Nearby Star Kronos Ate 15 Earths Worth Of Rocky Mass

About 350 light-years from Earth lies a binary star system whose stars have the official nomenclatures of HD 240430 and HD 240429. Nothing particularly unusual about that, except one of those stars, seems to have consumed terrestrial material worth the mass of 15 Earths.

HD 240430 is nicknamed Kronos because it is quite likely the planets it consumed were a part of its system. Kronos is a Greek mythological character, a Titan who ate his children lest one of them usurp him. Not that the planets, in this case, had any chance of threatening their parent star, but the analogy still applies. The companion star HD 240429 is named Krios, after Kronos’ brother.

Both Kronos and Krios are sun-like stars — yellow G-type — and are about 4 billion years old, orbiting each other about every 10,000 years. They have different chemical compositions — not an unknown phenomenon in binary star systems — but the scale of that difference is unusual, and seems even more bizarre when put in the context of the sun.

“Even if our sun ate the entire inner solar system, it wouldn’t come close to the anomaly we see in this star,” David Hogg, the group leader for astronomical data at the Center for Computational Astrophysics (CCA) at the Flatiron Institute, who co-authored a new study on the subject, said in a statement Thursday.

The terrestrial matter is rich in heavier elements that can make up rock-forming minerals, such as magnesium, aluminum, silicon, iron, chromium, and yttrium. These materials are found in abundance in Kronos’ outer layers. The lighter elements like oxygen, carbon, nitrogen, and potassium are found in the gaseous form. And given the laws of chemistry, heavier elements formed in nature after the lighter ones.

Therefore, usually, a Kronos-like metal-rich star would contain “all the other elements enhanced at a similar level, whereas Kronos has volatile elements suppressed, which makes it really weird in the general context of stellar abundance patterns,” the study’s lead author Semyeong Oh, a graduate student at Princeton University, said in a statement Thursday.

The almost-chance discovery of the chemical composition discrepancy between the two companion stars puzzled researchers for some time before they ruled out various possibilities and narrowed down on what is the most likely cause — consumption of terrestrial planets by Kronos.

“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,” Oh explained.

Further calculations showed “that gaining this many rock-forming minerals without many volatiles would require engulfing roughly 15 Earth-mass planets.” But that doesn’t necessarily mean Kronos actually had 15 planets orbiting it (no known star system has that many Earth-sized planets in it).

Jessie Christiansen, an astronomer at the NASA Exoplanet Science Institute at the California Institute of Technology, who was not involved in the research, gave examples of Kepler-11 and HD 219134, stars that have over 22 and 15 Earth masses of material respectively on the planets that orbit them.

“I see no problem with there being more than 15 Earth masses of accretable material around a solar-type star,” Christiansen said in the Princeton statement.

Consuming large gas giants — such as Jupiter, which is thought to have a rocky core that could easily have the terrestrial mass of 15 Earths — would not produce the same chemical spectrum as seen on Kronos.

“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. To flip it around, you have to throw in a bunch of smaller planets,” Adrian Price-Whelan, a postdoctoral fellow at Princeton and co-author of the paper, explained.

The paper, titled “Kronos & Krios: Evidence for accretion of a massive, rocky planetary system in a comoving pair of solar-type stars,” is available on the preprint server arXiv but is not fully peer-reviewed yet.