The Oldest and Coldest White Dwarf Ever Found has Bizarre Dust Rings Around it

When stars like our Sun exhaust their hydrogen fuel, they enter what is known as their Red-Giant-Branch (RGB) phase. This is characterized by the star expanding to several times it original size, after which they shed their outer layers and become compact white dwarfs. Over the next few billion years, it is believed that these stars will slowly consume any objects and dust rings still close enough to be influenced by their gravity.

However, a citizen scientist named Melina Thévenot recently made a surprising discovery when observing a white dwarf system. Based on data from the Wide-field Infrared Survey Explorer (WISE) mission, this star has been a white dwarf for billions of years, but still has multiple rings of dust around it. Known as LSPM J0207+3331 (or J0207), this discovery could force researchers to reconsider models of planetary systems.

The discovery was made through Backyard Worlds: Planet 9, a project led by Marc Kuchner (an astrophysicist at NASA’s Goddard Space Flight Center) that relies on volunteers to sort through WISE data for new discoveries. Located around 145 light-years away in the constellation Capricornus, astronomers suspect that J0207 might be the first known example of a white dwarf with multiple dust rings, and the oldest.

The discovery was also the subject of a recent study published in The Astrophysical Journal (“A 3 Gyr White Dwarf with Warm Dust Discovered via the Backyard Worlds: Planet 9 Citizen Science Project“). As John Debes, an astronomer at the Space Telescope Science Institute in Baltimore and the lead author on the paper, revealed in a recent NASA press release:

“This white dwarf is so old that whatever process is feeding material into its rings must operate on billion-year timescales. Most of the models scientists have created to explain rings around white dwarfs only work well up to around 100 million years, so this star is really challenging our assumptions of how planetary systems evolve.”

The star was detected by NASA’s Wide-field Infrared Survey Explorer (WISE) mission which picked up a strong infrared signal, suggesting the presence of dust. Based on the rate at which white dwarf stars cool over time, Debes’ team calculated from its surface temperatures – just over 5,800° C (10,500° F) – that J0207 has been in its white dwarf phase for about 3 billion years.

These new findings contradict what astronomers have suspected for some time about the evolution of star systems. In the past, astronomers have observed how planets and asteroids that survive a star’s RGB phase will move farther away after it enters its white dwarf phase. This is due to the star having lost much of its mass, and therefore its gravitational influence on surrounding objects.

Astronomers anticipate that this is what will happen to our Solar System in about 5 billion years. After expanding to encompass Venus, Mercury, and Earth, our Sun will lose its outer layers and become a white dwarf. At this point, the remaining planets and objects (which will likely include the Main Asteroid Belt, the gas giants and the Kuiper Belt) will all drift outwards.

However, in 1 to 4% of cases, white dwarfs have shown infrared emissions that indicate that they are surrounded by dust disks or rings. Scientists theorize that this may be the result of asteroids and comets being kicked from their belts through gravitational interaction with displaced planets and sent towards the star. As these bodies approach the white dwarf, they are torn apart by tidal disruption caused by the star’s strong gravity.

The resulting debris would then form a dusty ring that slowly falls inward and is accreted onto the surface of the star. However, in all previous cases, dust disks and rings have only been observed around white dwarfs that were about one billion years old. This seemed consistent with the notion that older white dwarfs had effectively depleted their supply of asteroids and the resulting dust rings.

This latest discovery effectively makes J0207 the oldest and coldest white dwarf with dust ever to be observed. When she first noticed J0207’s infrared signals, Melina Thévenot thought it was bad data. At the time, she had been searching through the European Space Agency’s Gaia mission archives for brown dwarfs, which are barely detectable beyond their infrared emissions.

Artist’s concept of the Wide-field Infrared Survey Explorer as its orbit around Earth. Credit: NASA/JPL

After consulting WISE infrared data, she realized that it was too bright and distant to be a brown dwarf. Thévenot relayed these findings to the Backyard Worlds: Planet 9 team, which then obtained follow-up observations from the W. M. Keck Observatory in Hawaii. As Thévenot explained:

“That is a really motivating aspect of the search. The researchers will move their telescopes to look at worlds you have discovered. What I especially enjoy, though, is the interaction with the awesome research team. Everyone is very kind, and they are always trying to make the best out of our discoveries.”

What’s more, Debes and his colleagues suspect that J0207 may even have a system of rings. They suggest in their study that this would consist of two distinct components: a thin outer ring where the white dwarf’s gravity breaks up the asteroids, and a wider ring closer to the star. These new observations are likely to force astronomers to reconsider their models for how planetary systems evolve.

“We built Backyard Worlds: Planet 9 mostly to search for brown dwarfs and new planets in the Solar System,” Kuchner said. “But working with citizen scientists always leads to surprises. They are voracious – the project just celebrated its second birthday, and they’ve already discovered more than 1,000 likely brown dwarfs. Now that we’ve rebooted the website with double the amount of WISE data, we’re looking forward to even more exciting discoveries.”

The life-cycle of a Sun-like star from protostar (left side) to red giant (near the right side) to white dwarf (far right). Credit: ESO/M. Kornmesser

In addition to forcing a rethink of how planetary system evolve over time, this research could provide clues about the future of our own Solar System. Once our Sun become a white dwarf, it is likely to spend the next few billion years consuming asteroids and KBOs that are kicked out of their belts by the surviving gas giants. According to this latest study, it could also have a ring of dust around several billion years later.

And while this discovery was a boon for astronomers, it also demonstrates what is possible thanks to collaborations between scientific organizations and citizen scientists. In the age of exoplanet research and advanced astronomy, the sheer volume of data makes such collaborations not only necessary, but highly lucrative.

Further Reading: NASA, The Astrophysical Journal Letters