I am a Senior Research Fellow (Forskare) at the Department of Astronomy and Theoretical Physics at Lund University, Sweden. My work centres on understanding the dynamics of extrasolar planets. My main collaborators here in the theoretical astrophysics group are Melvyn B. Davies, Anders Johansen, and Ross Church.
I completed my Ph.D. at the Institute of Astronomy, Cambridge, where I studied the interactions between planets and debris discs with Mark Wyatt. I then moved to the Universidad Autónoma de Madrid for my postdoctoral work on the effects of stellar evolution on planetary systems with Eva Villaver, and thence to Lund at the start of 2014, where I continue to study many aspects of exoplanet dynamics.
I use computer simulations and mathematical analysis to understand how planets' orbits change with time, and work out the implications for our knowledge of how planets form. My specific areas of interest are:
- How does stellar evolution (stars becoming large red giants and then dead white dwarfs) affect a planetary system?
- What sets planetary system multiplicity? Why are some planets single but others are gregarious with many companions?
- How do planets interact with extra-Solar comets and asteroids (the building blocks of, and debris from, planet formation)?
- How did planets, asteroids and comets form and attain their current orbits in our own Solar System?
Exoplanets and their companions
Why are some planets single? Why are some multi-planet systems stable and others not?
Stars do not remain unchanged forever, and go through great changes as they use up their nuclear fuel, age and die. After they leave the "Main Sequence", stars swell up to great sizes, first as a red giant branch star and then as an asymptotic giant branch star: during these stages, the Sun's radius will extend to Earth's orbit. The large radius and high luminosity of a giant star means that its envelope is very weakly bound, and it will lose most of its material to space to form a planetary nebula, while the core of the star remains as a white dwarf.
Study of white dwarfs provides us with a unique way to discover the elemental compositions of extrasolar planets, asteroids and comets: elements heavier than helium sink rapidly out of a white dwarf's atmosphere, and so spectroscopic detection of these elements implies recent or ongoing accretion of planetary, asteroidal or cometary material onto the white dwarf. The composition of the accreted body can then be directly inferred from the observed elemental abundances. Fully understanding how the observed abundances relate to the progenitor planets and asteroids—are we seeing the remains of planets or asteroids? did these bodies undergo some processing during the late stages of stellar evolution? where are the source populations located, and what kinds of planetary system can deliver bodies to the white dwarf?—requires fully understanding the effects of stellar evolution on planets and asteroids.
These effects can be significant. I have studied the engulfment of planets by giant stars, as a result of the large stellar radius and the decay of the planet's orbit as it raises tides on the star (similar to tides in the Earth–Moon system). Loss of mass from the star strengthens gravitational interactions between planets and can change their orbital evolution, and I have conducted consistent computer simulations of systems of planets and asteroids through their whole lives to model this process and understand the origins of the asteroidal and planetary debris found in white dwarf atmospheres. I have also studied the history of the mysterious and controversial planets orbiting evolved binary stars.
The planets in our own Solar System are currently on stable, almost unchanging orbits. But there is evidence that in the past the Solar System experienced a strong dynamical instability that changed the orbits of the outer planets and the Kuiper Belt. Moreover, many exoplanets are on eccentric orbits, which may have arisen from orbital instabilities earlier in their history. What determines whether or not a planetary system is stable? If an instability happens, what does that mean for the number of planets in a system, or other properties such as orbital inclinations or spacings? When we look at exoplanets, either system-by-system or as a whole population, how much of what we see is sculpted by past orbital instabilities?
I have worked on the chaotic dynamics underpinning orbital stability, and on the effects of stellar evolution on the stability of orbiting planets. Most recently, I addressed the origins of hot Jupiters (giant planets very close to their stars). These typically are not accompanied by lower-mass super-Earths, despite the fact that super-Earths are found orbiting around half of all stars. I showed that the lack of super-Earth companions to hot Jupiters arises naturally if hot Jupiters move close to their host star by "high-eccentricity migration" following an orbital instability or other dynamical effect, thus addressing the debate on their origins that has raged since their discovery 20 years ago.
Many stars are surrounded by comets, asteroids, and the dust that is generated when they collide or sublimate: "debris discs". Around nearby stars, these discs can be imaged, which reveals a fascinating diversity of morphologies, including inner gaps and sharp edges, warps, and clumps. Many of these features can arise from interactions with sometimes unseen planets.
I have worked on several aspects of planet–debris disc interactions. I have shown how planets can excite the large collision velocities needed to generate the large quantities of dust observed. I have studied how planets capture smaller bodies into orbital configurations (mean motion resonances) that may lead to "blobby" structure in the discs, and how planets clear out holes in debris discs. All this helps to guide future observational searches for wide-orbit exoplanets in these disc systems.
The outer regions of the Solar System may host a mysterious Planet 9, at distances around 10 times greater than Pluto's. How might a planet attain such a wide orbit? I recently proposed that the planet might have been captured from another star when the Solar System was very young.
Stars are born in clusters, and evidence suggests that the Sun's cluster may have held 1000 ‒ 10000 stars. The densities of stars in such clusters is high, and stars will often pass each other at close distances (at a few hundred times the Earth–Sun separation). In these encounters, planets on wide orbits like Planet 9 can be exchanged between the stars. We outlined a formation history for Planet 9 in which it is first scattered onto a wide orbit around its original host star, before being captured by the Sun, and demonstrated its plausibility with computer simulations.
This research was covered in New Scientist and Forbes, among other news outlets. You can read a popular summary of the paper by my co-author Sean Raymond here. A preprint of the paper itself is on arXiv.