Detecting the earliest planets
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.
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.