How stable are planetary systems? Will the Earth and its seven siblings always continue on their celestial ways, or could we one day be accidentally thrown out of our cosmic home?

Physicists understand the rules that govern the orbits of two celestial bodies, but when a third is added (not to mention a fourth, fifth, or hundredth) the dynamics become much more complex. Unpredictable instabilities arise when an object is randomly thrown into space or lands on its host star.

The so-called “three-body problem” has plagued scientists for centuries (and recently became the basis for a best-selling series of science fiction novels and a new Netflix movie). One obstacle to understanding this is that we know relatively little about how often such catastrophic instabilities occur.

In a new study published in Nature, we and our colleagues shed light on this question. When we looked at nearby stars, we found that every dozen pairs of stars could have swallowed a planet, probably because the planet “wobbled” in its orbit and fell onto the star.

Teaching twins

Our study shows that at least 8% of stellar binaries in our sample show chemical anomalies, indicating that a star once absorbed planetary material it orbited. To detect this subtle signal, we had to rule out other possible explanations for these chemical patterns. That’s why we focused on “twin stars” that are known to be born at the same time from a mixture of the same materials.

This approach can eliminate confounding factors, just as twin studies are sometimes used in sociological or medical research. The conclusion comes from a study of twin stars called C3PO, which one of the authors (Ting) initiated in the USA, and was later joined by Liu and others.

Our team collected an excellent sample of spectroscopic data from 91 pairs of twin stars; This was many times more than similar studies in the past. We found that some stars differ from their twins, exhibiting a different chemical structure with more of certain elements, such as iron, nickel and titanium, than others, such as carbon and oxygen. These differences point to strong evidence that the star has swallowed a planet.

Instability can be unexpectedly widespread

If the host star absorbs one or more members of the planetary system, this means that there is some instability in the dynamics of the system. Simulations suggest that such instability may be common early in the life of a planetary system (the first 100 million years or so). However, traces of the planets swallowed in this early period cannot be detected in the billions of years old stars we observe.

This suggests that the chemical anomalies we see are due to recent instability that has caused stars to consume some planets or planetary material.

This discovery is not entirely unexpected. Theorists who study planetary dynamics, including our co-author Bertram Beach, have noted that many planetary systems are known to be unstable, especially systems with a type of planet slightly larger than Earth but much smaller, called “super-Earths.” Than giants like Jupiter.

Systems containing super-Earths may be particularly unstable. Gravitational tug-of-war between the host star and its major planets can cause instability.

A delicate balance

Our research challenges us to rethink our place in the universe. Although we take for granted stability in our solar system, this may not be normal throughout the universe. Our research does not suggest that we might see such imbalances in our own solar system. However, it is important to recognize that despite our new results, planetary absorption and instability still occur in a very small number of cases.

We hope our research will inspire more people to study planetary systems and their relationships with their host stars. Our understanding of the dynamics of many-body systems is still very incomplete. As we continue to explore the mysteries of space, studies like this remind us of the delicate balance that allows life to thrive on Earth and the potential fragility of our cosmic home.