Research themes:

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Planetary system architectures

Stellar spin axis evolution

Giant planet formation

The origin of Mercury

Earth’s long-term habitability

Extinction dynamics

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How did the Solar system form? Do planetary systems around other stars follow similar rules? What was unusual about our own planet’s particular history? What factors drove the path of evolution here on Earth?

My research seeks to answer some versions of the questions above. This quest has taken me through several research topics, spanning many disciplines. However, the stories are all interwoven – physics drives biology, biology sculpts geology, and geology reflects astrophysics. It is all written in the language of Mathematics – my primary tool. I welcome comments, questions and ideas (christopher.spalding@princeton.edu).

Planetary system architectures

The closest planet to the Sun in our Solar system is Mercury, at about 4/10 the Earth distance (known as an Astronomical Unit, or AU). Whereas this proximity heats Mercury’s surface well beyond that of the Earth, it pales in comparison to the majority of planets known around other stars. These worlds often reside a mere 0.1 AU from their host stars, and are roasted to super-high temperatures (1000s of Kelvin). The existence of these fiery worlds conjures up 2 related questions:

1. How do extrasolar planets form on such hot orbits?
2. Why is our Solar system “inside-out”, in that it possesses almost no material closer than Mercury?

Mutual inclinations between planetary orbits: 

Not only are many planetary orbits misaligned with respect to their host stars, but oftentimes the orbits are misaligned with respect to each other. Initially dubbed the Kepler Dichotomy (it was revealed using data from the Kepler spacecraft) the mechanisms driving these misalignments has remained elusive. In my work, I investigated the possibility that a misaligned central star (as discussed above) might play a role. To that end, we 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 roughly half of systems who are flat, like the Solar system, are expected to orbit well-aligned stars. Indeed, we used this to constrain the misalignments of specific stars (like K2-38, less than about 30 degrees) where no other method is available.

The Solar wind’s role in the early Solar system

Above, I mentioned that our Solar system is devoid of material interior to Mercury’s orbit — why is that? One idea is that Jupiter underwent an inward then outward trek early in the Solar system’s history. This might have disrupted ready-made planets that existed there, but doesn’t explain why subsequent planet formation only extended as far in as Mercury. I have suggested that the Solar wind may have been to blame. The young Sun spun faster and had a stronger wind. These two combine to generate a strong tailwind, felt by objects leftover from Jupiter’s destructive trek, clearing out the inner Solar system before planets have time to form. We are continuing to explore under appreciated aspects for this potentially strong early wind for the Solar system’s evolution.

Stellar spin axis evolution

In our Solar system, the Sun spins once every month or so. You can think of it as a wheel spinning around an axle, known as the rotation axis. This axis points almost perpendicular to the plane of the planetary orbits. Such a well-aligned configuration bolsters the long-held viewpoint that stars form alongside their planetary systems as the end result of the collapse of giant cloud of spinning gas and dust. The center of the spinning cloud becomes the spinning stars, and the outer regions of the spinning cloud collapse into a disk and form planets, orbiting in the same direction as the central star.

Close-in planetary systems look different from ours, might they also differ in the relative tilt of the stellar spin axes and the planetary orbital planes? Within the last 10 years, it became possible to measure this “spin-orbit misalignment” in many other planetary systems. Remarkably, the well-aligned configuration of our Solar System is overshadowed by systems with misalignments ranging all the way from 0 degrees to 180 degrees. Accordingly, 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.

Through my past research, I have explored the possibility that these misalignments are the result of gravitational interactions of nearby stars to the planetary systems. In particular, young stars rarely form in isolation, often possessing a stellar companion bound to it through gravity. The gravity of this companion tugs at the planet-forming disk of material, tilting it relative to the central star. In this way, the spin-orbit misalignments seen in other planetary systems may record the past presence of a stellar companion, and the alignment of our Solar system may suggest that our Sun may have always been single.

Giant planet formation

The first planet discovered around another Sun-like star was the size of Jupiter, but about 20 times closer to the star than the Earth: Jupiter-sized and hot — it was of course classified as a “hot Jupiter“. Many more such objects have since come forward, but their formation history remains debated. It was long thought difficult to agglomerate so much material within such close proximity to the host star. Giant planets are though to form by first accumulating about 10 times the mass of Earth in rocks, which then suck up all the gas needed to become Jupiter-sized. However, at such a hot location, rocks evaporate, making the existence of hot Jupiters mysterious.

There remains no consensus on the formation of hot Jupiters. My work, though, has attempted to address the mystery by comparing hot Jupiters to their slightly cooler counterparts — the so-called warm Jupiters. Essentially the only physical difference between these two types of planets is their temperatures. However, whereas hot Jupiters seem to never appear alongside other planets within a given system, warm Jupiters possess neighbours about half the time.

This seems crazy — why would hot Jupiters and warm Jupiters be different if they are essentially just the same object but displaced by different amounts relative to their stars. We proposed a solution: the hot Jupiters are lonely because the proximity to the host star’s non-symmetrical gravity kicked out smaller companion planets. The mechanism here is similar to the effect described above — young stars rotate fast and become oblate. The gravitational field of an oblate star causes orbits to precess over time. The combined effect of the stellar field with the field of the hot Jupiter itself leads to the tilting of an exterior, smaller planetary orbits, making them invisible along the line of sight used to detect the giant planet. Thus, the very closeness of hot Jupiters to their host stars is what leads to their loneliness.

The origin of Mercury

Mercury is the smallest planet in our Solar system, but is one of the most mysterious. Its orbit is the most eccentric (at about 0.2), and it spins 3 times on its axis for every 2 orbits (i.e., it’s locked in a 3:2 spin-orbit resonance). The mystery that I have tried to tackle is why Mercury has such a large iron core.

Specifically, the radius of Mercury’s core takes up about 80% of the planet’s radius, in contrast to 50% for the Earth. Earth is about as dense as Mercury, but this is because Earth is much larger, compressing the rocks and core to greater densities. If both planets were “decompressed,” then Mercury would be much denser than Earth.

Why does Mercury have such a large core? Many ideas exist–but one favourite is that Mercury once looked a bit like Mars, with a large mantle of lighter chemicals (like silicates). Some time later on, a giant collision removed Mercury’s outer layers, leaving behind a dense ball of mostly iron. Whereas this idea is appealing, it suffers from the great law of nature that “what god sup, must come down.” I.e., the material launched from Mercury tends to fall back to its surface over millions of years.

We have shown that if the Sun’s wind was only about 10 times stronger in the early days of the Solar system (as discussed a little further up on this page), then the material blasted off Mercury can be cleared away before falling back to the surface, thus saving the giant impact theory. Much work is left to be done, however.

For a start, why is Mercury’s surface so low on oxygen? Is this tied to its impactor history? Does it disprove the giant impact theory? We’re working on it!

Earth’s long-term habitability

The search for alien worlds is often motivated by the hunt for “Earth 2.0” — a habitable world that might possess life like ours. However, the habitability of our own Earth 1.0 is still an unsolved mystery. Why? Well, if you look at 3.5 billion year old rocks, they tell a tale of oceans and life, just like today. But this evidence of early water faces a remarkable contradiction: stellar models predict that the Sun was much fainter during this early epoch. The Earth, and Mars, “ought” to have been frozen over — but they were not. This discrepancy was pointed out most famously by Carl Sagan, and referred to as the “Faint Young Sun Paradox“.

Most solutions to the Faint Young Sun paradox have proposed that the young Earth and Mars were pumped full of greenhouse gases, keeping both warm under the early Sun. Whereas this could work, geological proxies for the early atmospheric composition on either world are severely lacking. Given the critical importance of the early atmospheric composition to the evolution of Earth’s earliest life, it is important to fully consider the consequences of things we know to be different, and from that deduce how much greenhouse gas is needed.

My work has focused on tackling two aspects of the Faint Young Sun problem. I first looked into the effect of Earth’s rotation, which is known to have been faster. This traps more heat at the equator and makes it more difficult to entirely remove all of the ice form the planet. However, this effect alone cannot solve the paradox for Mars.

A separate way to solve the Faint Young Sun paradox is by proposing that the Sun was more massive 3.5 billion years ago — about 5% more massive. This is not a new idea, but as with greenhouse gases, it’s extremely difficult to measure the ancient mass of the Sun. Difficult, maybe, but not impossible. Our work has shown that the ancient Sun may be effectively “weighed” by measuring the time between bands of sediment in the ancient rock records of Earth and Mars. Though not likely to be accomplished in the near future, this avenue serves as an exciting potential method to validate, or rule out, the massive early Sun hypothesis.

Extinction dynamics

Nearly all species that have ever lived are now extinct (like the dinosaurs to the right)… It’s somewhat sobering. For all the startling variety of biological entities that grace the planet, the variety locked up in our planet’s remarkable past is so vast as to be beyond comprehension. Which forces guided the path of extinction, and similarly, the path of evolution?

My work has sought to identify new mechanisms governing extinction rates. Specifically, suppose a species is hit by a bad event at some random interval — say a hurricane once every few years. Eventually the species will be wiped out. How does the extinction time depend upon the frequency of those bad events? We showed that more frequency does not always equal worse, there is actually a worst frequency to get hit by bad events — a “most catastrophic catastrophe” — that minimises the time to extinction.

We are currently interested in extending this work to deduce whether the variability of the environment may more generally drive the rate of evolution.

Closer to home, how can we understand the ongoing pattern of extinctions globally within the context of the fossil record? This is not a trivial task. The fossil  record, as much as it has inspired generations of budding cowboy hat-donned fossil collectors, is rather incomplete. By some estimates, only about 10% of the species currently threatened with extinction would actually have had a good chance of being discovered as fossils in the first place. This means that when we look back at fossils, it is not necessarily true that we have known about the extinction of things like amphibians and reptiles that are threatened today. We are looking into ways to quantify this bias, and to understand the underlying structure of extinctions dynamics throughout our planet’s past, in order to hopefully learn about what to expect as we move into the coming century.

Macroevolution of Biomineralization

About 540 million years ago, the fossil record exploded with animal forms — the Cambrian explosion. The reason for this rapid enhancement in preserved organisms is the relatively sudden and widespread acquisition of hard skeletons and shells. Such hard parts are preserved better than squishier bits, enhancing their preservation potential in the fossil record. The manufacture of these shells is known as biomineralization.

As far as big animals go, these shells were often made of similar materials as those of  most shellfish and coral today: calcium carbonate. Calcium carbonate dissolves in acid, which is the reason why ocean acidification is such a major problem in today’s oceans. If human actions cause too acidic of an ocean, then marine life may suffer. Throughout life’s history, the relative success of biomineralizers has waxed and waned — seemingly with a complicated relationship to ocean chemistry. My research has sought to simplify this complexity; to extract some general relationships between the cost of making a shell and the type of ocean the Earth possesses during any given geological epoch.

Our work has suggested that the dependence of total shell cost upon the acidification state of the ocean may be relatively modest — shells may be about 10% more expensive in 2100 than today. This could be bad, but perhaps worse is the effect upon the maximum rate of shell-production possible in more acidic oceans. We showed that larvae of marine organisms may suffer the most — these little creatures are highly time-constrained in development. Their shells will cost more, but they also will end up with 10% less time in which to make them. Thus, we highlight larval development as key in driving any given species’ susceptibility to acidic oceans.

My ongoing work seeks to identify the costs associated with other forms of biomineralization, such as those organisms produce silica shells (e.g. diatoms). These organisms are almost always microscopic — why do small things make silica and big things use calcium carbonate carbonate?