Science

Astronomers Reveal New Details of How Stars Devour Planets

Astronomers have been witnessing the ends of worlds for millennia. Even in antiquity, sky watchers noted the rare star suddenly bursting into brightness and then fading away over months or years. These outbursts are supernovae, explosive stellar deaths that can also annihilate a star’s accompanying planets. Today modern researchers can see black holes shredding entire stars (alongside any unseen companions) and find evidence for debris from shattered rocky worlds raining down on white dwarf stars, which are the cinderlike corpses of expired suns.

Such events only hint at a planet’s death throes, however. Now results from a new preprint study are pointing the way to more direct observations of annihilation, forecasting the types of worlds that can be engulfed by their stars and the resulting boosts in stellar brightness that ensue. Before long, astronomers may actually see the telltale flaring produced during a planet’s final moments as it is swallowed by its star. And they may even find some that manage to survive the fiery stellar plunge.

Planetary engulfment “is something that is probably common,” says Ricardo Yarza, first author of the new research and a graduate student at the University of California, Santa Cruz. “It’s also not understood in detail.” Yarza and his colleagues modeled how planets of various masses would interact with the outer layers of dying stars known as red giants.

Their results suggest that the largest planets—at least 10 times as massive as Jupiter—can survive by blowing off their star’s outer layers, increasing that star’s brightness for a period anywhere between a few hours to a few thousand years. Smaller worlds can cause observable effects, too, such as a brief stellar flickering. But once engulfed, they should not escape a hungry star’s grasp. The team first presented its research at the 240th meeting of the American Astronomical Society in Pasadena, Calif.

Red giants are a closing phase of life for most solar-mass and heavier stars; our own sun is slated to become one some five billion years from now. They emerge when a star runs out of hydrogen fuel at its core and begins burning helium instead, releasing excess energy that causes the star to swell to 100 times or more its previous size. As it expands, the star can engulf any close-orbiting worlds. (This will happen to Mercury and Venus.) But as the star puffs up, it tends to expel its outer layers, too, progressively losing mass and diluting its gravitational grip. This allows some planets to avoid destruction by drifting outward in their orbit. (Earth may eventually escape engulfment in this way.) At the same time, this shrinking stellar mass also amplifies the gravitational tug the ballooning star feels from nearby planets, raising tides on the star that can siphon away the tugging planet’s momentum, causing the world to spiral in to its potential doom. But while its fate may seem sealed once consumed by its star, not every planet is destined for catastrophe.

The Point of No Return

Although bulky planets are more likely to survive, whether any particular world succumbs or endures comes down to how exactly it interacts with the red giant once it has been engulfed. Inside a star’s tempestuous atmosphere, a planet’s motions can be dominated by gravitational forces, making it sink toward the stellar core, where it will be completely shredded. At the same time, the friction generated by its journey through the stellar atmosphere slows the planet’s descent and dumps energy into the gas. If, through friction, a planet can sufficiently stir up the gas enveloping it at a star’s outskirts, the star’s outermost layer can expand and billow away, setting the planet free.

“If it’s able to blow away the layers before it reaches the point of no return, then it will survive,” says Catriona McDonald, a graduate student who studies white dwarfs at the University of Warwick in England, who was not part of the new study.

It’s not just about being big enough, however: to survive, a planet’s entry into its star must be precisely timed. If that entry occurs too early in the red giant transition, the star’s atmospheric layers will be dense, making them far more resistant to being tossed away. As an evolving star continues to shed material, its outer layers become more diffuse. All else being equal, a planet entering a star later in the red giant phase is more likely to survive because it can more easily peel away the star’s tenuous outer layers.

How long the planet is engulfed by the star can play an important role, too. “The more time the planet spends inside the star, the longer it’s going to be affected by the drag forces, and the more energy it’s going to be able to deposit,” Yarza says. Extended exposure to atmospheric drag in a star’s outer layers slows the planet’s journey, giving it more time to dump energy required to blow off the stellar envelope. Only the biggest, bulkiest planets are sizable enough to benefit from this effect.

Gone but Not Forgotten

Whether big or small, however, once irredeemably swallowed, a planet can still leave behind visible traces of its fate.

For example, astronomers have noticed for decades that some giant stars are rotating much faster than expected. Devoured planets are one likely culprit for those souped-up stellar spins.

“The more massive the object that is being eaten, the larger the effect,” says Smadar Naoz, a researcher at the University of California, Los Angeles, who was not a part of the study. In some cases, Naoz says, eating a planet could cause a star to spin so fast that it begins shedding its outer layers of gas; as many as 40 percent of the fast-rotating stars in open clusters—young, loosely bound collections of stars—are consistent with having eaten planets, she says.

Red giants may also carry crumbs of recently consumed worlds in their outer layers. Roughly 1 percent of observed red giants are anomalously rich in lithium, Yarza says. “This is very strange because lithium is very easy to burn [in stars],” he says. But this overabundance can be explained by modeling how a doomed planet gradually disintegrates as it falls deeper into a star. Plumes of stellar material welling up from beneath can carry lithium-rich pieces of the destroyed world back to the red giant’s exterior, making them detectable.

A Brighter Glow

Previous studies have predicted that stars eating their planets could briefly shine brighter by several orders of magnitude. By calculating how the energy of an engulfed planet is transferred into a star, Yarza and his colleagues confirmed and refined these earlier predictions from the planetary perspective. While the largest worlds could cause their stars to brighten for thousands of years, Yarza says, even a Jupiter-sized planet could cause a blip lasting a few hours.

“It’s nice to corroborate that the authors have now reached the same conclusion with different methods,” says Eva Villaver, a professor at Spain’s Center for Astrobiology, who studies how stars interact with their environments.

The brightening effect should be detectable with current instruments such as the Zwicky Transient Factory or with upcoming facilities such as the Vera C. Rubin Observatory, according to astronomer Jason Nordhaus, a researcher at the Rochester Institute of Technology, who was not a part of the study. “We are getting to the point where, if you’ve got a decade of images, you can start going through and looking for something that sort of matches the increased luminosity,” he says.

“The probability of catching a star at this time is very difficult,” Villaver says. “But we may get lucky.” As fleeting as these episodes of stellar brightening may be, astronomers have sheer numbers on their side: basic extrapolations from the decades of surveys and the thousands of exoplanets now known suggest that most every star is accompanied by at least one planet. Sooner or later, it seems, telescopes will clearly reveal some red giants’ unexpected flarings—and with them, the grisly fates of swallowed worlds.

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