Artist's impression of debris around a white dwarf star (Source: NASA)

Detecting the earliest planets

The white dwarf G29-38 (Source: NASA)

Professor Boris Gäniscke from Warwick University came by to give a seminar yesterday and sparked my interest with his findings from looking at white dwarfs: by looking at the chemistry of these old, burned out stars, he can find out the make-up of planets that formed way back in the early history of the Universe!

It works like this: absorption lines and emission lines in a star’s spectrum indicate what elements are present in its atmosphere. In a white dwarf, you’d just expect to see hydrogen or helium because the star’s high gravity causes heavier elements to sink towards the centre.

A white dwarf surrounded by rocky debris (artist's impression)

A white dwarf surrounded by rocky debris (artist's impression) (Source: Mark A. Garlick/space-art.co.uk/University of Warwick)

But in some white dwarfs, we see other elements: oxygen, iron, silicon and magnesium. The only way they could be there is if this is debris being actively accreted now. The source of this debris: the star’s original planetary system.

His argument for this is twofold. Firstly, it’s reasonable to expect there to be such debris: the outer planets are displaced outwards by a dying star and this movement leads to tidal forces that break up asteroids and Kuiper-belt-like objects and cause collisions with any surviving terrestrial planets. Secondly, the chemistry fits: this material is volatile-depleted, so it’s rocky material, and it has the kind of ratios between elements you see in the meteorites and planets in our Solar System.

And the really exciting bit? For a star to be born, go through its life-cycle and get to the age of star he’s looking at takes a good fraction of the age of the Universe. So if he can get the composition of planets that formed around those stars, he’s got a window onto the earliest planets of all.