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Most stars may have much more time to form planets than previously thought

Good news for late bloomers: Planets may have millions of years more time to rise around most stars than previously thought.

Planet-forming disks around young stars typically last 5 million to 10 million years, researchers report in a study posted Oct. 6 on The disk’s lifetime, based on a survey of nearby young star clusters, is well beyond the previous estimate of 1 million to 3 million years.

“One to three megayears is a very strong constraint on planet formation,” says astrophysicist Susanne Pfalzner of Forschungszentrum Jülich in Germany. “The discovery that we have a lot of time just relaxes everything” for building planets around young stars.

Large and small planets develop in the discs of gas and dust swirling around young stars (SN: 5/20/20). Once a disk disappears, it’s too late to create new worlds.

Previous studies have estimated disk lifespans by looking at the fraction of young stars of various ages that still have disks, particularly by observing star clusters of known ages. But Pfalzner and her colleagues discovered something strange: The farther a star cluster is from Earth, the shorter the disk’s estimated lifespan. That didn’t make sense, she says, because why would a protoplanetary disk’s lifetime depend on how far away it is from us?

The answer is quite simple: it doesn’t. But in clusters farther away, it’s harder to see most stars. “If you look at greater distances, you’ll see stars with a higher mass,” Pfalzner says, because those stars are brighter and easier to see. “You don’t actually see the low-mass stars.” But the stars with the lowest masses make up the vast majority. These stars, orange and red dwarfs, are cooler, smaller and fainter than the Sun.

So Pfalzner and her 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 “the low-mass stars have a much longer disk life, between 5 and 10 mega years,” than astronomers realized, she says. In contrast, disks around higher-mass stars are known to spread more quickly, perhaps because the bright light from their suns pushes the gas and dust away more quickly.

“I wouldn’t say this is definitive proof” for such long disk life around orange and red dwarfs, said Álvaro Ribas, an astronomer at the University of Cambridge who was not involved in the work. “But it’s pretty convincing.”

To bolster the result, he would like to see observations from more distant star clusters — perhaps with the James Webb Space Telescope — to determine the fraction of the faintest stars that have preserved their planet-forming disks between 5 million and 20 million years (SN: 10/11/22).

If the disks around the lowest-mass stars are indeed long-lived, that could explain a difference between our solar system and those of most red dwarfs, Pfalzner says. The latter often miss gas giants such as Jupiter and Saturn, which are about 10 times the diameter of Earth. Instead, those stars often have numerous ice giants like Uranus and Neptune, about four times the diameter of Earth. Perhaps Neptune-sized planets would appear in greater numbers if a planet-making disk lasts longer, Pfalzner says, explaining why these worlds tend to be in abundance around smaller stars.

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