A dead star caught violently tearing apart the planetary system

Both rocky and icy objects have been identified among the debris on the surface of a white dwarf star

“Take out your dead!” Loops in the air in the classic movie “Monty Python and the Holy Grail”, a parallel scene of what happens around[{” attribute=””>white dwarf star in a nearby planetary system. The dead star is “ringing” its own bell, calling out to the “dead” to collect at its footsteps. The white dwarf is all that remains after a Sun-like star has exhausted its nuclear fuel and expelled most of its outer material – decimating objects in the planetary system that orbit it. What’s left is a band of players with unpredictable orbits that – despite protests that they “aren’t dead yet!” – will ultimately be captured by the central star.

How do we know? The bodies consumed by the star leave telltale “fingerprints” – caught by the Hubble Space Telescope and other NASA observatories – on its surface. The spectral evidence shows that the white dwarf is siphoning off both rocky-metallic and icy material – debris from both its system’s inner and outer reaches. Uncovering evidence of icy bodies is intriguing, since it implies that a “water reservoir” might be common on the edges of planetary systems, improving the chances for the emergence of life as we know it.

The pain of a star’s death has so violently disrupted the planetary system that the dead star it left behind, called a white dwarf, is pulling debris from both the system’s inner and outer cusps. This is the first time astronomers have observed a white dwarf star consuming both mineral and icy rocky materials, which are the components of planets. Archival data from NASA’s Hubble Space Telescope and other NASA observatories were essential in diagnosing this case of cosmic cannibalism. The results help describe the violent nature of advanced planetary systems and could tell astronomers about the makeup of newly formed systems. credit: NASA’s Goddard Space Flight Center; Main Producer: Paul Morris

The dead star caught tearing apart the planetary system

The pain of a star’s death has so violently disrupted the planetary system that the dead star it left behind, called a white dwarf, is pulling debris from both the system’s inner and outer cusps. This is the first time astronomers have observed a white dwarf star consuming both mineral and icy rocky materials, which are the components of planets.

Archival data from NASA’s Hubble Space Telescope and other NASA observatories were essential in diagnosing this case of cosmic cannibalism. The results help describe the violent nature of advanced planetary systems and could tell astronomers about the makeup of newly formed systems.

The results are based on the analysis of material captured by the atmosphere of the nearby white dwarf star G238-44. A white dwarf is what’s left of a star like our sun after it strips away its outer layers and stops burning fuel through nuclear fusion. “We’ve never seen these two types of bodies accumulating on a white dwarf at the same time,” said Ted Johnson, principal investigator and a recent graduate of the University of California, Los Angeles (UCLA). “By studying these white dwarfs, we hope to gain a better understanding of the still intact planetary systems.”

This illustration of the G238-44 planetary system traces its destruction. A small white dwarf star is at the center of the action. The extremely faint accretion disk consists of bits of tattered bodies falling on the white dwarf. Asteroids and the remaining planetary bodies form a reservoir of material surrounding the star. The larger gas giant planets may still be in the system. Much further away is a belt of icy bodies such as comets, which also eventually feed the dead star. Credit: NASA, ESA, Joseph Olmsted (STScI)

The findings are also interesting because small, icy bodies are credited with colliding and “irrigating” the dry, rocky planets in our solar system. Billions of years ago, comets and asteroids are believed to have delivered water to Earth, creating the conditions necessary for life as we know it. The composition of the bodies observed raining on the white dwarf suggests that icy reservoirs may be common among planetary systems, Johnson said.

“Life as we know it requires a rocky planet covered with a variety of elements such as carbon, nitrogen and oxygen,” said Benjamin Zuckerman, a UCLA professor and co-author. “The abundance of elements we see on this white dwarf appears to require a main body that is rocky and rich in volatility – the first example we found among studies of hundreds of white dwarfs.”

demolish derby

Planetary system evolution theories describe the transition between the red giant star and the white dwarf phases as a chaotic process. A star is rapidly losing its outer layers and the orbits of its planets change dramatically. Small objects, such as asteroids and dwarf planets, can venture close to giant planets and fall toward the star. This study confirms the true scale of this violent chaotic phase, showing that within 100 million years after the onset of the white dwarf phase, the star is able to simultaneously capture and consume material from the asteroid belt and Kuiper belt-like regions.

The estimated total mass devoured by the white dwarf in this study may not be greater than the mass of an asteroid or a small moon. While the presence of at least two objects consumed by the white dwarf is not directly measured, it is likely that one is as rich in minerals as an asteroid and the other is an icy object similar to what is found on the fringes of our solar system in the Kuiper Belt.

Although astronomers have classified more than 5,000 exoplanets, Earth is the only planet for which we have some direct knowledge of its internal makeup. White dwarf cannibalism provides a unique opportunity to break apart planets and learn what they were made of when they first formed around the star.

The team measured the presence of nitrogen, oxygen, magnesium, silicon and iron, among other elements. The discovery of iron in very large quantities is evidence of the existence of the metal cores of terrestrial planets, such as Earth,[{” attribute=””>Venus, Mars, and Mercury. Unexpectedly high nitrogen abundances led them to conclude the presence of icy bodies. “The best fit for our data was a nearly two-to-one mix of Mercury-like material and comet-like material, which is made up of ice and dust,” Johnson said. “Iron metal and nitrogen ice each suggest wildly different conditions of planetary formation. There is no known solar system object with so much of both.”

Death of a Planetary System

When a star like our Sun expands into a bloated red giant late in its life, it will shed mass by puffing off its outer layers. One consequence of this can be the gravitational scattering of small objects like asteroids, comets, and moons by any remaining large planets. Like pinballs in an arcade game, the surviving objects can be thrown into highly eccentric orbits.

“After the red giant phase, the white dwarf star that remains is compact – no larger than Earth. The wayward planets end up getting very close to the star and experience powerful tidal forces that tear them apart, creating a gaseous and dusty disk that eventually falls onto the white dwarf’s surface,” Johnson explained.

The researchers are looking at the ultimate scenario for the Sun’s evolution, 5 billion years from now. Earth might be completely vaporized along with the inner planets. But the orbits of many of the asteroids in the main asteroid belt will be gravitationally perturbed by Jupiter and will eventually fall onto the white dwarf that the remnant Sun will become.

For over two years, the research group at UCLA, the University of California, San Diego, and the Kiel University in Germany, has worked to unravel this mystery by analyzing the elements detected on the white dwarf star cataloged as G238-44. Their analysis includes data from NASA’s retired Far Ultraviolet Spectroscopic Explorer (FUSE), the Keck Observatory’s High Resolution Echelle Spectrometer (HIRES) in Hawaii, and the Hubble Space Telescope’s Cosmic Origins Spectrograph (COS) and Space Telescope Imaging Spectrograph (STIS).

The team’s results were presented at an American Astronomical Society (AAS) press conference on Wednesday, June 15, 2022.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C.