Remnants of planetary systems around white dwarfs
We know over 3000 extrasolar planets, with the overwhelming majority orbiting solar-like stars. What is the future of these planetary systems as their host stars evolve off the main sequence? And what will happen to our solar system once the Sun dies? As the planet host stars evolve into red giants, and eventually into white dwarfs, they lose substantial amounts of mass. This lost layers of mass can be seen for some time as planetary nebulae. Consequently, the orbits of the planets, and all minor bodies, asteroids, and comets, will widen, with many of them moving far beyond the maximum radius that the star will reach during the red giant phase. In our solar system, Mars, the asteroid belt, and all the giant planets will escape evaporation. Thus, what we expect to be left behind are white dwarfs, bearing the remnants of planetary systems.
While directly detecting planets around white dwarfs is beyond the reach of current telescopes and instruments, we have recently detected extrasolar asteroids around a number of white dwarfs. Just as we know about sun-grazing asteroids, every now and then an extrasolar asteroid will have its orbit perturbed by a larger body, venture too close to the white dwarf, and by tidally shredded and form a debris disk of dust and/or gas. Those dust disks are detected as infrared excess emission, the gas disks are identified through highly unusual emission lines of metals such as Ca or Fe in their optical spectra.
|Artist impression of the gaseous debris disk formed from the disruption of a rocky asteroid in the strong graviational field of a white dwarf. It is very likely that the solar system will undergo episodes like this once the Sun has evolved into a white dwarf in ~6 Gyr from now - studying systems like this allows a glimpse into the remote future of our own planetary system.|
More spectacularly, as the dust and gas falls onto the the white dwarf, it pollutes its otherwise pristine hydrogen (or in some case helium) atmosphere, and becomes spectroscopically measurable. In other words, through spectroscopy of asteroid-debris bearing white dwarfs, we can measure the chemical bulk composition of extrasolar rocky material, the building blocks of terrestrial planets like our Earth – an insight that can not be learned from “normal” extrasolar planets. "Metal-polluted" white dwarfs have been known for almost a century, their link to the remnants of rocky, terrestrial planetary systems has become clear only over the past few years. A really exciting recent discovery was the detection of transits from debris orbiting at the tidal disruption radius of the white dwarf WD1145+017, a star that has both infrared excess from dust and metals within its atmosphere. Our high-time resolution photometry obtained with ULTRASPEC shows that the debris cloud is rapidly evolving, providing a "live feed" into the disruption of an exo-asteroid.
Within your PhD, you can come involved in the rapid prgress in this new and exciting research area: currently, the number of evolved planetary systems containing white dwarfs contaminated by sufficient amounts of debris to allow mult-element abundance studies is limited to a few dozen. This is a simple consequence of the small size of white dwarfs (comparable to the size of the Earth), meaning that these stellar remnants are much, much fainter than stars on the main sequence - and hence hard to pick out against a sea of brighter, more distant background stars. Using the accurate astrometry of the Gaia mission, we have identified ~260.000 white dwarfs, an increase of an order of magnitude on the current sample size, and we will obtain spectroscopic follow-up observations using three large-area multi-object spectroscopic surveys: DESI, WEAVE, and SDSS-V. You will be one of a small number of astronomers to be part all three surveys, providing you instant access to the spectroscopy of the white dwarfs, develop algorithms to identify debris-accreting white dwarfs, and model their spectra to measure the bulk composition of the disrupted planetesimals. The goal is to assemble accurate composition measurements for ~1000 exo-plantesimals (comparable to the sample size of meteorite samples in the solar system) which will provide critically needed input into planet formation models.
Supervisor: Boris Gänsicke
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