“The way I think about engulfment is to think of the star as a cup of coffee and the planet as a spoon,” Yarza said. “Once you put the spoon inside the cup of coffee and you start stirring it, you’re obviously making the coffee rotate. So once the planet enters the star, it’s kind of stirring it up from the inside.” This could in turn, explaining the faster-than-expected rotation of some evolved stars, which may have eaten their planets and consequently been stirred up from the inside.
Engulfment could also explain odd planetary systems littered throughout our galaxy: White dwarfs — the still-glowing embers of Sun-like stars — with closely orbiting planets or brown dwarfs. In this case, “If you stir the coffee cup hard enough with the spoon, some of the coffee’s gonna spill out,” Yarza said. Essentially, a massive enough planet or brown dwarf stirs the star up so much that its outer layers are ejected from the system entirely. The energy exchange also shrinks the object’s orbit, and “you end up with a planet or brown dwarf orbiting what’s left at the center.”
One particularly interesting conclusion of the new work, Yarza said, is that the simulated planets injected energy into their stars at rates different than expected based on pen-and-paper models. The differences, he said, were as large as a factor of 10 and dictate the effects engulfment has on the structure of the star. The simulations suggest that chowing down on its planets can increase the brightness of a star by up to 10,000x for a short (cosmically speaking) period of time lasting between about one thousand and a few thousand years.
As is often the case, the devil is in the details. According to these new calculations, what happens when the planet is engulfed — including whether or not it survives and how it affects its parent star — depends on several factors.
An object must be at least 100 times the mass of Jupiter (putting it firmly into brown dwarf territory) to liberate material from the star’s outer envelope — if the star has thus far only expanded to less than 10 times the radius of the Sun. At later times, after the star has expanded further, a planet just 10 times Jupiter’s mass could free material from the star. This also shrinks the planet’s orbit significantly, dropping it closer to the final stellar remnant when the dust and gas of the red giant phase clear.
The researchers hope their work can provide a starting point for future studies focused on the interactions between aging stars and swallowed planets. By first understanding this smaller, zoomed-in model, other teams can then expand on the work to take a look at the bigger picture to visualize the entire structure of the parent star.
“A significant fraction of planetary systems will experience engulfment,” Yarza said, in which at least one planet is engulfed. That means better understanding this process — and how it affects the host star — is key to understanding the evolution of the stars that serve as building blocks for everything else in the universe.