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 instances their younger ones.

These stars may broaden sufficient to eat a number 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 techniques have close by planets a lot bigger than Mercury or Venus. Many of those techniques will finally see no less than 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 sort of stirs it up inside.”

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

A planetary swallow may additionally clarify why some stars are unusually 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 levels 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, reminiscent of a big planet or a brown dwarf (a star too chilly hardly bigger than an enormous planet).


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

These, he says, have been shaped from a substance that was expelled from the purple large when it tried to swallow a really giant planet. “If you happen to stir the espresso vigorously sufficient with the spoon, a number of the espresso will spill out,” Yarza says. Within the star’s case, the spilled materials can be materials from its outer layers, which may then coalesce to kind a brand new physique.

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

However his group 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 enhance its brightness by an element of 10,000.

Does this imply we’d be capable to detect such a course of in motion by discovering stars hundreds of instances brighter?

In idea, maybe. In apply, Yarza says, it may be difficult, as a result of these will increase in brightness are astronomically short-lived, lasting only some thousand years. “I believe it could be troublesome to detect,” he says.

Yarza’s examine is offered on-line at arxiv.org/abs/2203.11227.