Good news for late bloomers. It is possible that planets have millions more years to appear around stars than previously believed.
Planet-making disks around young stars Typically, they last between 5 million and 10 million years.According to researchers, the study was published on Oct. 6 at arXiv.org. This disk lifetime is based on a survey by nearby young star clusters and is significantly longer than the previous estimate that it was between 1 million and 3 million years.
“One to three megayears is a really strong constraint for forming planets,” says astrophysicist Susanne Pfalzner of Forschungszentrum Jülich in Germany. “Finding that we have a lot of time just relaxes everything” for building planets around young stars.
Large and small planets develop in the Disks of gas- and dust-rich material that swirl around young stars (SN: 5/20/20). Once a disk vanishes, it’s too late to make any more new worlds.
Past studies have estimated disk lifetimes by looking at the fraction of young stars of different ages that still have disks — in particular, by observing star clusters with known ages. Pfalzner and her coworkers discovered something strange: The disk lifetime of a star cluster that is farther from Earth is shorter. She says that this was absurd because the life expectancy of a protoplanetary disc should not depend on its distance from Earth.
The answer is quite simple: It doesn’t. But in clusters that are farther away, it’s harder to see most stars. “When you look at larger distances, you see higher-mass stars,” Pfalzner says, because those stars are brighter and easier to see. “You basically don’t see the low-mass stars.” But the lowest-mass stars constitute the vast majority. These stars, also known as orange and red dwarfs are smaller, fainter, and cooler than the sun.
Pfalzner, along with her colleagues, examined only young star clusters that were within 650 light-years from Earth. They found that the fraction stars that had planet-making disks was significantly higher than the previous reports. This analysis showed that “the low-mass stars have much longer disk lifetimes, between 5 and 10 megayears,” than astronomers realized, she says. In contrast, disks around higher-mass stars are known to disperse faster than this, perhaps because their suns’ brighter light pushes the gas and dust away more quickly.
“I wouldn’t say that this is definite proof” for such long disk lifetimes around orange and red dwarfs, says Álvaro Ribas, an astronomer at the University of Cambridge who was not involved with the work. “But it’s quite convincing.”
To bolster the result, he’d like to see observations of more distant star clusters — perhaps with the James Webb Space Telescope — to determine the fraction of the faintest stars that have preserved their planet-making disks between 5 million and 20 million years (SN: 10/11/22).
Pfalzner believes that if the disks surrounding the lowest mass stars have long lives, this could explain the difference in our solar system from most red dwarfs. These stars are often missing gas giants such as Saturn and Jupiter, which are approximately 10 times larger than Earth. These stars are often home to ice giants, like Neptune and Uranus, which are approximately four times as large as Earth. Pfalzner suggests that Neptune-sized worlds may be more common if a planet-making disc lasts longer. This could explain why these worlds tend not to abound around smaller stars.
