard to detect and hard to divert or destroy. However, they should be a rare class of object, and we estimate globally catastrophic collisions should only occur about once per billion years.
Many stars are surrounded by disks of dusty debris formed in the collisions of asteroids, comets, and dwarf planets, but is gas also released in such events? Observations at submillimeter wavelengths of the archetypal debris disk around β Pictoris show that 0.3% of a Moon mass of carbon monoxide orbits in its debris belt. The gas distribution is highly asymmetric, with 30% found in a single clump 85 astronomical units from the star, in a plane closely aligned with the orbit of the inner planet, β Pictoris b. This gas clump delineates a region of enhanced collisions, either from a mean motion resonance with an unseen giant planet or from the remnants of a collision of Mars-mass planets.
We consider the observational signatures of giant impacts between planetary embryos. While the debris released in the impact remains in a clump for only a single orbit, there is a much longer lasting asymmetry caused by the fact that all debris must pass through the collision-point. The resulting asymmetry is stationary, it does not orbit the star. The debris is concentrated in a clump at the collision-point, with a more diffuse structure on the opposite side. The asymmetry lasts for typically around 1000 orbital periods of the progenitor, which can be several Myr at distances of ~50 au. We describe how the appearance of the asymmetric disc depends on the mass and eccentricity of the progenitor, as well as viewing orientation. The wavelength of observation, which determines the grain sizes probed, is also important. Notably, the increased collision rate of the debris at the collision-point makes this the dominant production site for any secondary dust and gas created. For dust small enough to be removed by radiation pressure, and gas with a short lifetime, this causes their distribution to resemble a jet emanating from the (stationary) collision-point. We suggest that the asymmetries seen at large separations in some debris discs, like Beta Pictoris, could be the result of giant impacts. If so, this would indicate that planetary embryos are present and continuing to grow at several tens of au at ages of up to tens of Myr.
Large impacts in the outer parts of a planetary system will produce debris discs that display a strong, distinctive, asymmetry, which will last for 105 year timescales. Debris resulting from a large impact may be able to explain the asymmetries in some known debris discs that have hitherto been difficult to understand.
We study the evolution of debris created in the giant impacts expected during the final stages of terrestrial planet formation. The starting point is the debris created in a simulation of the Moon-forming impact. The dynamical evolution is followed for 10 Myr including the effects of Earth, Venus, Mars and Jupiter. The spatial distribution evolves from a clump in the first few months to an asymmetric ring for the first 10 kyr and finally becoming an axisymmetric ring by about 1 Myr after the impact. By 10 Myr after the impact 20 per cent of the particles have been accreted on to Earth and 17 per cent on to Venus, with 8 per cent ejected by Jupiter and other bodies playing minor roles. However, the fate of the debris also depends strongly on how fast it is collisionally depleted, which depends on the poorly constrained size distribution of the impact debris. Assuming that the debris is made up of 30 per cent by mass mm-cm-sized vapour condensates and 70 per cent boulders up to 500 km, we find that the condensates deplete rapidly on ~1000 yr time-scales, whereas the boulders deplete predominantly dynamically. By considering the luminosity of dust produced in collisions within the boulder-debris distribution we find that the Moon-forming impact would have been readily detectable around other stars in Spitzer 24 μm surveys for around 25 Myr after the impact, with levels of emission comparable to many known hot dust systems. The vapour condensates meanwhile produce a short-lived, optically thick, spike of emission. We use these surveys to make an estimate of the fraction of stars that form terrestrial planets, FTPF. Since current terrestrial planet formation models invoke multiple giant impacts, the low fraction of 10-100 Myr stars found to have warm (>~150 K) dust implies that FTPF≲10 per cent. For this number to be higher, it would require that either terrestrial planets are largely fully formed when the protoplanetary disc disperses, or that impact generated debris consists purely of sub-km objects such that its signature is short-lived.
We consider the evaporation of close-in planets by the star's intrinsic extreme-ultraviolet (EUV) and X-ray radiation. We calculate evaporation rates by solving the hydrodynamical problem for planetary evaporation including heating from both X-ray and EUV radiation. We show that most close-in planets (a < 0.1 au) are evaporating hydrodynamically, with the evaporation occurring in two distinct regimes: X-ray driven, in which the X-ray heated flow contains a sonic point, and EUV driven, in which the X-ray region is entirely sub-sonic. The mass-loss rates scale as LX/a2 for X-ray driven evaporation, and as Φ*1/2/a for EUV driven evaporation at early times, with mass-loss rates of the order of 1010-1014 g s-1. No exact scaling exists for the mass-loss rate with planet mass and planet radius; however, in general evaporation proceeds more rapidly for planets with lower densities and higher masses. Furthermore, we find that in general the transition from X-ray driven to EUV driven evaporation occurs at lower X-ray luminosities for planets closer to their parent stars and for planets with lower densities.
Coupling our evaporation models to the evolution of the high-energy radiation - which falls with time - we are able to follow the evolution of evaporating planets. We find that most planets start off evaporating in the X-ray driven regime, but switch to EUV driven once the X-ray luminosity falls below a critical value. The evolution models suggest that while 'hot Jupiters' are evaporating, they are not evaporating at a rate sufficient to remove the entire gaseous envelope on Gyr time-scales. However, we do find that close in Neptune mass planets are more susceptible to complete evaporation of their envelopes. Thus we conclude that planetary evaporation is more important for lower mass planets, particularly those in the 'hot Neptune'/'super Earth' regime.
We study the relationship between coronal X-ray emission and stellar age for late-type stars, and the variation of this relationship with spectral type. We select 717 stars from 13 open clusters and find that the ratio of X-ray to bolometric luminosity during the saturated phase of coronal emission decreases from 10-3.1 for late K dwarfs to 10-4.3 for early-F-type stars [across the range 0.29 ≤ (B-V)0 < 1.41]. Our determined saturation time-scales vary between 107.8 and 108.3 yr, though with no clear trend across the whole FGK range. We apply our X-ray emission-age relations to the investigation of the evaporation history of 121 known transiting exoplanets using a simple energy-limited model of evaporation and taking into consideration Roche lobe effects and different heating/evaporation efficiencies. We confirm that a linear cut-off of the planet distribution in the M2/R3 versus a-2 plane is an expected result of population modification by evaporation and show that the known transiting exoplanets display such a cut-off. We find that for an evaporation efficiency of 25 per cent we expect around one in ten of the known transiting exoplanets to have lost ≥5 per cent of their mass since formation. In addition we provide estimates of the minimum formation mass for which a planet could be expected to survive for 4 Gyr for a range of stellar and planetary parameters. We emphasize the importance of the earliest periods of a planet's life for its evaporation history with 75 per cent expected to occur within the first Gyr. This raises the possibility of using evaporation histories to distinguish between different migration mechanisms. For planets with spin-orbit angles available from measurements of the Rossiter-McLaughlin effect, no difference is found between the distributions of planets with misaligned orbits and those with aligned orbits. This suggests that dynamical effects accounting for misalignment occur early in the life of the planetary system, although additional data are required to test this.
Image credit: NASA/ESA, the debris disk around Fomalhaut as seen in scattered light by the STIS instrument on the Hubble Space Telescope
Terrestrial planet formation is a very active field, and the formation of Earth and other planets has undergone a revolution in terms of solar system theories and extrasolar planetary exploration. In this course students learnt how we think the planets formed as dynamical, physical, and thermodynamical systems, with an emphasis on the physical process of accretion and its clues in geochemistry, planetary structure and orbital architecture. The course was quantitative and included regular problem sets and reading assignments, culminating in a short research paper or literature review and a final written exam.
My role as a supervisor was to go through the solutions to the weekly problem sets with the students in groups of 2-3.
Fluids are ubiquitous in the Universe on all scales. As well as obvious fluids (e.g. the gas that is in stars or clouds in the interstellar medium) a variety of other systems are amenable to a fluid dynamical description - including the dust that makes up the rings of Saturn and even the orbits of stars in the galactic potential. Although some of the techniques of conventional (terrestrial) fluid dynamics are relevant to astrophysical fluids, there are some important differences: astronomical objects are often self-gravitating or else may be accelerated by powerful gravitational fields to highly supersonic velocities. In the latter case, the flows are highly compressible and strong shock fronts are often observed (for example, the spiral shocks that are so prominent in the gas of galaxies like the Milky Way).
In this course, we consider a wide range of topical issues in astronomy, such as the propagation of supernova shock waves through the interstellar medium, the internal structure of stars and the variety of instabilities that affect interstellar/intergalactic gas. These include, perhaps most importantly, the Jeans instability whose action is responsible for the formation of every star and galaxy in the Universe. We also deal with exotic astronomical environments, such as the orbiting discs of gas which feed black holes.
Image credit: Me, Merton College, Oxford - rear of Fellows quad
It has now been widely recognised for some time that giant impacts are far from being simple perfect mergers and display a diverse range of outcomes (e.g. Agnor & Asphaug, 2004; Asphaug, 2010; Leinhardt & Stewart, 2012). Incorporating the effect of these non-perfect mergers into full N- body simulations of planet formation remains extremely challenging however, such that many studies still use the perfect merger model (e.g. Raymond et al., 2009), and the small number of studies that have attempted to investigate the influence of non-perfect mergers all have significant limitations.
Our group at ASU has a database of over 1500 smoothed-particle hydrodynamic (SPH) simulations that represents an excellent resource to help tackle this problem. This database will allow us to construct parameter space maps of the behaviour and properties of giant impact products similar to those of Leinhardt & Stewart 2012, Stewart & Leinhardt 2012 but with greater parameter resolution, and especially improving our knowledge of the smaller debris that has been less well studied in the past and is subject to scale-dependent effects. In addition we will provide constraints on the initial orbits of the impactors and probability distributions for the post-impact trajectories of the impact products. These two components combined will then allow us to construct a statistical model of the giant impact phase which will be enormously beneficial in allowing us to gain insights into the planet formation process without recourse to computationally expensive N-body simulations.
This project is being conducted in collaboration with Erik Asphaug and Travis Gabriel at SESE.
Image credit: NASA, artist's impression of the giant impact that formed the Pluto-Charon system
Large amounts of the debris released in a giant impact is eventually re-accreted, either by the progenitor body or by other planets in the system. The amount of debris generated during the course of terrestrial planet formation is significant, each individual impact releases several per cent of the mass of the colliding bodies as debris, and in total over the course of the formation of Earth-size planets debris equal to about 10-15 per cent of their final mass will have been generated (e.g. Stewart & Leinhardt 2012). As this debris will have been heavily processed within a large planetary body, as well as subjected to strong shocks during the collision, it is likely to differ chemically from material that has never been incorporated into a large body. Accretion of, and impacts with, this debris material thus might have detectable influences on the final bodies in the system, especially in the case of our own solar system. Indeed Bottke et al 2015 showed that impacts with debris from the Moon-forming impact left a detectable signature on the asteroid belt in Ar-Ar ages. I am especially interested in the possibility of debris from the final giant impacts in the solar system having left impact crater records on bodies in the inner solar system with old surfaces, such as the Moon, Mars and Mercury, as well as the asteroid belt. For the progenitor body of the debris in particular re-impacting debris would interact with any magma ocean present on the body and influence the cooling of the magma ocean and the evolution of the planetary surface.
I am the Principal Investigator for this project, which is a collaboration with Erik Asphaug and Linda Elkins-Tanton at SESE and David Minton at Purdue University, and is funded by NASA's Emerging Worlds program, grant number NNX16AI31G.
Image credit: Tim Wetherell, Australian National University, artist's impression of the Moon during the Late Heavy Bombardment
Giant impacts are violent events and the debris that is launched is subjected to strong shocks. The energy deposited in the debris is sufficient to vaporise substantial quantities of the material. The vapour cloud expands away from the planet and cools, eventually condensing to form droplets that are typically millimetres to centimetres in size (Melosh & Vickery 1991; Johnson & Melosh 2012, 2014). The total surface area of these vapour condensates is enormous, and can easily result in the debris cloud being optically thick. Optically thick debris is not something that is commonly considered for debris disks, and can lead to complex, variable behaviour. This variability can serve as a unique fingerprint encoding the properties of the progenitor body and its orbit and the high brightness of the vapour condensate phase may allow us to see collisions involving smaller bodies (perhaps only the size of Ceres) that would otherwise be inaccessible. The remarkable ID8 system (light-curve shown in the thumbnail) may be an example of a system in which we are seeing this kind of behaviour. This phenomenon also has important consequences for planet formation in the solar system and in exoplanetary systems and our ability to correctly recognise the debris byproducts of planet formation processes.
This project is a collaboration with Kate Su, George Rieke and Andras Gaspar at the University of Arizona.
Image credit: NASA/JPL-Caltech/University of Arizona, graphic of the light curve of the system ID8 in 2012-2013
Image credit: NASA/JPL-caltech/SETI institute, Galileo mosaic of Europa
I am always happy to talk to you if you have questions about my research or the field in general.
I divide my time between the Scarborough (UTSC) and St. George (downtown) campuses of the University of Toronto. Typically you can find me at UTSC Monday-Wednesday, and downtown Thursday-Friday. My UTSC office is SW504E in the Science Wing and my downtown office is 1202C in the McLennan laboratory tower.
If I am not in my office, just drop me an email.
Image credit: ASU/SESE, ISTB4