What happens when a red giant star devours a planet?

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

These stars may develop sufficient to devour a few of the planets orbiting them, says Ricardo Yarza, a graduate pupil 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, perhaps 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 programs have close by planets a lot bigger than Mercury or Venus. Many of those programs will finally see not less than one planet swallowed up, Yarza mentioned 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 place 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 form of stirs it up inside.”

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

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

That is unusual, Yarza says, as a result of lithium is definitely 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 lately 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 an enormous planet).


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Lastly, he says, some white dwarfs — the remaining stars that arose when the crimson 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 fashioned from a substance that was expelled from the crimson large when it tried to swallow a really giant planet. “When 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 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 staff 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 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 have the ability to detect such a course of in motion by discovering stars 1000’s of occasions brighter?

In concept, maybe. In observe, 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 feel it might be tough to detect,” he says.

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