Exo-planetary archaeology with white dwarfs

Tuesday 18th February, 2020

Our speaker was Dr Christopher Manser from Warwick University who came to talk to us about "Exoplanetary archaeology with white dwarfs". He introduced himself by saying that he had studied for his undergraduate degree and doctorate at Warwick University and this would be his tenth year there. At the end of this year he would be moving to Imperial College, London to seek a more tenured post but continuing his research into white dwarfs.

Dr Manser began by saying that a white dwarf star is roughly the size of the Earth with about 60% of the Sun's mass, making it an extremely dense object. As our own Sun ages it will become a white dwarf after it has gone through a phase of its life as a red giant star. Unfortunately, for most of the inner planets of our Solar System when our Sun becomes a red giant it will expand and swallow up Mercury, Venus and probably the Earth, leaving only Mars and the planets beyond to witness its metamorphosis into a white dwarf.

He then explained that using the Gran Telescopio Canarias, a large professional telescope on the island of La Palma, with its 10.4-metre diameter mirror, his team spotted what they think is the core of a planet orbiting a white dwarf star. This small remnant of a planet is so close to the white dwarf that it only takes only a couple of hours to complete one orbit.

They propose that the planet may have formed further out but somehow was displaced by the gravitational pull of a larger outer planet into this new close orbit. As it approached the central white dwarf its outer layers — the crust and mantle — would have been stripped away by the gravitational pull of white dwarf.

Dr Manser continued by saying that all that would be left of the planet would be a dense, iron core similar to the metallic asteroid 16-Psyche that orbits amongst other asteroids in the asteroid belt between Mars and Jupiter. In fact NASA is planning to send a spacecraft (called Psyche) to study 16-Psyche and it will launch in 2022, reaching it four years later.

He then said that even though they had used the world's largest single-aperture telescope, they were not able to observe this tiny planet directly. The clue to the existence of the planet was that the white dwarf's light showed it had been contaminated by certain elements that should not have been there in its upper atmosphere. For a white dwarf's upper layers usually consist of hydrogen and helium as the heavier elements will have sunk towards its centre. Further in-depth analysis of the light from the central white dwarf has revealed that the planet is emitting a stream of gas and dust. This planetary debris is, in turn, sitting within a flattened disc of debris around the white dwarf star.

Dr Manser concluded his talk by saying that this is only the second discovery of a planetary body in a close orbit around a white dwarf. The previous discovery was of a much larger Neptune-sized planet that was losing its atmosphere due to the gravitational pull of the white dwarf star it was closely orbiting. This atmospheric outflow was then forming a disc of gas around the central star.