The Hubble Space Telescope has discovered rocky remains of planetary material ‘polluting’ the atmospheres of two white dwarfs — a sign that these stars likely have (or had) planetary systems and that asteroids are currently being shredded by extreme tidal forces.
Although white dwarfs with polluted atmospheres have been observed before, this is the first time evidence of planetary systems have been discovered in stars belonging to a relatively young cluster of stars.
By turning their attention to the Hyades star cluster some 150 light-years from Earth, Jay Farihi of the University of Cambridge and his team used Hubble’s Cosmic Origins Spectrograph (COS) to detect the faint signature of carbon and silicon in the two white dwarfs — called WD 0421+162 and WD 0431+126. The ratio of these elements detected are suggestive that these “dead” stars are consuming rocky material of a similar chemical composition as Earth.
White dwarfs form after stars like our sun have exhausted all their hydrogen fuel, puffed up as a red giant and blown apart as a planetary nebula. What’s left behind is a small white dwarf that can persist for billions more years.
Any planetary system that was once in orbit around the star will be severely disrupted during the red giant phase, but any rocky material — such as asteroids, even entire planets — can survive. But the extreme tidal stresses the newly formed white dwarf can exert will rip apart any orbiting body, grinding it to dust. The dust is then dragged into the white dwarf’s atmosphere, polluting it.
This pollution, however, is very useful to astronomers. The chemical makeup of the material the old star system contains can then be identified by their chemical “fingerprint” in the star’s spectrum.
“We have identified chemical evidence for the building blocks of rocky planets,” said Farihi in a Hubble news release. “When these stars were born, they built planets, and there’s a good chance that they currently retain some of them. The signs of rocky debris we are seeing are evidence of this — it is at least as rocky as the most primitive terrestrial bodies in our Solar System.”
Where Planets Go To Die
Of the 800+ exoplanets discovered to date, only four are known to orbit stars in star clusters. Astronomers believe the vast majority of stars evolved in clusters, but why have so few exoplanets been discovered too?
By focusing on the Hyades cluster, Farihi and co analyzed the light from two of the “retired” stars — i.e. white dwarfs that have gone through the complete stellar cycle. Finding evidence of pulverized rocky material in the stars’ atmospheres are a sure sign that, yes, stars in cluster do host planetary material. By the researchers’ reckoning, the dust originates from asteroids less than 160 kilometers across being tidally ripped apart. Where there’s asteroids, there are likely exoplanets.
“The one thing the white dwarf pollution technique gives us that we won’t get with any other planet detection technique is the chemistry of solid planets,” said Farihi. “Based on the silicon-to-carbon ratio in our study, for example, we can actually say that this material is basically Earth-like.”
So, by analyzing the light from white dwarfs, we aren’t only able to see evidence for planetary systems around stars in star clusters, we’re also looking into the future of our solar system. In a few billion years time, alien astronomers my be analyzing the light from our “retired” white dwarf sun, and deduce that they are looking at a planetary graveyard.
Image: Artist’s impression of the thin, rocky debris disc discovered around the two Hyades white dwarfs. Credit: NASA, ESA, STScI, and G. Bacon (STScI)