This is the first case of cosmic cannibalism in which astronomers have observed a white dwarf consuming both rocky-metallic material, likely from a nearby asteroid, and icy material, presumed to be from a body similar to those found in the Kuiper belt at the fringe of our own solar system.

“We have never seen both of these kinds of objects accreting onto a white dwarf at the same time,” said lead researcher Ted Johnson, a physics and astronomy major at UCLA who graduated last week. “By studying these white dwarfs, we hope to gain a better understanding of planetary systems that are still intact.”

The findings are based on an analysis of materials captured by the atmosphere of G238-44, a white dwarf some 86 light-years from Earth, using archival data from the Hubble Space Telescope and additional NASA satellites and observatories. A white dwarf is the burned-out core that remains after a star like our sun sheds its outer layers and stops burning fuel through nuclear fusion.

As surprising as the white dwarf’s wide-ranging diet is, the findings are also intriguing because astronomers believe icy objects crashed into and irrigated dry, rocky planets in our solar system — including Earth. Billions of years ago, comets and asteroids are thought to have delivered water to our planet, sparking the conditions necessary for life. The makeup of the material detected raining onto G238-44 implies that icy reservoirs might be common among planetary systems, said research co-author Benjamin Zuckerman, a UCLA professor of physics and astronomy.

“Life as we know it requires a rocky planet covered with a variety of volatile elements like carbon, nitrogen and oxygen,” Zuckerman said. “The abundances of the elements we see on this white dwarf appear to have come from both a rocky parent body and a volatile-rich parent body — the first example we’ve found among studies of hundreds of white dwarfs.”

Chaos and destruction: From living star to red giant to white dwarf

Theories of planetary-system evolution describe the demise of a star as a turbulent, chaotic event, one that begins when it first balloons exponentially into what is known as a red giant and then quickly loses its outer layers, collapsing into a white dwarf — a super-dense star about the size of Earth, with a mass of our sun. The process dramatically disrupts the remaining planets’ orbits, and smaller objects — asteroids, comets, moons — that venture too close to them can be scattered like pinballs and sent hurtling toward the white dwarf.

This study confirms the true scale of the chaos, showing that within 100 million years after the beginning of its white dwarf phase, the star is able to simultaneously capture and consume material from its nearby asteroid belt and its far-flung Kuiper belt-like regions.

Though astronomers have cataloged more than 5,000 planets outside our solar system, the only planet whose interior makeup we have some direct knowledge of is Earth. Because the materials accreting onto G238-44 are representative of the building blocks of major planets, this white dwarf cannibalism provides a unique opportunity to take planets apart and see what they were made of when they first formed around the star, said UCLA astronomy researcher Beth Klein, a member of the team.

The team measured the presence of nitrogen, oxygen, magnesium, silicon and iron, among other elements, in the white dwarf’s atmosphere. Their detection of iron in very high abundance is evidence for metallic cores of terrestrial planets, like Earth, Venus, Mars and Mercury, Johnson said. Unexpectedly high nitrogen abundances led them to conclude that icy bodies were also present.

“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.”

The researchers say the ultimate scenario for our own sun some 5 billion years from now will likely be quite similar to what has been seen with G238-44. During the sun’s red giant phase, the Earth might be completely vaporized along with the inner planets, they predict.

The orbits of many of the asteroids in our solar system’s main asteroid belt will be gravitationally perturbed by Jupiter and will also fall onto the white dwarf remnant that the sun will become, he said.

For more than two years, the research group at UCLA, 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 the elements detected on the white dwarf star.

Their analysis included data from NASA’s retired Far Ultraviolet Spectroscopic Explorer, the Keck Observatory’s High Resolution Echelle Spectrometer in Hawaii and the Hubble Space Telescope’s Cosmic Origins Spectrograph and Space Telescope Imaging Spectrograph. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency.

The team’s results were presented at an American Astronomical Society press conference on June 15.

The discovery forms part of a treasure trove of data made available in Gaia’s Data Release 3, which provides the most detailed survey of our galactic neighbourhood to date.

Observations made by the Gaia observatory, which orbits a point in space about 1.5 million km from Earth, will allow astronomers to create the most accurate and complete multi-dimensional map of the Milky Way and better understand our place in the Universe.

Martin Barstow, Professor of Astrophysics and Space Science at the University of Leicester and Director of Strategic Partnerships for Space Park Leicester, is part of the Gaia collaboration to have co-authored multiple papers using the new data.

Other highlights of Data Release 3 include description of ‘starquakes’, stellar DNA and a new binary star catalogue of more than 800,000 binary systems. Before Gaia launched in 2013, only around 30,000 binaries were known in our galaxy.

Professor Barstow said:

“Gaia data has been moving through astronomy like a tidal wave. It’s the most productive observatory we have available to use right now, and it’s transforming both astronomy and our understanding of the Universe.

“This Data Release 3 is a complete step change. All the data we have catalogued using spectra — stellar distances, ages, composition, and more — adds an extra dimension to what we know about the stars in our galaxy, and represents a huge leap forward.”

Data reveals a ‘super Jupiter’ companion

Observations of exoplanets orbiting white dwarfs is notoriously difficult. White dwarfs are the core remnant of stars not massive enough to become a black hole or neutron star.

However, by analysing the motion of the metal-rich white dwarf WD 0141-675 and noting a ‘wobble’ in its orbit, researchers inferred the existence of a companion object with a mass around nine times that of Jupiter. Too small to be a star, this must be an exoplanet.

This ‘super Jupiter’ is only the third known exoplanet to orbit a white dwarf, and makes WD 0141-675 the closest white dwarf to Earth to host a planet.

Stars in their eyes: Gaia more than doubles number of known binaries

Gaia’s Data Release 3 also expands astronomers’ understanding of binary systems, where two stars are gravitationally bound to one another. Sirius, the brightest star visible from Earth (after our Sun), is a binary system comprising a main sequence star, Sirius A, and a faint white dwarf companion, Sirius B.

These new datasets both refine the stellar catalogue of known binaries and add many more new such systems, bringing the total of known binary systems from around 300,000 before Data Release 3 to more than 800,000. Researchers detect binaries using Gaia’s radial velocity spectrometer and a variety of techniques;

• astrometry: by detecting the motion of source objects which are not uniform and are observed to ‘wobble’ or otherwise change direction from what would otherwise be expected;
• photometry: when aligned with Gaia’s line of sight, where one star is observed to pass in front of another and periodically ‘eclipse’ its companion;
• and spectroscopy: these binaries have a radial velocity that varies periodically, depending on whether a star approaches or recedes from our viewpoint at Earth.

Professor Barstow continued:

“We have so much more data on binaries with this release and, crucially, that data is so much more precise than what has gone before.

“Once you have more precise data about a binary system you can work out all sorts of things such as ages and composition: all aspects we haven’t had information on before.

“And by answering those questions, we can start to understand more of the fundamentals about how our Universe works, including how stars live and die.”

HD 53143 — a roughly billion-year-old Sun-like star located 59.8 light-years from Earth in the Carina constellation — was first observed with the coronagraphic Advanced Camera for Surveys on the Hubble Space Telescope (HST) in 2006. It also is surrounded by a debris disk — a belt of comets orbiting a star that are constantly colliding and grinding down into smaller dust and debris — that scientists previously believed to be a face-on ring similar to the debris disk surrounding our Sun, more commonly known as the Kuiper Belt.

The new observations were made of HD 53143 using the highly-sensitive Band 6 receivers on ALMA, an observatory co-operated by the U.S. National Science Foundation’s National Radio Astronomy Observatory (NRAO), and have revealed that the star system’s debris disk is actually highly eccentric. In ring-shaped debris disks, the star is typically located at or near the center of the disk. But in elliptically-shaped eccentric disks, the star resides at one focus of the ellipse, far away from the disk’s center. Such is the case with HD 53143, which wasn’t seen in previous coronagraphic studies because coronagraphs purposely block the light of a star in order to more clearly see nearby objects. The star system may also be harboring a second disk and at least one planet.

“Until now, scientists had never seen a debris disk with such a complicated structure. In addition to being an ellipse with a star at one focus, it also likely has a second inner disk that is misaligned or tilted relative to the outer disk,” said Meredith MacGregor, an assistant professor at the Center for Astrophysics and Space Astronomy (CASA) and Department of Astrophysical and Planetary Sciences (APS) at CU Boulder, and the lead author on the study. “In order to produce this structure, there must be a planet or planets in the system that are gravitationally perturbing the material in the disk.”

This level of eccentricity, MacGregor said, makes HD 53143 the most eccentric debris disk observed to date, being twice as eccentric as the Fomalhaut debris disk, which MacGregor fully imaged at millimeter wavelengths using ALMA in 2017. “So far, we have not found many disks with a significant eccentricity. In general, we don’t expect disks to be very eccentric unless something, like a planet, is sculpting them and forcing them to be eccentric. Without that force, orbits tend to circularize, like what we see in our own Solar System.”

Importantly, MacGregor notes that debris disks aren’t just collections of dust and rocks in space. They are a historical record of planetary formation and how planetary systems evolve over time. and provide a peek into their futures. “We can’t study the formation of Earth and the Solar System directly, but we can study other systems that appear similar to but younger than our own. It’s a bit like looking back in time,” she said. “Debris disks are the fossil record of planet formation, and this new result is confirmation that there is much more to be learned from these systems and that knowledge may provide a glimpse into the complicated dynamics of young star systems similar to our own Solar System.”

Dr. Joe Pesce, NSF program officer for ALMA, added, “We are finding planets everywhere we look, and these fabulous results by ALMA are showing us how planets form — both those around other stars and in our own Solar System. This research demonstrates how astronomy works and how progress is made, informing not only what we know about the field but also about ourselves.”

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