A Quarter of Stars Like Our Sun Eat Their Planets, Study Finds

Stars show chemical traces of past "planetary ingestion" that can be used to search for Earth-like planets.
August 30, 2021, 3:00pm
Stars show chemical traces of past "planetary ingestion" that can be used to search for Earth-like planets.
Artist concept of planetary rocks falling in
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It’s practically a childhood rite of passage to eat forbidden treats and then deny it when caught, even though the evidence is smeared all over your face. If you’ve ever been guilty of this transgression, rest assured that you have something in common with a surprisingly large population of stars similar to our Sun that devoured their own planets, leaving them with telltale chemical signatures that can be spotted from Earth. 

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That’s the result of new research led by Lorenzo Spina, an astrophysicist and data scientist at the Astronomical Observatory of Padua in Italy, which estimates that about 20 to 35 percent of Sun-like stars have committed these acts of “planetary ingestion,” according to a study published on Monday in Nature Astronomy.

This gnarly discovery is interesting in its own right, but it could also “drive a generational advancement in astrophysics” that can help in the search for a habitable Earth-like planet by resolving “one of the most contradictory examples in stellar astrophysics and a source of tension between theory and observations,” according to the study.

“Theoretical models of evolution of planetary systems predict that planet engulfment events can occur around Sun-like stars,” Spina said in an email. “However, we didn't have any clear prediction about the occurrence of these dramatic events. That is exactly why our result is so important. In fact, our findings provide constraints to theoretical models that were not available before.”

“Besides all that, I was very surprised to find that a significant fraction of planetary systems around Sun-like stars underwent a very dynamic and chaotic past, unlike our solar system,” he added. “It suggests that, although planetary systems are common in the galaxy, many of them must be in many ways quite different from the solar system.”

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To reach this finding, Spina and his colleagues set out to resolve a question that has plagued scientists for years: why do some binary systems contain stars with different chemical profiles?

Current models of star formation suggest that stars are born from molecular “protostellar” clouds with chemically homogeneous ingredients. As a consequence, twin stars born from the same cosmic brew should end up with more or less the same chemical ingredients. Yet scientists have observed many binaries that are inexplicably chemically distinct.

One explanation for these ill-matched binary stars is that protostellar clouds simply aren’t as uniform as scientists expected, a possibility that would upend our understanding of how stars are born. A more dramatic possibility is that some systems develop dynamical instabilities that fling planets into their host stars. As a result, the outer atmospheres of those stars become enriched by planetary ingredients such as iron and lithium, making them distinct from their more stable binary partners. 

“In the last decade, there are some previous works in the literature on chemical anomalies in binary pairs,” Spina noted. “However, the origin of these anomalies has always remained elusive. In fact, in addition to the planet engulfment scenario, many alternative theories have been proposed to explain the chemical anomalies.” 

“Nevertheless, compelling evidence in favor (or against) the planet engulfment scenario would have greatly improved our understanding of the evolution of planetary systems and the probability of finding analogues of our solar system,” he continued. “For that reason we considered it important to fill this gap in knowledge once and for all.”

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To that end, the researchers conducted a statistical study of 107 binary pairs of Sun-like stars with similar effective temperatures, including 74 pairs with matching chemical abundances and 33 pairs with noticeably different concentrations of iron. 

The team’s models revealed that planetary engulfment events could not only explain the anomalous pairs, but that a star’s temperature also correlated to its chemical profile. In other words, hotter stars were more likely to display signs of past planetary ingestion because their outer atmospheres are thinner, creating a more obvious view of the remnant planet bits convecting inside them. 

“We demonstrate that the probability of finding a chemically anomalous binary increases with the average temperature of the pair,” said the team in the study. “This result cannot be explained by hypothetical inhomogeneities of the protostellar cloud.”

Needless to say, this is just one more reason why we Earthlings should be grateful to live in such a calm and stable solar system. But even as these hungry stars offer an apocalyptic glimpse of worlds swallowed whole, they may ultimately help us find Earth-like planets that could potentially host life.

Any hypothetical Earth twins orbiting Sun-like stars will be tricky to track down, in part because it takes them hundreds of days to complete an orbit, lowering the odds that our telescopes will catch a glimpse of them as they pass in front of their stars.

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However, scanning Sun-like stars for signs of past planetary ingestion could narrow down the search, because it would hint that such a system is, or was, dynamically unstable. These systems can be deprioritized as they are less likely to be habitable; after all, any Earth twins that might have evolved within them could have ended up on their host star’s menu.

“One of the key scientific challenges for the current decade is to find planets similar to the Earth orbiting stars similar to the Sun,” Spina said. “We established that planet engulfment events can change the chemical composition of Sun-like stars. Therefore, this result potentially opens to the possibility of using a chemical analysis of stars to identify those that are more likely (or less likely) to host true analogues of our calm solar system.” 

“Why is that important? Because there are millions of nearby stars similar to the Sun out there: without a method to identify the most promising targets, the search for the Earth 2.0 will look like the proverbial needle in a haystack,” he added.

This new “upstream method” of searching for Earth-like planets is not the only advance that the team hopes will stem from their research. Next-generation instruments from the European Southern Observatory, such as the 4-metre Multi-Object Spectrograph Telescope (4MOST) and Multi-Object Optical and Near-infrared Spectrograph (MOONS), will be able to observe chemically inhomogeneous Sun-like pairs in even more detail. 

Future surveys could reveal the hidden secrets of these systems, such as the root drivers of their instability or whether certain classes of planets might be more likely to be thrown into the stellar frying pan.

“Our study establishes that planet engulfment events can change the chemical composition of Sun-like stars,” Spina said. “Therefore, in the future, we should observe more binary systems in order to derive a more precise percentage of engulfment events. Also, in the future we will have to search for any correlation between the chemical signatures of planet engulfment events in the hosting star and the architecture of planetary systems.” 

“Are the systems with a more chaotic architecture hosted by stars showing signatures of planetary engulfment events? We must answer this question,” he concluded.