1) The formation of planetary systems orbiting other stars.
2) The forces driving Earth's biodiversity over geological history.
1. The origin of planetary systems around other stars
Spin-orbit misalignments
Planets encircling stars much heavier than the Sun often possess orbits that are greatly inclined with respect to the stellar spin axis, including many retrograde systems. In general, lighter stars show reduced misalignments. My work has demonstrated that spin-orbit misalignments can arise when stellar companions gravitationally torque protoplanetary disks out of alignment with their host stars, with the planets inheriting the disk's orientation. Lower mass stars possess strong dipole magnetic fields that are capable of undoing misalignments. The more massive star's fields are too weak to do so, reproducing the observed mass-misalignment trend.
In our Solar System, the Sun spins about an axis which is misaligned with the planetary orbital planes by only 7 degrees. Such close alignment between star and planetary oribts comes as a direct consequence of one of the longest-standing correct hypotheses about planet formation - that planetary systems and stars collapse out of giant cloud of gas and dust. I should say, however, that even 7 degrees requires explaining. As it turns out, the gravitational torques induced by a distant, massive planet - the famed Planet 9 - may be capable of tilting the solar system by the required 7 degrees.
within the last 10 years, it became possible to measure the spin-orbit misalignment in many other planetary systems. Remarkably, the 7 degree misalignment of our Solar System is overshadowed by systems with misalignments closer to 90 degrees. Indeed, extra-solar planets (Exoplanets) sometimes appear to defy reason by orbiting in the opposite direction from their star's spin - which I refer to as "upside-down" solar systems. My goal has been to understand where such misalignments come from and when they arose.
Telescopes can now literally see disks and the clouds from which they collapsed, suggesting that spin-orbit misalignments do not conflict with the picture of stars and planets sharing a common cloud source. Therefore some mechanism either turns the planet's orbit over or flips the star on its head. I explore the idea that disks themselves may become misaligned with their host stars. To see how, note that, more often than not, any given star-disk system will exist in the vicinity of another star - a companion. The gravity of this companion star forces the protoplanetary disk of the first star to precess, just like the Earth's gravity causes a spinning top to precess on a table. A chain reaction ensues - the precessing disk tugs on the star around which it orbits, causing the host star to sway too and fro. At some point, the disk-hosting star is swung back and forth at just the right frequency, a resonant frequency, to become flipped on its side, or even upside down. In this way, the entire observed range of misalignments can be reproduced.
...but the Sun is aligned
Importantly, gross misalignments are only common within planetary systems where the host star is over about 1.2 times heavier than the Sun. I have suggested a link between this trend and the magnetic field strengths of low-mass young stars, whose dipole fields appear to be stronger than their more massive cousins. My work shows that stronger fields are sufficient to magnetically shackle lower-mass stars to their disk's planes for the duration of the planet formation process, thereby cancelling out the misalignments described above.
Close-in giant planets - the "hot Jupiters"
Above I talked about systems that I call "upside-down". However, the weirdness of exoplanetary systems does not stop there (indeed, it probably won't stop anywhere in the near future). Take a look at our solar system and you'll see that the inner regions are remarkably empty. Inside of Maercury's orbit, there are roughly 30 million miles of nothing, until you hit the Sun. In constrast, about half of solar systems around other stars possess planets well interior to this region, with their masses ranging all the way from Mercury up to beyond Jupiter. The largest of these are known as "hot Jupiters," because they are blisteringly close to their host stars, yet as large as Jupiter.
The first exoplanets discovered around Sun-like stars were of the hot Jupiter ilk, largely because they are so huge and close that they tug the star itself around enough that we easily see a "wobble" in the star's light. Despite knowing they exist for about 25 years now, we still do not know exactly how they formed.
The problem with hot Jupiters lies in their size. In order to form a giant planet such as Jupiter, you first need to stick together a solid core of rock and ice in order for its gravity to be strong enough to suck up the gas in its vicinity within its gaseous formation environment. In the frigidly cold outer reaches of planetary systems, there will be lots of ice around to stick rocks together and form this huge core. However, at 1/20 the Earth-Sun distance, where hot Jupiters live, water is evaporated and you are simply left with too few solid to build the core - the foundations of a giant planet cannot be constructed at the location we see them to exist. The solution? They must have formed further out and somehow migrated inwards.
Generally, two migration pathways can explain the hot Jupiters. One happens early-on, when the planet-forming disk is still around. I mention it again below, but essentially the giant planet interacts with its natal disk in such a ways as to lose orbital energy and shrink its orbit closer to the star. The second pathway occurs later and relies on some violent interaction to throw the planet inward close to the star, with tides evening out the orbit into a nice, close-in circle. The violent pathway predicts spin-orbit misalignments mentioned above, but I have shown that they may also arise from migration throw a tilted disk. However, one addtional feature signals that hot Jupiters might have formed from a violent history - they very rarely occur alongside other planets.
As with spin-orbit misalignments, I decided to revisit the idea that "loneliness" of giant planets can only come about through violent migration casting out companion planets. Essentially, I found that if a small planet were to have formed outside the hot Jupiter during the disk-hosting stage, interactions between the two planets and the host star would have tilted the outer companion or removed it altogether. Thus, in most cases a system that forms a hot Jupiter will appear alone when we search for it because the best measurements are obtained for line-of-sight planetary orbits, so the tilted companions would be invisible most of the time.
Protoplanetary Disk Processes
All planets are thought to form within a protoplanetary disk - a flattened blanket of gas and dust gravitationally bound to encircle a star for up to ten million years. Through processes that remain somewhat mysterious, the dust and gas collect into gargantuan bodies like Jupiter and Saturn, or even larger. The gravitational pull of these giant planets is so great that Jupiter is sometimes speculated to shield the Earth from otherwise devastaing collision events heralding from the outer solar system. Therefore, it is not surprising that these planets were thought to interact strongly with the disk that formed them.
In a remarkable example of the counter-intuitive nature of disks, a massive body in a disk tends to repel the material around it. Indeed, Jupiter is thought to be sufficiently massive to have cleared a gap in the gas around it. Turbulence in the disk causes material to flow inwards (accrete) towards the central star. In the process the gap-clearing planet is swept inward with it. This process is known as Type II migration. This tendency for planets to move about in disks makes it particularly difficult to infer where any given planet formed.
The Kepler Dichotomy
Not only are many planetary orbits misaligned with respect to their host stasr, but about half of the time, orbits are misaligned with respect to each other. The other half are pretty flat, just like our solar system. Dubbed the Kepler Dichotomy - it was revealed using data from the Kepler spacecraft - the origin of these separate populations has remained elusive. In my work, I investigated the possibility that the misaligned central star might play a role. To that end, I showed that when the central star is misaligned with respect to a coplanar, multi-planet system, the stellar torques are capable of yanking the orbits out of alignment with each other - but only if the star is sufficiently misaligned. In light of this idea, the half of systems who are flat are expected to orbit well-aligned stars. As discussed above, well aligned systems tend to orbit smaller, cooler stars. The figure to the left is a test of such a prediction - there does seem to be a larger occurence of multi-planet systems (which is equivalent to being flat when detected via small transits of the stellar surface) around cooler stars.
The Dynamical Loss of Exomoons
There are many potential consequences of disk-driven migration for planetary systems. I am considering what happens to a moon that is in orbit around a migrating planet. In particular, the inward motion of the planet causes the moon's orbit to become increasingly subject to gravitational influences from the central star. When the stellar influence becomes sufficiently strong, the lunar orbit is stretched (made eccentric) enough such that the moon is cast inexorably into the planet and destroyed. The process by which lunar eccentricity rises is known as the "evection resonance."
2. The forces driving Earth's biodiversity
Extinction risks from ocean acidification
Roughly 251 million years ago, 2 million square km of Siberia turned into a searing flood of molten lava. Carbon dioxide rushed into the air and oceans, causing ocean acidification alongside global warming and ocean anoxia. Thus began the end-Permian mass extinction. Organisms relying upon calcium carbonate shells to survive suffered greatly, but some more than others. My work has shown that the differential susceptibility within such organisms is most likely linked to inter-species differences in the lifestyles of larvae and juveniles. Specifically, I show that, from a chemical point of view, acidification can only increase the relative costs of shells by about ten percent. This fraction is expected to be much more detrimental during younger, more energy-constrained life-stages than in adults. As humans continue to acidify the oceans, the species that need the most protection are likely to be those with the greatest time and energy-constraints whilst larvae.
The most severe mass extinction event of the past half a billion years occurred about 251 million years ago, at the end of what is known as the Permian Period. The cause is not entirely clear, but a lot of evidence points to extensive ocean acidification as a dominant driver of the disaster. For example, there was a clear prevalence for extinctions within groups of marine organisms that produce calcium carbonate skeletons as a part of their life cycle. Furthermore, sedentary organisms, who are not used to dealing with high levels of metabolically-derived carbon dioxide, appeared to suffer, suggesting high CO2 levels were present during the event. Other evidence includes the eruption of a "flood basalt" in Siberian, whereby millions of square kilometres burst into volcanic activity,injecting huge amounts of carbon into the atmosphere. Other factors were certainly at play, but the oceans at this time were surely under an acidic seige.
In modern times, it appears history is repeating - carbon dioxide levels are going up and the oceans are absorbing it. CO2 molecules make the ocean more acidic by stripping hydrogen ions off water molecules. It is these hydrogen ions that can attack the calcium carbonate shells of marine life. The end-Permian event demonstrated that some groups within the calcium carbonate producers were hit harder than others. Likewise, it becomes imperative to determine which groups will need the most assistance during the modern acidification event.
It may seem obvious that calcium carbonate-using creatures will harmed by the sudden cascade of hydrogen ions, stripping bits off their shells - but not necessarily. This is because most of the oceans are fully saturated in calcium carbonate - no more can dissolve - even after a century of acidification. Of course, the restricted regions that are under-saturated are growing, and this is a big problem. However, it is unclear how and why calcifiers might suffer when the oceans are acidified yet remain fully-saturated in calcium carbonate. The objective of my work has been to quantify, from a chemical point of view, just how much energy a shell costs to these organisms and how this cost changes as a result of ocean acidification.
My research suggests that there is a fundamental limit to just how high acidification can elevate shell-making costs. When CO2 dissolves in seawater, it breaks apart into bicarbonate and carbonate ions. Organisms make their shells by grabbing seawater and using specialised 'proton pumps' to convert bicarbonate ions into carbonate ions, which then forms their shells. Therefore, the amount of "heavy lifting" that must be done by their physiology is closely coupled to the concentration of bicarbonate in the water relative to carbonate. However, acidifcation or not, bicarbonate is already by far the dominant inorganic carbon component in seawater, comprising about 80% of dissolved inorganic (meaning, not things like sugars) carbon species. This fraction cannot go above 1 and indeed is unlikely to even get that high, so the acidifcation hammer can only strike with a force of 10 percent or so of the organism's current requirements.
Whether or not 10 percent is a really bad effect depends on what organism you are looking at, or more crucially, what life stage you look at. For many shellfish, the larval stages spend their earliest days (sometimes just 1 day) floating around in the plankton. Afterward, they must settle somewhere and, wihtout eating, spontaneously secrete a shell which can weigh more than themselves in a very short amount of time - hours to days. A ten percent hammer sounds like bad news to these early life stages. Cumulatively, we suggest that the most important factors determining a species' susceptibility to acidifcation are best defined with respect to their larval stages.
Population dynamics in fluctuating environments
Populations of organisms do not persist forever. Understanding how long it takes for populations to arise and go extinct has been a long-standing problem, and takes a more urgent position in the modern world where human influence is reducing habitats and polluting the global biosphere (with ocean acidification above being one prime example). I am explored the influence on random environmental fluctuations upon population persistence times, otherwise known as extinction times, in order to better undertand the expected times to extinction. My primary finding thus far is that the timescale of environmental fluctuations is key. Rare events are typically very harmful, but happen so rarely that something else will cause extinction before they hit. On the other end of the spectrum, very common events typically don't do a lot of damage and so don't cause extinctions too frequently. Somewhere in the middle, however, is what I call "the most catastrophic catastrophe". Ie, an event that comes just rarely enough to likely cause extinction, but frequntly enough to cause extinctions in a short amount of time. This finding has consequences for evolutionary rates, and might help explain patterns in the fossil record whereby the famous "mass extinctions" have not been responsible for most of the extinctions throughout Earth's history (Raup 1991) - the less severe, more frequent events are to blame.