Astronomers have discovered a star with an unusually high concentration of lithium, a sign that it may have recently consumed a planet
Study: Lithium Enrichment in a Subgiant Star with a Brown Dwarf Companion: A Planetary Engulfment Candidate (DOI: 10.3847/1538-4357/ae71bb)
A team of astronomers, led by Brooke Kotten of the University of Michigan, has shown that TOI-5882-a sun-like star located some 1,300 light-years away-has likely eaten one of its planets.
Although a star might seem like the ultimate incinerator for destroying evidence, the team still found telling clues in the chemical composition of TOI-5882, specifically in its unusually high lithium concentration.
"You are what you eat, right?" said Kotten, a graduate student researcher in the U-M Department of Astronomy and lead author of the new report in The Astrophysical Journal. "We know that there's much more lithium in planetary material than there is in stars. So if a star eats a planet, it's going to take on a bunch of lithium."
The research was supported, in part, by federal funding from NASA and the U.S. National Science Foundation.
When a star consumes a planet, astronomers call it engulfment. Astronomically speaking, the process is incredibly fast, taking weeks or even days. That means astronomers can't rely on seeing an engulfment event as it happens, Kotten said, which is why it's important to develop methods that can help researchers study engulfment events after the fact.
"That's what makes this field so exciting. You really are solving a mystery," said Kotten, who started working on the study as an undergraduate student as part of the Lamat Program at the University of California, Santa Cruz. "We can't just watch the crime happen, so we have to work with all the clues we're given to figure out whodunit."
In working to solve these cases, astronomers can better understand how common planetary engulfment is and the different ways that they can occur. For example, in roughly 5 billion years, our sun will enter the late stages of its lifetime and grow into what's called a red giant. As it swells, it will engulf Mercury, Venus and maybe Earth.
But TOI-5882 hasn't yet ballooned to the point where its expansion is a likely explanation of how it gulped down a planet. The team has floated an intriguing alternative, though: The star may have had an accomplice.
Also orbiting TOI-5882 is a giant ball of gas that's more than 20 times the mass of Jupiter, but still not quite big enough to ignite as a star. This object, called a brown dwarf, may have helped steer the engulfed planet into TOI-5882, but testing that theory will be the subject of its own separate study, Kotten said.
Building the case
Lithium is a powerful piece of forensic evidence because, although stars have some naturally, planets are heavily enriched in it, said Seth Jacobson, a senior author of the study and assistant professor at Michigan State University.
"Lithium atoms delivered by planetary engulfment to a star are like sports fans arriving at a stadium," he said. "There may already be a few early arriving fans present, representing the initial amount of lithium in the stellar atmosphere, but they are quickly outnumbered."
Based on the amount of lithium the researchers observed, they suspect the planet that TOI-5882 engulfed had a mass somewhere between a couple Earths and Neptune.
"The fact that we can look at a star 1,300 light-years away and say with confidence, 'This star has more lithium than you would expect,' is a testament to both the precision of modern instrumentation and the hard interpretive work that goes into making sense of that signal," said Melinda Soares-Furtado, a senior author of the study and assistant professor at the University of Wisconsin.
Fourteen experts from the U.S. and Chilé came together for this project. A technique known as spectroscopy enabled the team to analyze light from TOI-5882 for signatures of lithium. From the star's spectra, the researchers could tell it had a high lithium content, but they then had to prove it was anomalously high.
So they put together a lineup of 62 control stars that were comparable in a variety of criteria, including their age, mass and temperature. The team then compared TOI-5882 to that control group in multiple ways.
"And it's not like you have to cherry-pick the data to make it stand out. It's robust," Soares-Furtado said. "No matter how you slice it, TOI-5882 is so enriched in lithium it shows up as being at least in the 97th percentile."
The study also built on previous work by Soares-Furtado that indicated TOI-5882 could in fact be probed for signs of engulfment. She had explored the characteristics a star would need to have in order to show signs of engulfment events and, disappointingly, many stars do not fit the bill, she said. But TOI-5882 was one of the rare exceptions.
Interestingly, a few of the other stars within the control sample also showed high lithium concentrations, suggesting there may be other enrichment mechanisms at work for researchers to explore. As is often the case in science, answering one question can create new mysteries, which is exciting news for astronomers like Kotten.
"When I was growing up, I dreamed about becoming a private investigator," she said. "I think that explains a lot about where I ended up. I do feel like a detective."
