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An artist’s conception of a star engulfing a planet. The blue lines traces the path of the planet as it spirals toward the star and ultimately collides with it (the planet is partially as it crashes into the left-hand side of the star).
Image Credit: NASA, ESA, CSA, Ralf Crawford (STScI)
Scientific Frontline: Extended "At a Glance" Summary: Planetary Engulfment
The Core Concept: Planetary engulfment is an astronomical event in which a star consumes an orbiting planet. This process rapidly alters the star's chemical composition, leaving behind distinct and measurable elemental signatures.
Key Distinction/Mechanism: Because an engulfment event occurs very rapidly—often concluding within days or weeks—astronomers rarely observe it in real time. Instead, researchers detect it retroactively by analyzing a star's lithium concentration. Stars naturally possess low levels of lithium, whereas planets contain heavily enriched amounts; consequently, a star that devours a planet will exhibit an anomalously high lithium concentration in its atmosphere.
Major Frameworks/Components:
- Stellar Spectroscopy: The use of light spectrum analysis to identify anomalous chemical signatures, specifically lithium enrichment, within stellar atmospheres.
- Comparative Statistical Analysis: The establishment of baseline stellar chemical profiles. By comparing TOI-5882 against a control group of 62 stars matched by age, mass, and temperature, researchers proved the star's lithium levels were statistically anomalous (above the 97th percentile).
- Orbital Dynamics and Perturbation: The theoretical role of massive substellar companions in destabilizing planetary orbits. TOI-5882 is orbited by a massive brown dwarf, which may have gravitationally steered the terrestrial-to-Neptune-mass planet into the host star.
Branch of Science: Astronomy, Astrophysics, Astrochemistry.
Future Application: Refining spectroscopic screening techniques to locate post-engulfment stars across the galaxy and improving computational models that predict orbital decay and the long-term dynamical stability of multi-planet systems.
Why It Matters: Investigating historical engulfment events provides empirical data on the volatility of exoplanetary systems. This advances our understanding of solar system lifecycles, offering insight into the eventual fate of our own inner planets when the sun enters its red giant expansion phase.
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| The star TOI-5882 has a diameter that’s roughly twice that of our sun. Image Credit: NASA Eyes on Exoplanets |
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.”
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 cannot rely on seeing an engulfment event as it happens, Kotten said, which is why it is 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 it can occur. For example, in roughly 5 billion years, our Sun will enter the late stages of its lifetime and grow into what is called a red giant. As it swells, it will engulf Mercury, Venus, and maybe Earth.
But TOI-5882 has not 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 is more than twenty 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 of 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 US and Chile 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.
They put together a lineup of sixty-two 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.”
Funding: The research was supported, in part, by federal funding from NASA and the US National Science Foundation.
Title: Lithium Enrichment in a Subgiant Star with a Brown Dwarf Companion: A Planetary Engulfment Candidate
Authors: Brooke Kotten, Melinda Soares-Furtado, Ricardo Yarza, Andrew C. Nine, Seth A. Jacobson, Noah Vowell, Olivia Maynard, Allyson Bieryla, Andrew Vanderburg, Jack Schulte, Claudia Aguilera-Gómez, Enrico Ramirez-Ruiz, Joseph E. Rodriguez, and David W. Latham
Source/Credit: University of Michigan
Edited by: Scientific Frontline
Reference Number: astr061526_01
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