How the Sun Could Make a New Batch of Planets
Sometimes, stars get a second chance at world-building.
Stars like the sun die not with a bang, but a whimper. They slowly bloat into red bags of gas, too large to hold onto themselves. They lose wisps of their outer atmospheres to space, and their cores collapse. Gravity crushes them into white dwarf stars, while their outer shells inflate and form luminous nebulas that can be seen from thousands of light years away.
But for some stars, there’s creation before oblivion. Toward the end of their lives, some stars might make new planets.
Planet-building is a task generally left to young stars, those that are only a few million years old and still surrounded by the gas and dust that gave birth to them. But astronomers using the Very Large Telescope just caught the first glimpse of a hot disc of dust around a very old, dying star.
The disc looks like a glowing ring left by a coffee mug, and capturing it was a major technical feat, involving four different telescopes. At the center of the ring is the main star in this binary system, which is currently a red giant on its way to becoming a white dwarf. The ring around it is dust and gas made of the star’s own remnants. There’s also a smaller, younger companion in this system, about as far from the main star as Mars is from our sun. That star is about the same age and size as the sun.
Hans Van Winckel and Michel Hillen of the University of Leuven in Belgium study multiple-star systems, and they chose this target, which is about 4,000 light years from Earth, because there were promising signs that it was surrounded by dust. Astronomers try to study these dusty discs in many stages of a star’s life to understand how planets form, Van Winckel says.
It turns out that the discs of this dying star and those of infant stars look very similar, despite being separated in time by billions of years. Just as with young stars, the ring starts at a safe distance—any closer, and the star’s radiation would blow tiny dust grains apart. The grains are about a millimeter across, pretty big for space dust. The VLT interferometer’s remarkable sensitivity allowed the research team to see the whole cloud. For comparison’s sake, the resolution is high enough that they’d be able to distinguish the size and shape of a quarter seen from a distance of 1,250 miles away, according to Jacques Kluska of the University of Exeter in the U.K.
For astronomers who look at the discs around young stars, the result is intriguing, says Lisa Prato, an astronomer at Lowell Observatory who focuses on infant sun-like stars.
“Planets are really robust. If they can form around a dead star, they can form around any kind of star, you would think, as long as you have the material,” she says.
It’s not totally clear how much material surrounds this star, but the grains have been forming for only about 1,000 years, according to Van Winckel. At this stage in the star’s life, things change very quickly, so there may not be enough time left for planets to coalesce before the star winks out. “The discs are probably not living long enough for real planets to be formed,” he says. “There is simply not enough time.”
But the time scales for planet formation are still hotly debated, so we shouldn’t rule it out yet. Astronomers could use the so-called wobble method to scrutinize the star and look for signs of any planets, Prato says.
Does that mean the sun might sprout new planets in its death throes? Probably not, Prato says. The sun’s fate is to become a cool white dwarf, surrounded by a wispy halo of diffuse gas.
“It will cool off over many, many years and just die. I don’t think there’s enough material that will come off a star the mass of our sun to make any kind of disc, or any kind of system that you could form a new generation of planets,” Prato says. “I think we’re it.”