NASA's Hubble Space Telescope has found the building blocks for Earth-sized planets in an unlikely place, the atmospheres of a pair of burned-out stars called white dwarfs. The dwarfs are being polluted by asteroid-like debris falling onto them. This discovery suggests that rocky planet assembly is common in stars, say researchers.
The white dwarfs reside 150 light-years away in the Hyades star cluster, residing in the constellation Taurus the Bull. The cluster is relatively young, only 625 million years old.
Hubble's spectroscopic observations identified silicon in the white dwarfs' atmospheres, a major ingredient of the rocky material constituting Earth and other terrestrial planets in our solar system. The silicon may have come from asteroids that were shredded by the white dwarfs' gravity when they veered too close to the stars. The rocky debris likely formed a ring around the dead stars, which then funneled the material onto the stellar relics.
The material detected whirling around the white dwarfs suggests that terrestrial planets formed when these stars were born. After the stars collapsed to white dwarfs, surviving gas-giant planets may have gravitationally perturbed members of any leftover asteroid belts into star-grazing orbits.
"We have identified chemical evidence for the Lego building blocks of rocky planets," says Jay Farihi of the University of Cambridge in England, lead author of a new study that appeared in the May 2 issue of the Monthly Notices of the Royal Astronomical Society. "When these stars were born, they built planets, and there's a good chance they currently retain some of them. The material we are seeing is evidence of this. The debris is at least as rocky as the most primitive terrestrial bodies in our solar system."
Astronomers commonly believe that all stars formed in clusters. But searches for planets outside our solar system have only detected a handful of them orbiting cluster stars. Farihi suggested that it may be harder to make the precision measurements needed to find extrasolar planets in clusters because the stars are young and more active, producing stellar flares and other outbursts.
The team, therefore, searched planets around retired cluster stars. "Using Hubble to analyze the atmospheres of white dwarfs is the best method for finding the signatures of solid planet chemistry and determining their composition," Farihi explains. "Normally, white dwarfs are like blank pieces of paper, containing only the light elements hydrogen and helium. Heavy elements like silicon and carbon sink to the core."
Besides finding silicon in the Hyades stars' atmospheres, Hubble also detected low levels of carbon, another sign of the debris' rocky nature. Astronomers would expect carbon to be depleted or absent in rocky, Earth-like material. Carbon is a key element that helps astronomers determine the properties and origin of the planetary debris raining down onto white dwarfs. It leaves fingerprints only in ultraviolet light, which cannot be observed from ground-based telescopes. Finding its chemical signature required Hubble's Cosmic Origins Spectrograph (COS).
"The one thing the white dwarf pollution technique gives us that we just won't get with any other planet-detection technique is the chemistry of solid planets," Farihi says. "Based on the silicon-to-carbon ratio in our study, for example, we can actually say that this material is basically Earth-like. If you put this stuff into the hand of a child, or an adult, and you ask them, `What is this?' Any human being would be able to respond, ‘It's a rock!' They wouldn't need to be a scientist. They would know exactly what it is, as it's something familiar to all of us."
Farihi suggests that asteroids less than 100 miles (160 kilometers) across were probably gravitationally torn apart by the white dwarfs' strong tidal forces. The pulverized material may have been pulled into a ring that eventually fell onto the dead stars. "It's difficult to imagine another mechanism than gravity that causes material to get close enough to rain down onto the star," he says.
The team estimated each asteroid's size by measuring the amount of dust being gobbled up by the dead stars, about 10 million grams per second, equal to the flow rate of a small river. They then compared that data with measurements of material falling onto other white dwarfs.
The Hyades study offers insight into what will happen in our solar system when our Sun burns out 5 billion years from now. When the Sun exhausts its hydrogen fuel, it will puff up to a red giant and swallow Mercury and Venus, and perhaps the Earth. As the Sun begins to eject its outer layers, it loses mass. The balance of gravitational forces between the Sun and Jupiter changes, disrupting the main asteroid belt. Some of these asteroids could veer too close to the Sun, which breaks them up. The debris could be pulled into a ring around the dead Sun, similar to the inferred rings around the Hyades white dwarfs.
The two "polluted" Hyades white dwarfs are part of the team's search of planetary debris around more than 100 white dwarfs, led by Boris Gänsicke of the University of Warwick in England. Team member Detlev Koester of the University of Kiel in Germany is using sophisticated computer models of white dwarf atmospheres to determine the abundances of various elements that can be traced to planets in the COS data.
The team plans to analyze more white dwarfs using the same technique to identify not only the rocks' composition but also their parent bodies. "The beauty of this technique is that whatever the universe is doing, we'll be able to measure it," Farihi said. "We have been using our solar system as a kind of map, but I don't know what the universe does. Is there another recipe for Earth-like or habitable planets? The chemistry can tell us. Hopefully, with Hubble and its powerful ultraviolet-light camera COS, and with the upcoming ground-based 30- and 40-meter telescopes, we'll be able to tell a story. We hope to create a picture of hundreds of rocky planet building blocks and tell how often they look like Earth and how often they look different, or even exotic. Who knows, maybe we'll find some stuff we haven't thought of yet."
Space Telescope Science Institute, Baltimore, Md.