If there were such a thing as a photo album of the universe, it might include snapshots of pancake-like disks of gas and dust, swirling around newly formed stars across the Milky Way. Known as planet-forming disks, they are believed to be a short-lived feature around most, if not all, young stars, providing the raw materials for planets to form.

Most of these planetary nurseries are short-lived, typically lasting only about 10 million years – a fleeting existence by cosmic standards. Now, in a surprising find, researchers at the University of Arizona have discovered that disks can grace their host stars much longer than previously thought, provided the stars are small – one-tenth of the sun's mass or less. 

In a paper published in the Astrophysical Letters Journal, a research team led by Feng Long of the U of A Lunar and Planetary Laboratory, in the College of Science, reports a detailed observation of a protoplanetary disk at the ripe old age of 30 million years. Presenting the first detailed chemical analysis of a long-lived disk using NASA's James Webb Space Telescope, the paper provides new insights into planet formation and the habitability of planets outside our solar system. 

"In a sense, protoplanetary disks provide us with baby pictures of planetary systems, including a glimpse of what our solar system may have looked like in its infancy," said Long, the paper's lead author and a Sagan Fellow with the Lunar and Planetary Laboratory. 

As long as the star has a certain mass, high-energy radiation from the young star blows the gas and dust out of the disk, and it can no longer serve as raw material to build planets, Long explained. 

The team observed a star with the official designation WISE J044634.16–262756.1B – more conveniently known as J0446B – located in the constellation Columba (Latin for "dove") about 267 light-years from Earth. The researchers found that its planet-forming disk has lasted about three times longer than expected.

"Although we know that most disks disperse within 10 million to 20 million years, we are finding that for specific types of stars, their disks can last much longer," Long said. "Because materials in the disk provide the raw materials for planets, the disk's lifespan determines how much time the system has to form planets."

Even though tiny stars retain their disks longer, their disk's chemical makeup does not change significantly. The similar chemical composition regardless of age indicates that the chemistry does not change drastically even as a disk reaches an advanced age. Such a long-lived, stable chemical environment could provide planets around low-mass stars with more time to form. 

By analyzing the disk's gas content, the researchers ruled out the possibility that the disk around J0446B is a so-called debris disk, a longer-lasting type of disk that consists of second-generation material produced by collisions of asteroid-like bodies. 

"We detected gases like hydrogen and neon, which tells us that there is still primordial gas left in the disk around J0446B," said Chengyan Xie, a doctoral student at LPL who also contributed to the study.

The confirmed existence of long-lived disks rich in gases has implications for life outside our solar system, according to the authors. Of particular interest to researchers is the TRAPPIST-1 system, located 40 light-years from Earth, consisting of a red dwarf star and seven planets similar in size to Earth. Three of those planets are located in the "habitable zone," where conditions allow for liquid water to exist and offer the potential for life to form, at least in principle. 

Because stars with long-lived planetary disks fall into a similar mass category as the central star in the TRAPPIST-1 system, the existence of long-lived disks is especially interesting for the evolution of planetary systems, say Long and her co-authors. 

"To make the specific arrangement of orbits we see with TRAPPIST-1, planets need to migrate inside the disk, a process that requires the presence of gas," said Ilaria Pascucci, a professor of planetary sciences at LPL who co-authored the study. "The long presence of gas we find in those disks might be the reason behind TRAPPIST-1's unique arrangement."

Long-lived disks have not been found for high-mass stars such as the sun, since stars in such systems evolve much more quickly and planets have less time to form. Although our solar system took a different evolutionary route, long-lived disks can tell researchers a lot about the universe, the authors noted, because low-mass stars are believed to vastly outnumber sun-like stars. 

"Developing a better understanding of how low-mass star systems evolve and getting snapshots of long-lived disks might help pave the way to filling out the blanks in the photo album of the universe," Long said.