Colloquium

The Final Fate of Planetary Systems

We know over 1500 extra-solar planets, and the past few years have seen a rapid expansion of the parameter space to wider ranges in host star and planet mass, as well as larger orbital separations. Consequently, the architectures of the known planetary systems are becoming increasingly diverse, and so does the entire field of exo-planets. The questions that the study of white dwarfs can address are: What is the future of the known exo-planetary systems as their host stars evolve of the main-sequence? And what will happen to the solar system once the Sun dies? How can we detect evolved planetary systems? And what can they teach us? The strong surface gravity of white dwarfs implies that metals will sink out of the photosphere on time-scales that are orders of magnitude shorter than their cooling ages, and therefore white dwarfs are expected to have either pure hydrogen or helium atmospheres. Yet, the existence of metal-polluted white dwarfs has been a conundrum for nearly a century. We know now that these white dwarfs are polluted by accretion of rocky debris, remnants of a former planetary system. With hindsight, this is may not come as too much of a surprise, as our Sun will eventually evolve in a white dwarf orbited by Mars, the outer planets, and hosts of asteroids - and a similar fate awaits many of the known exo-planetary systems! The photospheric abundances of these white dwarfs mirror those of the debris they accrete, hence we can directly, and accurately measure the bulk abundance of extra-solar planetary material. In zeroth approximation, the abundance pattern measured so far are overall similar to those of the terrestrial planets in the Solar system. At a closer look, there is evidence for a variety of thermal processing and possibly differentiation in the parent bodies, and even for the accretion of water. Perhaps most astonishing are the lower limits on the mass of the parent bodies that were accreted, ranging up to 1e24g, i.e. well above the most massive asteroids in the Solar system. These chemical abundance analyses are currently, and for some time to come, by far the most precise studies of extra-solar planetary material. These recent discoveries raise many new questions: what is the architecture of planetary systems on the post-main sequence? What type of planetary bodies are delivered to the white dwarf, and how? What is the long-term evolution of these systems? A suite of recent simulations gives a glimpse into some possible answers.

Followed by the slow evolution of our own host star off the main sequence.

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