Dead stars covered in space debris could reveal the origins of planets

Dead stars covered in space debris could reveal the origins of planets

white dwarf stars are messy eaters, and the crumbs on their faces could reveal the origins of planetary cores.

When University of Cambridge astronomer Amy Bonsor and her colleagues studied the light spectrum of white dwarfs — the burnt-out remains of small stars — they noticed patches of heavier elements on the stars’ surfaces where there should have been only a glowing patch of helium and hydrogen. Astronomers realized that the stars’ surfaces were read with debris from asteroids and comets that had fallen into the stars and were visible on the surface just before sinking to depth.

The chemical composition of these planetary crumbs — visible in their spectra, the specific wavelengths of light each chemical emits — suggests that the building blocks of planets are as old as a star system itself, and not things that later form from the orbiting disk of material the star.

What’s new – It’s morbid but true: most stars eventually engulf 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, when planetary gravity knocks other planets — or smaller things, like asteroids and coming — out of their orbit. Some of these objects will be ejected from the solar system to begin new lives as rogue planets, but others will end up spiraling inward toward the immense gravity of the star at the heart of the system.

According to Bonsor, this appears to be more common in white dwarf star systems.

“The host star has lost mass, giving the planets a greater ‘impact’ over the comet or asteroid,” she says Turning back.

Between 30 and 50 percent of the white dwarfs whose spectra were measured by astronomers were caught with the crumbs of the engulfed planets still on their faces. Depending on the star’s temperature, composition, and surface gravity, it can take anywhere from days to millions of years for the material that falls into it to disappear 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 odd about the planetary crumbs that a small fraction of white dwarfs still guiltily carried, and it may reveal more details about how and when planets form.

Here is the background – In a huge cloud of gas called a nebula, matter sometimes clumps together until it collapses under its weight. When that happens, the heat and pressure at its center is enough to fuse hydrogen atoms into helium — the thermonuclear reaction at the heart of a star. Over time, the newborn star will attract more material. Some feed the growing star, but more end up in orbit around it. And gradually parts of this material begin to clump together.

Artist’s rendering of a disintegrating exoplanet orbiting in a planetary debris disk. MARK GARLICK/SCIENCE PHOTO LIBRARY/Science Photo Library/Getty Images

These clumps of dust are called planetesimals: they are the beginnings of planets that will later grow. Depending on chance, some planetesimals might eventually suck in 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 this potential. Many of the asteroids in our solar system are planetesimals that never grew past this very early stage.

However, astronomers are unsure of exactly when planetesimals will begin merging from the disk of material around a newborn star. Most models suggest this happens later, as the disk of material evolves to contain relatively less gas and more dust and ice. But the debris that Bonsor and her colleagues saw slowly sinking into the surfaces of some white dwarfs suggests that the planets’ building blocks begin to form very soon after the star they orbit.

Why it matters – Understanding the timing is crucial for scientists trying to understand why our homeworld, or any other planet, exhibits such a mix of elements.

A T Tauri star is a young stellar object no older than a few million years. MARK GARLICK/SCIENCE PHOTO LIBRARY/Science Photo Library/Getty Images

“Therefore, there is current debate in the literature about how important the fact that the building blocks of the Earth were likely differentiated (formed an iron core) is in the final composition of the Earth,” says Bonsor. “It can potentially change the final composition of the planet, including key species like uranium and thorium that provide internal heat.”

To go into detail – For the most part, white dwarfs feeding on hapless asteroids seem to have a balanced diet. The crumbs of planetary debris on their surface are an even mixture of metals and rocks. But on a small fraction of the stars, Bonsor and her colleagues noticed that the debris slowly sinking into their surfaces appeared to be composed mostly of metals like iron, chromium, or nickel — or mostly rocky material like magnesium and silicate.

It appeared these stars had been eating planetesimals, whose material had sorted itself into layers, with the densest material sinking toward the center of the planetesimal. If that was the case, then at some point the planetesimals must have completely melted (after all, clumps of solid material don’t settle into layers; liquids do).

That sometimes happens when a planet gets so big that its pressure heats its interior, but Bonsor and her colleagues’ simulations suggest that entire planets shouldn’t be very vulnerable to being 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 likely an isotope called aluminum-26, an aluminum atom with 26 protons and 26 neutrons. It’s radioactive, and as it decays it gives off enough heat to melt the iron and rock around it.

Scientists are fairly certain 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 much of a heater anymore.

What’s next – All of the work to date indicates that when these asteroids — the ones that smashed into white dwarfs and left planetesimal crumbs strewn across their surfaces — were melted by the heat of the decaying aluminum-26, as Bonsor and her colleagues believe is what happened If so, then this was likely to have happened within the first few hundred thousand years of the star’s birth — much sooner than scientists expected.

The next step for Bonsor and her colleagues will be to study more white dwarfs and see what remnants of planetesimals still cling to their surfaces.

“Gaia has identified hundreds of thousands of white dwarfs, many of which are readily accessible for ground-based spectroscopic observations,” she says.

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