Good news for latecomers: Planets may take millions of years longer to emerge around most stars than previously thought.
Planet-forming disks around young stars usually lasts 5 to 10 million years, the researchers report in a study published Oct. 6 on arXiv.org. That disk lifetime, based on a study of nearby young star clusters, is much longer than the previous estimate of 1 million to 3 million years.
“One to three megayears is a really strong constraint on planet formation,” says astrophysicist Susanne Pfalzner of Forschungszentrum Jülich in Germany. “Finding out that we have a lot of time just relaxes everything” to build planets around young stars.
Large and small planets develop in the disks of gas and dust that revolve around young stars (Serial number: 05/20/20). Once a disk is gone, it’s too late to create any more new worlds.
Previous studies have estimated the lifetime of disks by looking at the fraction of young stars of different ages that still have disks, in particular, by looking at clusters of stars with known ages. But Pfalzner and his colleagues discovered something strange: the farther a star cluster is from Earth, the shorter the disk’s estimated lifetime. That doesn’t make sense, she says, because why should a protoplanetary disk’s lifespan depend on how far away it is from us?
The answer is quite simple: it doesn’t. But in clusters that are farther away, it’s harder to see most of the stars. “When you look at greater distances, you see higher mass stars,” says Pfalzner, because those stars are brighter and easier to see. “Basically, you don’t see the low-mass stars.” But lower-mass stars make up the vast majority. These orange and red dwarf stars are cooler, smaller, and fainter than the sun.
So, Pfalzner and colleagues examined only the closest young star clusters, those within 650 light-years of Earth, and found that the fraction of stars with planet-forming disks was much higher than reported in previous studies. This analysis showed that “low-mass stars have much longer disk lifetimes, between 5 and 10 megayears,” than astronomers previously believed, she says. In contrast, disks around higher-mass stars are known to scatter faster than this, perhaps because the brighter light from their suns pushes out gas and dust more quickly.
“I wouldn’t say this is definitive proof” of such long disk lifetimes around orange and red dwarfs, says Álvaro Ribas, an astronomer at the University of Cambridge who was not involved in the work. “But it’s pretty convincing.”
To reinforce the result, he would 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 retained their planet-forming disks between 5 and 20 million years (Serial Number: 10/11/22).
If the disks around lower-mass stars have long lifetimes, that may explain the difference between our solar system and those of most red dwarfs, says Pfalzner. The latter often lack gas giants like Jupiter and Saturn, which are about 10 times the diameter of Earth. Instead, those stars often have numerous ice giants like Uranus and Neptune, roughly four times the diameter of Earth. Perhaps Neptune-sized planets arise in greater numbers when a planet-creation disk lasts longer, says Pfalzner, which explains why these worlds tend to abound around smaller stars.