Astronomers just discovered 10 billion-year-old planetary debris with Earth-like properties

One of the oldest stars in the Milky Way has been located by astronomers from the University of Warwick.

The oldest rocky and icy planetary system identified is from the oldest white dwarf star in our galaxy and is accreting debris from circling planetesimals, according to a new study published in Monthly Notices of the Royal Astronomical Society on Saturday.

"We're finding the oldest stellar remnants in the Milky Way that are polluted by once Earth-like planets," said Abbigail Elms, lead author of the study, a Ph.D. student at the department of physics, University of Warwick.

"It's amazing to think that this happened on the scale of 10 billion years, and that those planets died way before the Earth was even formed."

"These metal-polluted stars show that Earth isn't unique, there are other planetary systems out there with planetary bodies similar to the Earth," he added.

All stars turn into white dwarfs

White dwarfs, which were created from the oldest stars in our galaxy, shed light on how planetary systems formed and developed around the oldest stars in the Milky Way.

They are so common in the universe—97 percent of all stars will eventually become one—making it crucial to comprehend them.

A white dwarf is a star that has used up all of its fuel, lost all of its outer layers, and is currently cooling and contracting. And most stars, including those like our sun, will eventually turn into white dwarfs.

Two peculiar white dwarfs observed

Two peculiar white dwarfs seen by the European Space Agency's GAIA space observatory were modeled for this study by a team of astronomers.

The researchers analyzed two stars, which are both contaminated by planetary material. One was discovered to be extraordinarily blue, while the other is the weakest and reddest star discovered to date in the nearby galactic neighborhood.

In order to determine how long the "red" star WDJ2147-4035 has been cooling, astronomers used spectroscopic and photometric data from GAIA, the Dark Energy Survey, and the X-Shooter instrument at the European Southern Observatory.

They discovered that the star is approximately 10.7 billion years old, of which 10.2 billion years have been spent cooling as a white dwarf.

Only marginally younger than WDJ2147-4035, the second "blue" star, WDJ1922+0233, was contaminated by planetary debris with a makeup close to the continental crust of Earth.

"The red star WDJ2147-4035 is a mystery as the accreted planetary debris are very lithium and potassium-rich and unlike anything known in our own solar system," Abbigail explained.

"This is a very interesting white dwarf as its ultra-cool surface temperature, the metals polluting it, its old age, and the fact that it is magnetic, makes it extremely rare."

The scientific team came to the conclusion that WDJ1922+peculiar 0233's mixed helium-hydrogen atmosphere is what gives it its unusually blue color despite its cold surface temperature.

The findings

Through the process of spectroscopy, which involves examining the star's light at various wavelengths, it is possible to identify and quantify the elements that are present in the star's atmosphere by detecting when they absorb light at particular wavelengths.

The team uncovered the oldest metal-polluted white dwarf by studying the spectra from WDJ2147-4035, which contains sodium, lithium, and potassium, as well as a possible carbon accretion.

The WDJ2147-4035 red star's debris, which was discovered in its otherwise nearly pure-helium and high-gravity atmosphere, came from an ancient planetary system that endured the star's transformation into a white dwarf, leading astronomers to claim that this is the oldest planetary system around a white dwarf ever found in the Milky Way.

"When these old stars formed more than 10 billion years ago, the universe was less metal-rich than it is now, since metals are formed in evolved stars and gigantic stellar explosions." said professor Pier-Emmanuel Tremblay of the Department of Physics at the University of Warwick.

"The two observed white dwarfs provide an exciting window into planetary formation in a metal-poor and gas-rich environment that was different to the conditions when the solar system was formed."

Study abstract:

We identify two ultra-cool (⁠Teff<4000 K) metal-polluted (DZ) white dwarfs WD J2147−4035 and WD J1922+0233 as the coolest and second coolest DZ stars known to date with Teff≈3050 K and Teff≈3340 K, respectively. Strong atmospheric collision-induced absorption (CIA) causes the suppression of red optical and infrared flux in WD J1922+0233, resulting in an unusually blue color given its low temperature. WD J2147−4035 has moderate infrared CIA yet has the reddest optical colors known for a DZ white dwarf. Microphysics improvements to the non-ideal effects and CIA opacities in our model atmosphere code yields reasonable solutions to observations of these ultra-cool stars. WD J2147−4035 has a cooling age of over 10 Gyr which is the largest known for a DZ white dwarf, whereas WD J1922+0233 is slightly younger with a cooling age of 9 Gyr. Galactic kinematics calculations from precise Gaia EDR3 astrometry reveal these ultra-cool DZ stars as likely members of the Galactic disc thus they could be pivotal objects in future studies constraining an upper age limit for the disc of the Milky Way. We present intermediate-resolution spectroscopy for both objects, which provides the first spectroscopic observations of WD J2147−4035. Detections of sodium and potassium are made in both white dwarfs, in addition to calcium in WD J1922+0233 and lithium in WD J2147−4035. We identify the magnetic nature of WD J2147−4035 from Zeeman splitting in the lithium line and also make a tentative detection of carbon, so we classify this star as DZQH. WD J1922+0233 likely accreted planetary crust debris, while the debris composition that polluted WD J2147−4035 remains unconstrained.