Stars that eat planets can start spinning so fast they rip apart

Stars sometimes bite off more than they can chew. When a star devours a planet, it can have strange effects on the star, including causing it to start falling apart. Understanding those effects could help us figure out how different kinds of planetary systems are formed.

Many planets across the universe probably end up falling into their stars, either because they stray too close, or because the stars expand as they age. We have seen some evidence of this, like clouds of leftover debris and stars full of elements they couldn’t maintain on their own.

Alexander Stephan at the University of California, Los Angeles, and his colleagues calculated how planets could affect the stars that eat them. They found that a planet falling into a star can make the star brighten for anything from centuries to millennia. And that the star can spin faster as the planet deposits its energy.

“The interactions between the planet and star may not be able to kill the star as such, but they can certainly mess it up,” Stephan says. Often, when a star eats a planet, the star can start to spin so quickly that it begins to rip apart, throwing its outer layers off into space where they form a strange, flattened nebula of dust and gas.

This brightening and the weird nebulae are signatures that we can look for to find stars that are in the course of swallowing their planets, or those that have just done so. That could help us figure out what’s going on with unusual objects in space – like Tabby’s star, which may behave strangely because it is surrounded by a cloud of debris.

It could also help us learn about other planetary systems across the universe. “When we look at planets, we only see the survivors – we can’t directly see the planets that have been destroyed,” says Stephan. “If we can nail this down and find these stars, we can then say something about an exoplanet population as it formed, before a bunch of the planets get eaten.”

Something similar may also happen in our solar system in about five billion years when the sun expands to become a red giant, swallowing up Mercury, Venus, and maybe even Earth. “These interactions will strip off some of the sun’s outer layers, and it will look cool for aliens,” Stephan says. “But we will all be dead long before that.”

Reference: arxiv.org/abs/1909.05259