Astronomers have a cosmic thriller on their fingers, investigating a celestial crime scene to find out if a distant star has eaten a super-Earth exoplanet. The star might have had an confederate — a failed star or “brown dwarf” companion — which can have steered the unlucky planet towards its fiery doom.
The workforce charged with investigating this thriller first found hints of the crime after they discovered the star, TOI-5882, situated round 1,300 light-years away, is surprisingly wealthy within the component lithium.
“You’re what you eat, proper?” workforce chief Brooke Kotten of the College of Michigan mentioned in a statement. “We all know that there is far more lithium in planetary materials than there’s in stars. So if a star eats a planet, it is going to tackle a bunch of lithium.”
So-called engulfment occasions similar to this one happen very quickly, on a timescale of some days to a few weeks, which suggests catching stellar beings within the act of having fun with a planetary meal is extraordinarily uncommon. Thus, astronomers must act as cosmic crime scene investigators to reconstruct these occasions with the proof at hand.
“That is what makes this discipline so thrilling. You actually are fixing a thriller,” Kotten mentioned. “We won’t simply watch the crime occur, so we’ve got to work with all of the clues we’re given to determine whodunit.”
One of many goals of those investigations is to find the methods by which a star can devour a planet. One of the crucial widespread engulfment situations occurs when a star runs out of hydrogen at its core on the finish of its main sequence lifetime. This results in it swelling out to up to 100 times its original diameter, engulfing its attendant planets during its so-called red giant phase. This will occur in the solar system in around 5 billion years when the sun will puff out to around the orbit of Mars, swallowing the inner rocky planets, including our own.
However, Kotten and colleagues know this isn’t what has happened in the TOI-5882 system, as this star hasn’t yet become a red giant. Instead, the researchers think the sun-like star had assistance from its brown dwarf companion.
Companion brown dwarf of partner in crime?
Brown dwarfs get their slightly unfortunate nickname of “failed stars” because, despite forming from collapsing clouds of gas and dust, just like stars, they fail to grow to the masses needed to trigger the nuclear fusion of hydrogen to helium in their cores, the process that defines what a main-sequence star is. They’re quite mysterious by existing in this sort of limbo between planet and star.
This particular brown dwarf has around 20 times the mass of Jupiter, or around 2% of the mass of the sun. That’s not massive enough to trigger nuclear fusion, but is massive enough for it to have enough of a gravitational influence over planets orbiting TOI-5882. That means the team suspects this brown dwarf could have perturbed the orbit of this unfortunate planet enough to send it plummeting into its star.
This is something the scientists will need to investigate further. They may not have enough information yet to determine this planet’s cause of death, but they do have some evidence that helps them identify the kind of world it would have been before it was obliterated. This comes from observations of the chemical composition and lithium content of 62 stars with similar ages and masses to TOI-5882.
“Lithium atoms delivered by planetary engulfment to a star are like sports fans arriving at a stadium,” team member Seth Jacobson of Michigan State University 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.”
From the lithium abundance they measured, the team has determined that this planet was a so-called super-Earth with a mass somewhere between two times that of our planet and the mass of the solar-system ice giant Neptune, which is around 18 times as massive as Earth.
“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. “And it’s not like you have to cherry-pick the data to make it stand out. It’s robust. No matter how you slice it, TOI-5882 is so enriched in lithium it shows up as being at least in the 97th percentile.”
Soares-Furtado added that TOI-5882 is one of the few stars she has seen demonstrating evidence of planetary engulfment, although a few of the other stars in the control sample were enriched in lithium, albeit not to the extent of TOI-5882. That leaves another mystery for the team to solve, something that Soares-Furtado may well be quite content with.
“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.”
The team’s research was published on Monday (June 15) in The Astrophysical Journal.
