White dwarf stars they are messy eaters, and the crumbs on their faces could reveal the origin of the 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 specks of heavier elements on the stars’ surfaces where there should have been only a glowing expanse of helium and hydrogen. . Astronomers realized that the surfaces of stars were littered with debris from asteroids and comets that had fallen onto the stars, visible on the surface for a short time before sinking into the depths.
The chemical composition of these planet crumbs—visible in their spectra, the specific wavelengths of light each chemical emits—suggests that the building blocks of the planets are as ancient as a star system, rather than things that formed later from the orbiting disk of material. 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 the planets’ gravity knocking other planets — or smaller things like asteroids and comets — out of their path. Some of these objects are ejected from the solar system to start a new life as rogue planets, but others end up spiraling inward toward the massive gravity of the star at the heart of the system.
This seems to happen more often in white dwarf systems, according to Bonsor.
“The host star has lost mass, giving the planets more ‘clout’ on comets or asteroids,” he says. Inverse.
Of the white dwarfs whose spectra have been measured by astronomers, between 30 percent and 50 percent were caught with the crumbs of devoured planets still on their faces. Depending on the star’s temperature, composition, and surface gravity, the infalling material can take anywhere from days to millions of years to sink out of sight below the surface. And in the meantime, astronomers studying the star can measure elements like silicon, magnesium, iron, chromium, nickel and more.
Bonsor and her colleagues noticed something strange about the planet crumbs that a small fraction of white dwarfs still guiltily wear, and it could reveal more details about how and when planets form.
Here is the background – In a huge cloud of gas called a nebula, material sometimes collects until it collapses under its own weight. When this happens, the heat and pressure at its center is enough to start fusing hydrogen atoms into helium – the thermonuclear reaction at the heart of a star. Over time, the newborn star pumps out more material. Some feed the growing star, but more end up orbiting it. And gradually, pieces of this material begin to come together.
These clumps of dust are called planetesimals: they are the beginnings of the planets that will develop later. Depending on the sheer probability, some planetesimals could eventually draw on enough nearby gas and dust to grow into planets—perhaps a dwarf planet like Pluto or a gas giant like Jupiter. Others never live up to these potentials. Many of the asteroids in our Solar System are planetesimals that never developed beyond this very early stage.
What astronomers aren’t sure about, however, is exactly when planetesimals begin to coalesce from the disk of material around a newborn star. Most models suggest that this happens later, as the disc of material evolves to contain relatively less gas and more dust and ice. But the debris that Bonsor and her colleagues saw slowly sink into the surfaces of some white dwarfs suggests that the building blocks of planets begin to form very soon after the star they orbit.
Why it matters – For scientists who want to understand why our home world, or any other planet, has the mix of elements it does, understanding timing is critical.
“So there is debate in the literature right now about how important it is to the final composition of the Earth that the building blocks of the Earth probably differentiated (formed an iron core),” says Bonsor. “It can potentially change the final composition of the planet, including key species like uranium and thorium that provide internal heat.”
Digging into the details — Most of the time, white dwarfs feasting on unlucky asteroids seem to have a balanced diet. The crumbs of planetary debris on their surfaces are a uniform mixture of metals and rock. But in a small fraction of stars, Bonsor and her colleagues noticed that the debris slowly sinking to their surfaces appeared to be mostly metals like iron, chromium or nickel — or else mostly rocky material like magnesium and silicate.
It looked like these stars were chewing up planetoids, whose material had been sorted into layers, with the densest material sinking towards the middle of the planetoid. If this were the case, then the planets must have completely melted at some point (after all, clumps of solid material don’t layer; liquids do).
This sometimes happens when a planet gets big enough that its pressure heats its interior, but Bonsor and her colleagues’ simulations suggest that entire planets shouldn’t be too prone to being swallowed by white dwarfs. This is a more likely fate for smaller pieces of rock and metal – planetoids. And that means something had to have heated them to the point of melting.
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 fairly certain that this 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 quickly, with a half-life of 700,000 years. And after a half-life or two, it’s not a very good heater anymore.
What’s next – All the work so far suggests that if these asteroids — the ones that crash into white dwarfs and leave planetary debris scattered across their surface — were melted by the heat of decaying aluminum-26, as Bonsor and her colleagues believe, then it had to it happened within the first hundreds of thousands of 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 planetoid debris is still attached to their surface.
“Gaia has detected hundreds of thousands of white dwarfs, many of which are easily accessible to ground-based spectroscopic observations,” he says.