The story of the insatiable star that swallowed 15 Earths
There are stories in the stars, so they say, and the story of Kronos is a doozy. Astronomers from Princeton noticed some marked chemical differences between the star and its binary partner, Krios, and on closer inspection they determined the most likely cause: Kronos has devoured the equivalent of 15 Earths.
Sitting about 350 light-years from Earth and two light-years apart, Kronos and Krios (officially known as HD 240430 and HD 240429) are loosely locked in orbit around each other. The pair is about 4 billion years old and are both yellow G-type stars, like our Sun. But that's about where the similarities end: Kronos and Krios, it turns out, are very different beasts.
Binaries don't always have the same chemistry, but the Princeton scientists noticed that the differences in this case were unheard of. Kronos is rich in many rock-forming minerals like magnesium, aluminum, silicon, iron, chromium and yttrium. That in itself isn't too out of the ordinary, but normally that would be matched with equally high levels of volatile compounds like oxygen, carbon, nitrogen and potassium. That makes it an outlier compared not only to Krios, but to any known star.
The researchers got to work trying to piece together the puzzle. First they had to confirm that Kronos and Krios actually were a binary system – being so far apart makes it hard to tell. They did that by checking that their radial velocities matched, meaning they had the same "wobble" as they orbited each other, moving towards and away from Earth in sync.
Then they ruled out other possibilities, such as the unlikely scenario that their accretion disks – dust and rock that form around stars, eventually clumping into planets – arose at different points in their life cycle. Also checked off the list was the idea that they might have initially come from different binary systems, swapping partners at some point in the past.
The breakthrough came when the researchers plotted out the condensation temperatures of the elements in Kronos. They found that the star was low in minerals that solidify below 1,200 Kelvin (927º C/1,700º F), and abundant in those that solidify at warmer temperatures. Since rocky planets tend to form in the warmer regions close to a star, the observation suggests that Kronos has eaten several Earth-like worlds.
"All of the elements that would make up a rocky planet are exactly the elements that are enhanced on Kronos, and the volatile elements are not enhanced, so that provides a strong argument for a planet engulfment scenario, instead of something else," says Semyeong Oh, lead author of the study.
Next, the researchers calculated just how many planets Kronos had to chow down on to get that kind of chemical fingerprint, and found it would take the equivalent mass of 15 Earths. That doesn't necessarily mean it swallowed up that many planets; Kronos could have swallowed up fewer, bigger planets to the same effect. Gas giants like Jupiter weren't on the menu though, because the huge gaseous atmospheres would have driven up levels of volatile compounds.
The star's voracious appetite earned it the name Kronos, after the Titan of Greek mythology who ate several of his own children to prevent a prophecy that foretold his son would overthrow him. But besides being a fascinating story, the findings may have important implications for our understanding of how stars and planets form.
"It's difficult to directly observe planet formation around young stars — they are typically shrouded in dust, and the stars themselves are very active, which makes it hard to disentangle any signals from the planets," says Jessie Christiansen, an astronomer at Caltech who wasn't involved in the study. "So we have to infer what we can from the limited information we have. If borne out, this new window onto the masses and compositions of the material in the early stages of planetary systems may provide crucial constraints for planet formation theories."
The research was published online at ArXiv.