One-third of Sun-like stars may have eaten their planets

2021-08-30

Like the Greek god Chronos, a good number of stars devour their children. As many as one-third of them have swallowed one or more of their own planets, a new study suggests. The findings could help astronomers rule out stellar systems unlikely to contain Earth-like worlds.

“This will probably end up being one of the classic papers on this subject,” says Eric Mamajek, an astronomer at NASA’s Jet Propulsion Laboratory who was not involved in the work.

Researchers have known for decades that stars could, on occasion, engulf their progeny. Rocky planets are rich in heavy elements such as iron, silicon, and titanium, whereas stars contain mostly lighter material like hydrogen, helium, oxygen, and carbon. When a planet is swallowed, its heavy elements spread out in the star’s outer layers, leaving telltale absorption signatures in its light.

“If a star is anomalously rich in iron but not in other elements such as carbon and oxygen, this can be interpreted as a signature of planetary engulfment,” says Lorenzo Spina, an astrophysicist at the Astronomical Observatory of Padua who led the study.

He and his colleagues investigated how often this happens by looking at 107 binary systems containing two Sun-like stars—akin to the fictional two-sunned world Tatooine in Star Wars. Binary stars are born from the same cloud of gas and dust, so their chemical compositions should be nearly identical. The team also chose partners that were extremely close in mass and temperature to one another—essentially twins.

In 33 of these pairs, one of the companions showed elevated levels of iron compared with the other, a sign of planetary cannibalism. These same partners were also rich in lithium, giving further credence to the world-munching hypothesis. Although Sun-like stars are born with substantial amounts of lithium, they burn it away within the first 100 million years of their lives, so seeing it in the older stars in the study sample indicated it likely came from a planet.

The team also found that abnormal chemical signatures showed up more often in the hottest stars. That makes sense, Spina says, because hot stars have thin outer layers—and a planet’s material would be concentrated in a smaller volume, leaving a starker signature.

Using these different lines of evidence, the team was able to model that between 20% and 35% of Sun-like stars consume a few Earths’ worth of their offspring. Such events could happen in systems where gravitational interactions among the planets would either fling one into the central star or bring it close enough for the star to slowly vaporize and devour it. The results appear today in Nature Astronomy.

“This is clearly a strong trend,” Mamajek says. Planetary ingestion has been studied before, he says, but the new paper provides a much larger sample size and clear statistical evidence for the phenomenon.

Spina thinks it is unlikely our Sun ever swallowed any planets, because it’s depleted in heavy elements compared with others in its class. Though that fact could help astronomers find Earth 2.0: If they spot an alien sun that appears to have eaten its offspring, they’ll probably want to point their telescopes elsewhere.

science.org, 30 August 2021
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