The Ring Nebula is a well-known space icon shaped like a doughnut about 2,500 light-years away in the constellation Lyra.
But astronomers think it's been holding a secret that only the penetrating gaze of the James Webb Space Telescope, a joint project of NASA and the European and Canadian space agencies, could reveal. Using Webb's infrared-sensing MIRI instrument, scientists got a clear sightline to the small but scorching star at its center.
The shriveled core — a white dwarf — is all that remains of the star on its deathbed, having molted its outermost layers. When researchers zoomed in, they got a surprise. Surrounding the withering star was a disk of dust, much like the kind found around new stars that are in their prime planet-birthing years. It was kind of like seeing a pregnant octogenarian in a nursing home.
This is just the second time scientists have observed such a disk around a star at the end of its life. And though they can't actually see baby planets in their nebula images, they are now wondering if these space environments could trigger a second generation of planets, long after the original brood of worlds formed around the star.
The discovery, published in The Astrophysical Journal, is "raising questions about their nature, formation, longevity, and potentially a second phase of planet formation," the authors wrote.
Unlike giant stars that explode into a supernova and collapse into a black hole when their time is up, a medium star gradually runs out of nuclear fuel and suffers a more prolonged death. These stars, like our midsize sun in perhaps 5 billion years, decline into so-called planetary nebulas, a confusing misnomer for the phenomenon because they have more to do with aging stars than planets.
Scientists have discovered a few thousand planetary nebulas in the Milky Way. Those include the Ring Nebula, aka NGC 6720 and Messier 57. With Webb, the researchers could see a compact dust cloud around the central white dwarf creating it.
"These are first seen as the gas-and-dust-rich planet-forming disks in young stellar objects, and are an integral part of the star formation process itself," the authors wrote. "Remarkably, dusty disks or disk-like structures manifest themselves again as these stars reach the ends of their lives."
Such a rare sighting is believed to have happened once before, when astronomers got a look at the Southern Ring Nebula with Webb, at wavelengths far beyond what people can see with their eyes. In the first images, astronomers were amazed to spot the true source of the nebula, said Karl Gordon, an astronomer at the Space Telescope Science Institute in Baltimore.
"We knew this was a binary star (beforehand), but we effectively didn't really see much of the actual star that produced the nebula," he said during a 2022 news conference. "But now in MIRI, this star glows red because it has dust around it."
The dust cloud around the Ring Nebula is composed of tiny grains of amorphous silicate, a glass-like substance, according to the new paper. These particles are miniscule, perhaps less than one-thousandth the thickness of a human hair. The cloud itself stretches thousands of times wider than the distance between the Earth and the sun.
The researchers noticed something else intriguing at the center of the nebula. The white dwarf's brightness keeps changing. That might be a clue that another star is lurking in the shadows nearby, perhaps a small-but-feisty red dwarf star. While scientists haven’t directly spotted the buddy yet, they can infer it's there from patterns in the nebula.
The study could help confirm earlier findings that suggested the star had two companions — one far in the outskirts of the system and another nearby. A close star could explain the strange arcs and rings seen around the nebula.
