“At the moment, we are still at the stage of piecing together different observations to determine how and when exoplanets form,” said Christiansen. “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. 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.”
Other co-moving star pairs have had different chemistries, Oh explained, but none as dramatic as Kronos and Krios.
“Other processes that change the abundance of elements generically throughout the galaxy don’t give you a trend like that,” said Price-Whelan. “They would selectively enhance certain elements, and it would appear random if you plotted it versus condensation temperatures. The fact that there’s a trend there hinted towards something related to planet formation rather than galactic chemical evolution.”
In mythology, the Titan Kronos devoured his children, including Poseidon (better known as the planet Neptune), Hades (Pluto) and three daughters.
Kronos and Krios are far enough apart that some astronomers have questioned whether the two were in fact a binary pair. Both are about 4 billion years old, and like our own, slightly older sun, both are yellow G-type stars. They orbit each other infrequently, on the order of every 10,000 years or so. An earlier researcher, Jean-Louis Halbwachs of the Observatoire Astronomique of Strasbourg, had identified them as co-moving—moving together—in his 1986 survey, but Oh independently identified them as co-moving based on two-dimensional astrometric information from the European Space Agency’s Gaia mission.
That was her “Aha!” moment, Oh said. “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.”
Eating a gas giant wouldn’t give the same result, Price-Whelan explained. Jupiter, for example, has an inner rocky core that could easily have 15 Earth masses of rocky material, but “if you were to take Jupiter and throw it into a star, Jupiter also has this huge gaseous envelope, so you’d also enhance carbon, nitrogen—the volatiles that Semyeong mentioned,” he said. “To flip it around, you have to throw in a bunch of smaller planets.”
Maybe they only started moving together more recently, after trading partners with another pair of binary stars, a process known as binary exchange. Oh ruled that out with “a simple calculation,” she said. “She’s very modest,” Price-Whelan noted.
She immediately observed that all of the minerals that solidify below 1200 Kelvin were the ones Kronos was low in, while all the minerals that solidify at warmer temperatures were abundant.
Devourer of planets? Researchers dub star ‘Kronos’ In mythology, the Titan Kronos devoured his children, including Poseidon (better known as the planet Neptune), Hades (Pluto) and three