Astronomers have observed a white dwarf star devouring both rocky metal material (possibly from a nearby asteroid) and icy material (presumably from an object similar to the Kuiper Belt at the edge of our solar system), the first observed in the universe.
“We’ve never seen these two objects accreting to a white dwarf at the same time,” said lead researcher Ted Johnson. “By studying these white dwarfs, we hope to better understand planetary systems that are still intact.” A UCLA physics and astronomy student who just graduated.
Those conclusions are based on an examination of material trapped in the atmosphere of G238-44, a white dwarf star about 86 light-years from Earth, and the researchers used archived data from the Hubble Space Telescope and other NASA satellites and observatories. White dwarfs are the burned-out cores that remain after a star similar to our sun sheds its outer layers and stops burning fuel through nuclear fusion.
While the broad diet of white dwarfs is surprising, the findings are also intriguing, as astronomers believe icy objects crashed into and irrigated the dry, rocky planets in our solar system — including Earth. Billions of years ago, comets and asteroids were thought to have delivered water to our planet, triggering the conditions necessary for life. Study co-author Benjamin Zuckerman, a UCLA professor of physics and astronomy, said the composition of matter detected on G238-44 means ice reservoirs may be very common in planetary systems.
“Life as we know it requires a rocky planet covered in a variety of volatile elements such as carbon, nitrogen and oxygen,” Zuckerman said. “The elemental abundances we see on this white dwarf appear to be from a rocky parent and volatile-rich Parent body — this is the first example we’ve found in our study of hundreds of white dwarfs.”
Chaos and Destruction: From Living Stars to Red Giants to White Dwarfs
Theories of planetary system evolution describe the demise of a star as a turbulent, chaotic event that begins with a dramatic expansion into a so-called red giant before rapidly losing its outer layers and collapsing into a white dwarf — a An ultra-dense star about the size of Earth with a mass equivalent to our sun. This process greatly disrupts the orbits of the remaining planets, while smaller objects — asteroids, comets, moons — can be hit like pinballs and sent toward the white dwarf if they venture too close to them.
The study confirms the true scale of the chaos, showing that within 100 million years of the start of the white dwarf phase, the star can capture and consume material from both its nearby asteroid belt and the distant Kuiper belt-like region.
Although astronomers have catalogued more than 5,000 planets beyond our solar system, the only planet with some direct knowledge of its inner composition is Earth. UCLA astronomy researcher Beth Klein noted that since the material that joins G238-44 is a building block for major planets, this white dwarf cannibalism presents a unique opportunity to take planets apart and see how they originally orbited What are stars made of when they form.
The team measured elements such as nitrogen, oxygen, magnesium, silicon and iron present in the white dwarf’s atmosphere. The abundance of iron they detected was very high, Johnson said, evidence for the metallic cores of terrestrial planets such as Earth, Venus, Mars and Mercury. The unexpectedly high nitrogen abundance led them to conclude that ice bodies were also present.
“The closest match to our data is an almost two-to-one mix of Mercury-like material and comet-like material, the latter consisting of ice and dust. The metallic iron and nitrogen ice, respectively, suggest that the conditions under which the planets formed were very different. No known solar system body has so many of these two substances,” Johnson said.
The researchers note that our own sun will likely end up in about 5 billion years from what G238-44 sees. They predict that during the sun’s red giant phase, Earth may be completely vaporized along with the inner planets.
Johnson noted that many of the asteroids in our solar system’s main asteroid belt will have their orbits disturbed by Jupiter’s gravitational pull, while also falling on the remnants of the white dwarf that will be turned from the sun.
For more than two years, the UCLA team, along with colleagues at UC San Diego and the University of Kiel in Germany, has worked to unravel the mystery of G238-44 by analyzing elements detected on the white dwarf.
Their analysis included data from NASA’s decommissioned Far Ultraviolet Spectrometer, the High-Resolution Echelon Spectrometer at the Keck Observatory in Hawaii, and the Hubble Space Telescope’s Cosmic Origins and Space Telescope Imaging Spectrometers. The Hubble Space Telescope is an international collaboration between NASA and ESA.
The team’s results were presented at a June 15 American Astronomical Society press conference.