Pulverized dwarf planet could point to once-habitable star system

Data coaxed from a star 170 light-years away suggest that the system once might have had at least one planet with a potential for hosting life.

The star is girded by a debris disk, the remnant of a dwarf planet, according to a team of astronomers that conducted the analysis. The researchers estimate the disk's parent body – long ago pulverized by the star's gravity – to have been at least 55 miles across (and probably much larger). The signatures it has left behind in the star's light suggest that it was water rich and that the system had the means to build larger planets also rich with water.

This is the first time anyone has detected water in connection with a rocky body outside the solar system, the researchers say.

The star is a white dwarf known as GD 61. At one time, GD 61 had about three times the sun's mass. But about 200 million years ago, the star ended its red-giant phase, shed its outer envelope of hot gas, and left behind a core that packs most of its original mass into an object roughly the size of Earth.

The star's enormous gravitational tug destroyed the once-rocky body, and the star is now consuming the disk of debris that remains.

"This planetary graveyard swirling around the embers of its parent star is a rich source of information about its former life," said Boris Gansicke, an astronomer at the University of Warwick in Britain and a member of the team reporting the results in Friday's issue of the journal Science.

"In these remnants lie chemical clues which point towards a previous existence as a water-rich terrestrial body, " he said in a prepared statement.

The team found those clues by looking at the chemical makeup of the star.

As a disk spirals into a white dwarf, it adds its cornucopia of additional elements to the star, whose outer layers consist of hydrogen and helium. In the case of GD 61, the team used the Hubble Space Telescope's Cosmic Origins Spectrograph to detect oxygen, magnesium, aluminum, silicon, calcium, and iron – the basic ingredients for rocks.

The team assumed that these elements were present in the disk's parent body as oxides, given that oxides are seen in rocks in our solar system. But the data showed excess oxygen. That suggested that the parent object for the disk was 26 percent water by mass, much like the dwarf planet Ceres in our solar system. The team estimated the object's mass as comparable to the smaller Vesta, which is about 326 miles across.

Researchers have observed disks around 30 other white dwarfs. This is the first time evidence for water has emerged, the team notes.

If the GD 61 system had objects like that, it surely had larger and similarly water-rich objects, too, the researchers reasoned.

"A system cannot create things as big as asteroids and avoid building planets, and GD 61 had the ingredients to deliver lots of water to their surfaces," said Jay Farihi of Cambridge University's Institute of Astronomy, who led the team. "The finding of water in a large asteroid means the building blocks of habitable planets existed – and maybe still exist – in the GD 61 system, and likely also around substantial number of similar parent stars."

"Our results demonstrate that there was definitely potential for habitable planets in this exoplanetary system."