Remnants of planetary systems around white dwarfs
We know over 4000 extrasolar planets, with the overwhelming majority orbiting solar-like stars. Thinking about the long-term evolution of these planets, some of the following questions come to mind: 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?
The future evolution of planetary systems: 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. In other words, in ~6Gyr, the solar system will consist of a white dwarf with one rocky planet, four giant planets, and countless asteroids and comets. Thus, what we expect to be left behind by the 1000s of known planetary systems are white dwarfs, bearing the remnants of planetary systems.
Extrasolar asteroids & the bulk composition of planets: The first detections of evolved planetary systems came not in the form of their planets, but asteroids. Just as we know about sun-grazing asteroids, ever 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 disc of dust and/or gas. Those dust discs are detected as infrared excess emission, the gas discs are identified through highly unusual emission lines of metals such as Ca or Fe in their optical spectra.
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.
The whole concept of shredded asteroids being accreted by white dwarfs was corroborated by 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. More recently, we have identified a solid planetesimal still orbiting within one of these debris discs, and propose that this is the dense (iron-rich) core of a planet that has been stripped of its crust and mantle to form the disc.
Artist impression of the gaseous debris disc formed from the disruption of a rocky asteroid in the strong gravitational 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.
(c) Mark Garlick and University of Warwick.
Planets orbiting white dwarfs: Whereas 100s of white dwarfs are known to still have asteroid belts, the detection of planets has proven more difficult. The reason is that the standard methods to identify planets are much, much harder in the case of white dwarfs. Measuring the Doppler wobble requires high-resolution spectroscopy, and most white dwarfs are too faint for current instruments (that will change with the E-ELT). Similarly, the detection of transits requires a line of sight very closely aligned with the orbital plane of the planet. Because the chances of that are relatively low, vast numbers of stars need to be monitored photometrically, and, again because of their faintness, that has remained a challenge (but see below).
In 2019, we discovered the first evidence for a giant planet in a close orbit around a white dwarf, that is being evaporated by the strong ultraviolet radiation from the white dwarf. This led us to think again more carefully about the future of the solar system, and we realised that the white dwarf left behind by the Sun will be be so luminous in the ultraviolet that it will drive detectable amounts of mass loss from Jupiter, Saturn, Uranus and Neptune. Detectable by some alien astronomer who, in 6Gyr, points their telescope at the solar white dwarf.
This animation shows the white dwarf WDJ0914+1914 and its Neptune-like exoplanet. Since the icy giant orbits the hot white dwarf at close range, the extreme ultraviolet radiation from the star strips away the planet’s atmosphere. While most of this stripped gas escapes, giving the planet a comet-like tail, some of it swirls into a disc, itself accreting onto the white dwarf.
(c) ESO/M. Kornmesser
Even more recently, the first transiting giant planet candidate has been found from the analysis of TESS observations of a white dwarf. This is extremely exciting, as this white dwarf is only ~25pc away, so such systems may not be so rare after all, and the Zwicky Transient Factory, and later the Rubin Observatory have the potential to find many more.
Be part of this exciting area of exo-planet research: Within your PhD, you will become involved in this new and rapidly growing research area, identifying more of these relatively rare white dwarfs with dust and gas discs, as well as transiting planets and planetesimals from large astronomical surveys. We are leading spectroscopic surveys of white dwarfs within DESI, WEAVE, and SDSS-V, all three projects will begin delivering data in 2021. We expect to observe over 100 white dwarfs per night - of which, on average, one should be suitable for detailed bulk composition studies. You will have the opportunity to study in detail the disruption of planetesimals, and the formation and evolution of debris discs using observations of individual stars obtained with the Very Large Telescope and ultraviolet spectroscopy from the Hubble Space Telescope. You will learn about the physical processes within white dwarf atmospheres, as well as the fundamental concepts of cosmo-chemistry. This PhD project will contribute to our overall understanding of the formation of planets, by establishing detailed statistical information of the compositions of their building blocks and fragments.
Supervisor: Boris Gänsicke