Pressing Challenges in Post-Main-Sequence Planetary Science

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MOTIVATIONWhat are rocky planets made of? How are planets born, and how will they die? The composition, formation and fate of planets is of intrinsic human interest, and touches upon some of the most fundamental questions in astrophysics. Our Sun is currently a middle-aged star and will eventually end its life in dramatic fashion. About 6 billion years from now, the Sun will expand and engulf the innermost two planets, Mercury and Venus, and disrupt if not destroy the Earth. During this violent phase, the swollen Sun will vaporise passing asteroids and comets, and leave rocky debris strewn throughout the Solar System amongst the surviving planets. The Sun will then end its life by contracting and cooling as a white dwarf (WD). A WD is small -- the size of the Earth -- but contains half of the mass of the Sun, and 100,000 times the gravity of the Earth.Millions of worlds orbit Sun-like stars that will become WDs. Observations of WDs show a striking feature that links these dying stars with their orbiting rocky remnants: polluted atmospheres. These WD atmospheres are being constantly force-fed a steady stream of rocky debris which reveal the compositions of the building blocks of exo-planets. Detailed observations of the feeding exist, but so far modeling has failed to explain its origin, amount and timescale. These fundamental shortcomings prevent us from painting a a self-consistent picture of planetary evolution.OBJECTIVEI will spearhead the theoretical modeling of old planetary systems, remedying these crucial shortcomings. I will link planetary and stellar formation with their fate. I will model current observations of WDs which host planetary remnants, and make specific predictions to motivate future observations. MY BACKGROUNDI see myself on a well-traced trajectory to consolidate my leadership role in the modelling/theoretical aspects of dying planetary systems, where there is a dearth of and need for research initiatives. As a PhD student, before the later discovery of exoplanets beyond 5 au, I demonstrated how planets formed in inner regions could migrate outwards through either disc-planet interactions or gravitational scattering. As a postdoctoral associate, I demonstrated how direct imaging campaigns can observe populations of these planets. This background instilled in me a desire to be at the forefront of pioneering new research, and motivated and empowered me to extend my interests to a new, growing area of astrophysics: the end state of planetary systems. METHODOLOGYModeling old planetary systems is challenging both numerically and analytically because both the planet and star undergo significant orbital and physical changes. I have already developed numerical codes that incorporate both types of variations self-consistently, and derived analytical formulae to model these changes. I will employ and expand upon this foundation to accomplish the tasks in this proposal.WHY THIS RESEARCH IS IMPORTANTAnalysis of polluted WD atmospheres represent the *only* substantial way to probe the bulk chemical composition of exo-planets. Understanding the fate of planetary systems is the missing piece in our knowledge of the full life cycle of these systems.IMPACTThe project results will transform our understanding about the life cycle of planetary systems, answering fundamental questions about the future of our world and other worlds. The proposed theoretical work is necessary to fully exploit the scientific investment into experimental facilities. By considering the end state rather than the formation of planetary systems, the project will bring a fresh perspective scientifically and for the lay person. Also, the topic of planetary death garners significant public interest, as demonstrated by media attention that the topic has received (http://www.bbc.co.uk/news/science-environment-24491845), providing pathways for outreach activities.