Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse
Objects: Polluted white dwarfs
Diet: Small rocky planets
As the old, bloated star neared death, it started to devour its planets – and they were delicious. Even after it had collapsed into a small, white corpse, the craving for planets continued. It spun restlessly, an undead star fixated on getting another taste of those rocky little worlds. At last, its patience was rewarded.
It's a grisly tale, but also one that represents a boon to astronomers. They can observe white dwarf stars spattered with the remains of their planetary meals to perform a kind of cosmic autopsy – one that reveals the types of worlds that existed when the star was young and healthy.
When stars like our sun die, they go through a series of changes that can be devastating to their planets. First they puff up into red giants and engulf anything too close. Powerful stellar winds cause the dying stars to slough off much of their mass, exposing dense stellar cores called white dwarfs.
Heavy pollution
Elements heavier than hydrogen and helium sink down into a white dwarf's core, leaving behind an atmosphere of pure hydrogen and helium. But in 1987, the atmosphere of a white dwarf called G29-38, in the constellation Pisces, was found to be polluted with heavier elements.
Today about a quarter of all white dwarfs are known to have more heavy elements in their atmospheres than they should. Dusty discs, like the rings of Saturn, surround some of these white dwarfs.
The discs are probably the remnants of planets or asteroids that came too close to the white dwarfs and were ripped apart by gravitational forces, says Eric Becklin of the University of California, Los Angeles (UCLA). The remnant stars feed on the pulverised remains, so their atmospheres become laced with heavier elements.
This process could help us figure out what rocky extrasolar planets are made of. Recently the hunt for planets around other stars has yielded hundreds of confirmed finds and thousands of possible candidates. But we had no way of determining the compositions of smaller worlds, especially what is inside them.
Chemical signatures
Studies of the chemical signatures in starlight from polluted dwarfs may be just the tools for the job, says Michael Jura, also of UCLA. His team looked at 60 polluted white dwarfs and examined the proportions of different elements in their atmospheres.
Presenting this week at a meeting of the American Astronomical Society in Long Beach, California, Jura showed that oxygen, magnesium, silicon and iron are present in similar proportions to what exists on Earth, collectively making up 85 per cent of the mass of the heavy elements seen in their atmospheres. The team also showed that the stars had, on average, swallowed an amount of matter equal in mass to that of the asteroid belt in our solar system.
The finding, gleaned through observations in visible and ultraviolet light, suggests that rocky planets and asteroids in these far-off systems are made in a similar vein to objects in our solar system. "We are witnessing the building blocks of extrasolar rocky planets," Jura says.
"It's fantastic," says Becklin, who was on the team that discovered G29-38 but was not involved in the new research. "You can study what a solar system is like around other stars, and get inside the bodies. We didn't have the slightest idea of the composition of the planets in those exo-solar systems. This is actually showing there's a way to determine that."
The process must be ongoing, adds Becklin, or else the dwarfs should have long ago run out of victims. Gravity from surviving planets farther out may be constantly tossing fresh asteroids inwards, keeping the stars' feeding frenzy going.
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Astrophile: Zombie stars feed on Earth-like exoplanets
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Astrophile: Zombie stars feed on Earth-like exoplanets
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Astrophile: Zombie stars feed on Earth-like exoplanets
