What happens when a red giant star devours a planet?

Astrophysicists have long known that as stars like the Sun age, they reach a stage where they swell into large, bulging stars called red giants, whose diameters are many times their young ones.

These stars could expand enough to consume some of the planets orbiting them, says Ricardo Yarza, a graduate student in astronomy and astrophysics at the University of California, Santa Cruz, US. “This is something we know will happen to Mercury and Venus in our solar system,” he says.

This is clearly not good for a planet. But what happens to a star when it swallows a planet?

If the planet is small, maybe not so much. In our solar system, comets often fall into the sun, with little apparent effect. And although Mercury and Venus are much larger than comets, they are still small compared to the Sun itself.

But many planetary systems have nearby planets much larger than Mercury or Venus. Many of these systems will eventually see at least one planet swallowed up, Yarza said at a recent meeting of the American Astronomical Society in Pasadena, California.

One effect is that the planet will transfer its orbital momentum to the star as it sinks into its interior. “Think of the star as a cup of coffee and the planet as a spoon,” he says. “Once you put the spoon inside the coffee and you start stirring it, you’re obviously making the coffee spin. So, once a planet goes into the star, it kind of stirs it up inside.”

This, he says, may explain why some giant stars are spinning at an abnormal speed: “One explanation is that they swallowed a planet,” Yarza says.

A planetary swallow might also explain why some stars are strangely rich in lithium.

This is strange, Yarza says, because lithium is easily consumed in the nuclear furnace of a sun-sized star, so by the time such a star reaches the later stages of its life, there shouldn’t be much left of it. Unless, perhaps, it has recently consumed an object too small to have a nuclear furnace burning lithium, such as a large planet or a brown dwarf (a star too cold hardly larger than a giant planet).

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Finally, he says, some white dwarfs — the remaining stars that arose when the red giant planets eventually fell on themselves and collapsed to a more normal size — have brown dwarf planets or stars in close orbits around them.

These, he says, were formed from a substance that was expelled from the red giant when it tried to swallow a very large planet. “If you stir the coffee vigorously enough with the spoon, some of the coffee will spill out,” Yarza says. In the star’s case, the spilled material would be material from its outer layers, which could then coalesce to form a new body.

In an effort to look in detail, Yarza’s team modeled the sweep of giant planets of varying sizes. They found that it was indeed possible to make the star spin fast enough to slough off its outer layers, providing material from which a new planet or brown dwarf could form.

But his team also found that sweeping across a large planet can very quickly transfer a staggering amount of energy to the star – enough to briefly increase its brightness by a factor of 10,000.

Does this mean we might be able to detect such a process in action by finding stars thousands of times brighter?

In theory, perhaps. In practice, Yarza says, it can be challenging, because these increases in brightness are astronomically short-lived, lasting only a few thousand years. “I think it would be difficult to detect,” he says.

Yarza’s study is available online at arxiv.org/abs/2203.11227.