The fate of most stars, including our own Sun, is to become a white dwarf – a star that has burnt up all its fuel and shed its outer layers, often destroying its orbital bodies in the process. Over 300,000 white dwarf stars have been discovered in our galaxy alone, and researchers have long suspected that many of them are “feeding” on the scattered debris of planets and other objects that once orbited them.
But until recently, there has only been indirect evidence to support this theory.
Now, for the first time, researchers can confirm that the disintegrating remnants of planets are indeed destined to be swallowed up by the dying stars that once held them in their orbits.
The results, published in Nature, are the first direct measurement of such accretion of rocky material into a white dwarf.
Astronomers from the University of Warwick, in the UK, used x-rays to detect the collision and consumption of rocky and gaseous material from a collapsing planetary system with the white dwarf at its centre, watching the death of the system some billions of years after its initial formation. This is the first time that astronomers have seen the material actually being pulled into the star.
For several decades, astronomers have used spectroscopy at optical and ultraviolet wavelengths to measure the abundance of elements on the surface of white dwarf stars, deducing from this the composition of the objects they came from. From these indirect spectroscopic observations, astronomers could see that between a quarter and a half of all white dwarfs observed have heavy elements such as iron, calcium and magnesium polluting their atmospheres, which were presumed to have derived from the planetary bodies they were gobbling up.
Dr Tim Cunningham of the University of Warwick Department of Physics says that, until now, researchers have relied on “numerical models that calculate how quickly an element sinks out of the atmosphere into the star, and that tells you how much is falling into the atmosphere as an accretion rate. You can then work backwards and work out how much of an element was in the parent body, whether a planet, moon or asteroid.”
Finally having a direct confirmation of theory is thrilling, says Cunningham, noting that the degree of congruence between the established models and the new observations is “remarkable”.
The key to these new observations was the use of x-rays. As debris from crumbling planets is pulled into a white dwarf, the speed with which it slams into the star’s surface generates shock-heated plasma. This plasma, with a temperature between 100,000 and one million degrees kelvin, then settles on the surface, and as it cools it emits x-rays.
But detecting these x-rays is challenging. There are a number of bright x-ray sources scattered across our skies, and filtering out the very small amount reaching us from distant white dwarfs can be difficult.
To tackle the task, astronomers turned to the Chandra X-ray Observatory, a satellite telescope currently orbiting the Earth and normally used to detect x-rays from black holes and neutron stars that are accreting. This time they used it to analyse the nearby white dwarf G29-38.
With Chandra’s improved angular resolution over other telescopes, they could isolate the target star from other x-ray sources. They then viewed, for the first time, x-rays from an isolated white dwarf.
“What’s really exciting about this result is that we’re working at a different wavelength, x-rays, and that allows us to probe a completely different type of physics,” says Cunningham.
“This detection provides the first direct evidence that white dwarfs are currently accreting the remnants of old planetary systems. Probing accretion in this way provides a new technique by which we can study these systems, offering a glimpse into the likely fate of the thousands of known exoplanetary systems, including our own Solar system.”