The likely remains of a water-rich asteroid have been discovered orbiting a dead star 50 parsecs (150 light years) from Earth. The finding suggests that from cradle to grave, a wide range of stars might harbor rocky material rich in water — key ingredients for building a habitable planet.
Jay Farihi at the University of Cambridge, UK and his colleagues had been looking for several years at signs of rocky planets or their chemical building blocks around white dwarfs — the remains of stars between one and eight times the mass of the Sun crammed into a volume no bigger than Earth. White dwarfs are natural laboratories for studying the composition of exoplanets (planets outside the Solar System), as their strong gravitational pull shreds nearby asteroids and minor planets and pulls in their debris, Farihi notes. The remnants are relatively easy to detect because they pollute a white dwarf’s pristine atmosphere of hydrogen or helium with heavy elements. The team focused on the white dwarf GD 61, the compact core of a star about three times heavier than the Sun. Previous observations had revealed a disk of rocky debris orbiting GD 61 and an abundance of oxygen in its atmosphere, a clue that water might be present.
Using an ultraviolet spectrometer aboard the Hubble Space Telescope, the team took a more precise inventory of the oxygen content. Using the abundances of other elements measured by the Keck Observatory atop Hawaii’s Mauna Kea, the team determined how much oxygen should be present if it were all bound up in rock. The actual oxygen abundance revealed by Hubble was much greater, indicating that the additional oxygen was carried by either water or by carbonaceous material.
The white dwarf, however, contains very little carbon, leaving water as the only possible source of the oxygen abundance, Farihi says. The team calculates that if the debris disk surrounding GD 61 is the remains of a single object, then that would probably have been about the size of the Solar System asteroid Vesta, which has a diameter of around 500 kilometres, and would have possessed abundant water (26% by mass). That would make it similar to the asteroid Ceres, the type of asteroid thought to have ferried water to the young Earth.
“This is the first convincing case” that an object pulverized by a white dwarf was both rocky and water-rich, says astronomer Ben Zuckerman of the University of California, Los Angeles, who was not involved in the study.
The putative asteroid must have broken up relatively recently, because debris disks around white dwarfs last only for about a million years and material polluting their atmosphere sinks to the core in about 20,000 years or less, says Farihi.