Exoplanet Building Blocks Found Around White Dwarfs

This is Scientific American's 60-Second Space. I'm John Matson. Got a minute?
Watch for falling rocks.
Those are words to keep in mind when you're driving a mountain road.
Or, it seems, when you're cruising past a white dwarf star.
White dwarfs are small, dense remnants of normal stars that have expended their fuel.
Now researchers have identified rocky, asteroid-like material raining down on two white dwarfs.
The rocky material is akin to the building blocks of planets,
and its presence suggests that the two stars formed planetary systems before they expired and became white dwarfs.
That's the conclusion of a study in the Monthly Notices of the Royal Astronomical Society.
Astronomers identified the silicon-rich rocky material by training the Hubble Space Telescope on the nearby Hyades star cluster.
Finding rocky material means the stars once had the raw ingredients to make exoplanets,
although almost no known planets orbit stars inside of clusters.
The discovery of planetary material thus raises more questions.
Are star clusters indeed inhospitable to planets?
Or are planets around stars in clusters for some reason just harder to spot?
In the Hyades cluster, at least, it looks like there may be planets that have so far escaped our notice.
Thanks for the minute, for Scientific American's 60-Second Space. I'm John Matson.