Recently, "The Three-Body Problem" has once again become a hot topic. As an astronomer, I certainly won't miss such a great opportunity for science communication. In fact, I've always been curious about the stability of planetary systems.
In the early 17th century, Kepler published his three laws, revealing the orbital laws of celestial bodies. However, when multiple bodies exist in a system, dynamics can become unstable, leading to some objects being randomly ejected. This is also the astronomical basis of the story in "The Three-Body Problem."
One day, when I was just getting into astronomy, I had a wild thought: Could similar random ejections happen in the solar system where humans reside? To put it more clearly, could our solar system, in its early days, have had more than just "eight planets," but instead had more planets, with one or some of the early planets being randomly ejected?
The planets and dwarf planets of the solar system. The sizes in the image are drawn to scale only, and the distances are not to scale. - Wikipedia
The question has been lingering in my mind and has become a direction in my astronomical research. Why look for twin stars first? To unravel this mystery, we conducted a large-scale spectroscopic survey, delving deep into nearby stars. We focused particularly on 91 pairs of simultaneously formed and similar natured twin stars.
This approach bears some resemblance to the method in sociology of studying twin siblings. Chemically, a pair of twin siblings have roughly the same elements and proportions. Any differences between these "siblings" must be due to "environmental factors."
For example, the proportion of iron elements on Earth is higher than that in the Sun, while the proportions of hydrogen, helium, and other elements in the Sun are higher than those on Earth. Therefore, if we assume Earth fell into the Sun, the proportions of certain elements in the Sun would change – if the Sun had a twin sibling, then there would be chemical differences between the Sun and its twin.
Stellar ingestion of planets is not uncommon. Initially, we approached this with an exploratory mindset: finding a pair of different twin siblings would already be a huge success. But unexpectedly, we found not just one pair, but over a dozen pairs.
Of course, the differences between stars are not necessarily due to them consuming planets. We also considered differences in the mechanism of element precipitation in stellar atmospheres. To validate our hypothesis, we conducted detailed modeling and investigation.
Essentially, as mentioned above, relative to the Sun, Earth has a higher proportion of heavy elements to volatile elements. We found that in at least seven pairs of twin siblings with differences, the elemental differences could correspond to a scenario like "Earth falling into the Sun."
As Sherlock Holmes once said, when all the most probable situations have been eliminated, whatever remains, however improbable, must be the truth. This applies to detectives and, of course, astronomers too.
So, our conclusion is that in every twelve pairs of stars, at least one star has ingested material from planets orbiting it. This research was recently published on the cover of the journal Nature.
It's interesting how this phenomenon resembles something out of Liu Cixin's other work, "The Devourer", or perhaps akin to the infamous piece "Saturn Devouring His Son" from Francisco Goya's later "Black Paintings" series.
Spanish painter Francisco José de Goya y Lucientes's artwork "Saturn Devouring His Son" | Wikipedia
Our research findings indicate that, akin to the situations depicted in "The Three-Body Problem," many planetary systems exhibit varying degrees of instability. Unlike what's described in the book, such instability persists even without nearby companion stars.
In recent years, some scholars delving into planetary dynamical theories have pointed out that systems hosting massive "super-Earths" are inherently unstable. Gravitational disturbances from stars and massive planets could be sufficient to trigger instability. It's a bit like... if a household has many children, it's likely to be chaotic most of the time.
The probability of stars devouring planets is higher in reality
Despite some theoretical support, such a high probability of devouring indeed surprises us. This implies that the instability of planetary systems may be more widespread than previously thought. The signals we observe are likely just the tip of the iceberg.
