What happens when a red giant star devours a planet?

Astrophysicists have lengthy identified that as stars just like the Solar age, they attain a stage the place they swell into giant, bulging stars referred to as purple giants, whose diameters are many occasions their younger ones.

These stars might develop sufficient to eat a few of the planets orbiting them, says Ricardo Yarza, a graduate scholar in astronomy and astrophysics on the College of California, Santa Cruz, US. “That is one thing we all know will occur to Mercury and Venus in our photo voltaic system,” he says.

That is clearly not good for a planet. However what occurs to a star when it swallows a planet?

If the planet is small, possibly not a lot. In our photo voltaic system, comets typically fall into the solar, with little obvious impact. And though Mercury and Venus are a lot bigger than comets, they’re nonetheless small in comparison with the Solar itself.

However many planetary methods have close by planets a lot bigger than Mercury or Venus. Many of those methods will ultimately see a minimum of one planet swallowed up, Yarza stated at a current assembly of the American Astronomical Society in Pasadena, California.

One impact is that the planet will switch its orbital momentum to the star because it sinks into its inside. “Consider the star as a cup of espresso and the planet as a spoon,” he says. “As soon as you set the spoon contained in the espresso and also you begin stirring it, you are clearly making the espresso spin. So, as soon as a planet goes into the star, it type of stirs it up inside.”

This, he says, might clarify why some big stars are spinning at an irregular velocity: “One rationalization is that they swallowed a planet,” Yarza says.

A planetary swallow may also clarify why some stars are surprisingly wealthy in lithium.

That is unusual, Yarza says, as a result of lithium is well consumed within the nuclear furnace of a sun-sized star, so by the point such a star reaches the later phases of its life, there should not be a lot left of it. Except, maybe, it has not too long ago consumed an object too small to have a nuclear furnace burning lithium, comparable to a big planet or a brown dwarf (a star too chilly hardly bigger than a large planet).

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Lastly, he says, some white dwarfs — the remaining stars that arose when the purple big planets ultimately fell on themselves and collapsed to a extra regular measurement — have brown dwarf planets or stars in shut orbits round them.

These, he says, had been fashioned from a substance that was expelled from the purple big when it tried to swallow a really giant planet. “In case you stir the espresso vigorously sufficient with the spoon, a few of the espresso will spill out,” Yarza says. Within the star’s case, the spilled materials could be materials from its outer layers, which might then coalesce to kind a brand new physique.

In an effort to look intimately, Yarza’s staff modeled the sweep of big planets of various sizes. They discovered that it was certainly attainable to make the star spin quick sufficient to slough off its outer layers, offering materials from which a brand new planet or brown dwarf might kind.

However his staff additionally discovered that sweeping throughout a big planet can in a short time switch a staggering quantity of vitality to the star – sufficient to briefly improve its brightness by an element of 10,000.

Does this imply we would be capable of detect such a course of in motion by discovering stars 1000’s of occasions brighter?

In idea, maybe. In observe, Yarza says, it may be difficult, as a result of these will increase in brightness are astronomically short-lived, lasting just a few thousand years. “I feel it will be troublesome to detect,” he says.

Yarza’s research is out there on-line at arxiv.org/abs/2203.11227.