Astronomers: Many Sun-Like Stars Already Devoured Their Planets

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(Image: NASA/EASA/G. Bacon)The sun won’t consume the earth for another few billion years. This is either good or bad news, depending on how you look at it. But in the meantime, we can look at other sun-like stars and see what they’ve been up to – and even what they’ve eaten recently. Spoiler: a lot of planets. A new spectral survey of sun-like binary pairs published in Nature gives the odds of a star ingesting one or more of its planets at around one in four.

You can tell a lot about a star by the lines of its spectral “fingerprint,” starting with what chemical elements it must be made of. From there, we can draw conclusions about lots of stellar phenomena, including how old a star is, how hot it’s burning, how fast it’s moving, and whether it’s due to explode. That’s how we figured out that Betelgeuse — one of Orion’s shoulders — is a baby rogue star doing its best to live fast and die young.

Our sun is not like Betelgeuse, thankfully. Just like the overwhelming majority of known stars, Sol is a nice quiet stable main-sequence star. Main sequence refers to the period of a dwarf star’s life when it’s fusing hydrogen into helium. As a main-sequence star ages, a cascade of reactions and explosions creates heavier elements, with some of the raw materials flung away into space at every step. The debris from this unbroken sequence creates a forensic record that reliably links a main sequence star’s color and composition to its temperature and age.

Earth has yet to be consumed. (Photo: NASA)

Stars born in the same place at the same time should be, in the authors’ words, “chemically identical.” If the stars in a pair are of different kinds, that can tell us a binary pair came about as a capture. Trace differences in stellar composition can even tell us what a star has eaten recently.  Twin studies are important to science, and twin stars are no exception: Twins are also why the authors of this new study are able to so confidently assert these odds of planetary snackage. Based on how often they observed the presence of certain heavy elements in just one of the twins in a binary pair, the researchers estimated that up to one in three stars will eat “at least one” of their rocky planets.

“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,” explained lead study author Lorenzo Spina, an astrophysicist at the Astronomical Observatory of Padua. Furthermore, sun-like stars burn off their lithium pretty fast, but there’s lithium in the crust of planets – so lithium in an older star’s spectral signature is another tipoff that it may have consumed a planet. So too for metals, and silica from rock.

This all confirms our expectation of Sol eventually expanding to consume everything from Earth inward, right before it explodes into a planetary nebula. It could also help point us toward earthlike worlds outside our own star system.

It also means that there’s a timeline where one of the twin suns of Tatooine ate the planet long before Anakin could start whining about sand. And the only way anyone would know there ever was any sand is by staring straight at the system and noticing the barest glow of silica in the light of one slightly mismatched star. Just like Anakin would have wanted.

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