Decades later, a stellar mystery is solved

This artist’s rendering depicts two dense white dwarf starts that orbit each other once every 321 seconds in a binary star system about 1,600 light-years away.NASA

It's a phenomenon that has mystified astronomers for decades. White dwarf stars are often spotted far out in space ringed by vast clouds of debris. That much is clear. The source of that debris has been hard to place, however. Now new observations from a Harvard astronomer have finally pieced the story together, creating a dramatic picture of the violent forces at work in an end-of-days solar system.

"We've seen circumstantial evidence before," says Andrew Vanderburg a graduate student in astronomy at Harvard University and the lead author of the research, which appeared in Nature in October. "Now, we have the smoking gun."

Most stars, including the sun, will eventually end up as white dwarfs. When stars run out of nuclear fuel to burn, they puff up into red giants and then cool and contract into dense, hard white dwarfs. In this state, they have about half of their original mass and can be thought of as the dead core of the star.

In the 1980s, astronomers were using a tool called a spectrograph to analyze the light emitted by white dwarf stars. Every element emits a signature wavelength of light, so by analyzing the wavelengths emitted by a star, astronomers can infer what it's made of. When they looked at these white dwarfs, they expected to see nothing but hydrogen and helium. By the early 2000s, in about one-third of observed white dwarfs, they'd observed the presence of a far more diverse range of elements — calcium, magnesium, silicon, iron, aluminum, and titanium.

"[These elements] must have been put on the surface recently, and the question is where did they come from and how did they get sprinkled onto the surface of the star," Vanderburg says.

Astronomers work to create models of the universe that fit all objects and forces into a complete story. When they come across large amounts of material in a place where it seems like it shouldn't be, they want to know how it got there. Astronomers had long theorized that white dwarfs accumulated these dust belts by encountering odd bits of debris as they flew on their way through the interstellar medium. But the numbers never really added up.

Then, a dozen years ago, astronomer John Debes was one of the first to speculate that in the final days of a solar system's existence, the gravitational pull of very large orbiting planets might kick outlying asteroids in toward the white dwarf. Heat from the star would then evaporate elements from these approaching asteroids, at the same time that the star's gravity might rip and attract whole chunks from the asteroids.

If the circumstances were right, the same process could even be applied to whole rocky earth-like planets. Through these processes of evaporation and gravitational sundering, the white dwarf might attract a continually replenishing cloud of debris.

Debes says the conditions in the dust cloud around the white dwarf are hard to guess at, because the circumstances are so different from anything we're directly familiar with. But, he reasons that because the material in the dust cloud is so close to the white dwarf, it must be whipping around the star at incredibly high speeds. And because it's moving at such high speeds while also not turning into gas, it must be very fine and thin so as to avoid collisions that would cause the particles to vaporize.

"At these distances, the orbital speeds are thousands of kilometers per second, [nearly half] a percent the speed of light," he says. "It's very fast, and you can imagine that if dust is in this regular disc-like structure, it must be flat and thin because if it were puffy and things were colliding, they'd just turn into gas."

It was a good theory that, like all good theories, needed some evidence to support it. That's what Vanderburg has provided. In March of this year, he was looking through data collected by the Kepler space observatory on a white dwarf star in the constellation Virgo, 570 light-years away, close to home by interstellar standards. The Kepler data logged light emitted from the white dwarf star, and Vanderburg noticed that every 4.5 hours, that light dimmed. This dimming is called a "transit signature" and it suggested that a planetary body was orbiting the star — every time this body passed between the star and Kepler's mirror, the observed light dimmed.

The transit signature was provocative enough that Vanderburg decided to take a look at the white dwarf directly, from ground-based telescopes in Arizona and Chile. Yet when he trained the telescopes on the star, he initially saw no evidence of the dimming he'd picked up in the Kepler data. Then on his last day of observations, it appeared.

"We were almost ready to give up, thinking we must have been tricked by the Kepler data," he says. "On the last night before we gave up, we saw these two transits, 4.5 hours apart. They were very high amplitude signals, 20 times larger than the deepest planetary transit anyone had seen before."

Vanderburg realized that the object orbiting the white dwarf star was not a planet — it was likely a large asteroid with a tail of dust. In other words, it was exactly the kind of object Debes had imagined more than a decade earlier might be the source of the heavy debris sprinkled on the surface of white dwarfs.

Debes, not surprisingly, is thrilled.

"I've been working on this for many years. It's such a cool result. It's so nice to see something like this," he says.

This discovery also enhances the astronomical importance of white dwarf stars. Initially astronomers viewed them as boring because, as Debes says, "you don't think of dead stellar systems as interesting to study. For the longest time. it was like me and three other people who spent their time doing this kind of stuff."

But now that the debris which collects around white dwarf stars can be traced to objects from their own disintegrating solar systems, the stars can be thought of as repositories — places that collect the elements that made the planets that used to circle them.

And this is where the logic gets fast and heady. Because, if a good percentage of white dwarf stars have dust clouds, and if those dust clouds are made from the remnants of rocky bodies that used to orbit those stars, then, well, as Debes puts it: "Rocky planets must be very common around stars, which is exciting because if our own earth is rocky, maybe it means that other rocky earths are common as well."