White dwarf stars are messy eaters, and the crumbs on their faces could reveal the origins of planets’ cores.
When University of Cambridge astronomer Amy Bonsor and her colleagues studied the spectrum of light from white dwarfs — the burned-out remains of small stars — they noticed flecks of heavier elements on the stars’ surfaces where there should have been only a glowing expanse of helium and hydrogen. The astronomers realized the stars’ surfaces were littered with debris from asteroids and comets that had fallen into the stars, visible on the surface just briefly before sinking into the depths.
The chemical makeup of those planet crumbs — visible in their spectra, the specific wavelengths of light each chemical emits — suggests that the building blocks of planets are as ancient as a star system itself, rather than things that form later from the disk of material orbiting the star.
What’s New — It’s morbid but true: most stars eventually gobble up at least some of the planets and other chunks of space rock in their orbits. Solar systems can be dangerous places, especially in their early stages, with planets’ gravity bumping other planets — or smaller things, like asteroids and comets –—off their courses. Some of those objects get launched out of the solar system to start a new life as rogue planets, but others end up spiraling inward toward the immense gravity of the star at the heart of the system.
That seems to happen more often in white dwarf star systems, according to Bonsor.
“The host star has lost mass, giving the planets greater ‘influence’ over the comets or asteroids,” she tells EuroJournal.
Of the white dwarfs whose spectra astronomers have measured, between 30 percent and 50 percent got caught with the crumbs of devoured planets still on their faces. Depending on the temperature, composition, and surface gravity of the star, material that falls in may take anywhere from days to millions of years to sink out of sight beneath the surface. And in the meantime, astronomers studying the star can measure elements like silicon, magnesium, iron, chromium, nickel, and others.
Bonsor and her colleagues noticed something weird about the planet crumbs a small fraction of white dwarfs were still guiltily wearing, and it could reveal more details about how and when planets form.
Here’s the background — In an enormous cloud of gas called a nebula, sometimes material clumps together until it collapses under its weight. When that happens, the heat and pressure at its center are enough to start fusing hydrogen atoms into helium — the thermonuclear reaction at the heart of a star. Over time, the newborn star draws in more material. Some feed the growing star but more end up in orbit around it. And gradually, bits of that material start clumping together.
These clumps of dust are called planetesimals: they’re the beginnings of planets that will grow later. Depending on sheer chance, some planetesimals might eventually draw in enough nearby gas and dust to grow into planets — maybe a dwarf planet like Pluto or a gas giant like Jupiter. Others never live up to that potential. Many of the asteroids in our Solar System are planetesimals that never grew beyond this very early stage.
What astronomers aren’t sure about, however, is exactly when planetesimals start to coalesce out of the disk of material around a newborn star. Most models suggest that it happens later, as the disk of material evolves to contain relatively less gas and more dust and ice. But the debris Bonsor and her colleagues saw slowly sinking into some white dwarfs’ surfaces suggests that the building blocks of planets start forming very soon after the star they’re orbiting.
Why it matters — For scientists who want to understand why our homeworld, or any other planet, has the mix of elements that it does, understanding the timing is critical.
“So there is debate in the literature at the moment about how important the fact that Earth’s building blocks were likely differentiated (formed an iron core) is for the final composition of Earth,” says Bonsor. “It can potentially alter the final composition of the planet, including key species such as uranium and thorium that provide internal heat.”
Digging into the details — Most of the time, white dwarfs that feast on ill-fated asteroids seem to have a balanced diet. The crumbs of planet debris on their surfaces are an even mix of metals and rock. But on a small fraction of the stars, Bonsor and her colleagues noticed that the debris sinking slowly into their surfaces seemed to be mostly metals like iron, chromium, or nickel — or else mostly rocky material like magnesium and silicate.
It looked as if those stars had been munching on planetesimals whose material had sorted itself into layers, with the densest stuff sinking toward the middle of the planetesimal. If that was the case, then the planetesimals must have been completely molten at some point (after all, clumps of solid material don’t settle out into layers; liquids do that).
That sometimes happens when a planet gets large enough that its pressure heats its interior, but Bonsor and her colleagues’ simulations suggest whole planets shouldn’t be very prone to be swallowed up by white dwarfs. That’s a more likely fate for smaller chunks of rock and metal — planetesimals. And that means something had to have heated them to the melting point.
Bonsor and her colleagues say the culprit is probably an isotope called aluminum-26, an aluminum atom with 26 protons and 26 neutrons. It’s radioactive, and as it decays, it pumps out enough heat to melt the iron and rock around it.
Scientists are pretty sure that’s what happened in our Solar System, based on the fact that its decay products are scattered throughout the asteroid belt. But that’s the thing about aluminum-26 — it decays fast, with a half-life of 700,000 years. And after a half-life or two, it’s not much of a heater anymore.
What’s Next — All of the work so far suggests that if these asteroids — the ones smashing into white dwarfs and leaving planetesimal crumbs scattered across their surfaces — had been melted by the heat of decaying aluminum-26, as Bonsor and her colleagues think happened, then it had to have happened within the first few hundred thousand years after the star was born — much earlier than scientists expected.
The next step for Bonsor and her colleagues will be to study more white dwarfs and see what leftover bits of planetesimal are still stuck to their surfaces.
“Gaia has identified hundreds of thousands of white dwarfs, many of which are readily accessible to ground-based spectroscopic observations,” she says.