Latest discovery: There are not only "three bodies" in the universe, but also terrible "devourers"

Recently, The Three-Body Problem has become a hot topic again. As an astronomer, I certainly won’t miss such a good opportunity to popularize science. In fact, I have always been curious about the stability of planetary systems.

In the early 17th century, Kepler published his three laws, which revealed the orbital laws of celestial bodies. But when there are multiple celestial bodies in the system, the dynamics may become unstable, causing some objects to be randomly ejected. This is the astronomical basis of the story of "The Three-Body Problem".

One day when I first came into contact with astronomy, I suddenly had a thought: Could similar random ejections happen in the solar system where humans live? To be more specific, could it be that our solar system originally had far more than just the "eight planets" but more planets, but one or some of the early planets were randomly ejected?

Planets and dwarf planets in the solar system. Only the sizes are drawn to scale, not the distances. | Wikipedia

This question has always lingered in my mind and has become a direction of my astronomical research.

Why look for twin stars first?

To solve this mystery, we conducted a large-scale spectroscopic survey of nearby stars, focusing specifically on 91 identical twin binary stars that formed at the same time.

This is similar to the way twins are studied in sociology. In terms of chemical composition, a pair of twins have roughly the same elements and proportions. If there are any differences between the two "brothers/sisters", these changes must be caused by "acquired factors".

For example, the Earth has a higher percentage of iron than the Sun, while the Sun has a higher percentage of hydrogen, helium, and other elements than the Earth. Therefore, if the Earth falls into the Sun, the ratio of certain elements in the Sun will change - if the Sun has a twin brother/sister, then at this time the Sun and it have different chemical compositions.

It's not uncommon for stars to swallow planets

Initially, we were also exploratory: if we could find a pair of twin brothers with differences, it would be a huge success. But unexpectedly, we found not just one pair, but more than ten pairs.

Of course, the differences between stars are not necessarily due to the engulfment of their planets. We also considered differences in the mechanisms of element precipitation in stellar atmospheres. To test our hypothesis, we performed detailed modeling and investigation.

The main thing is that the Earth has a higher ratio of heavy elements to volatile elements than the Sun. We found that among the twin brothers and sisters with differences, there are at least seven sets whose element differences can correspond to the situation of "Earth falling into the Sun".

Sherlock Holmes once said that when all the most likely scenarios have been eliminated, whatever remains, even if it is of low probability, must be the truth. This is true for detectives, and of course for astronomers.

"So, our conclusion is that at least one of every twelve pairs of stars has swallowed up material from a planet orbiting it," said the study, which was recently published on the cover of the journal Nature.

Interestingly, this phenomenon looks a bit like the situation described by Liu Cixin in another work, The Devourer, or the famous work "Faun Devouring His Son" from the "Black Paintings" of the Spanish painter Goya in his later years.

Spanish painter Francisco José de Goya y Lucientes painting "Saturn Devouring His Son" | Wikipedia

Our results suggest that many planetary systems, like those in The Three-Body Problem, have some degree of instability. And unlike in the book, this instability persists even in the absence of a nearby companion star.

In recent years, some scholars who study planetary dynamics theory have also pointed out that systems with massive "super-Earths" are inherently unstable. The gravitational perturbations of stars and massive planets may be enough to cause instability. It's a bit like... if there are many children in a family, the house will probably be messy all day long.

In reality, the probability of stars swallowing planets is higher

Although there are some theoretical support, such a high swallowing probability is indeed beyond our expectations. This means that the instability of planetary systems may be more common than we previously thought. The signal we can observe is likely just the tip of the iceberg.

Illustration of a star swallowing a planet | Handout/NSF's NOIRLab/AFP

First of all, the ejected planets do not necessarily fall into the host star. In fact, the direction in which the planets are ejected may be inward or outward. If they are inward, they may be swallowed by the star. But it is more likely that these planets will be ejected outside the star system and become "wandering planets" drifting in the universe.

Furthermore, even if the planet is devoured, the trace may disappear over time due to convection in the star's atmosphere. This process is like pouring milk into coffee, which creates a unique texture on the surface of the coffee, but these textures change over time and eventually dissipate.

We discovered the one in 12 "eaters" through such observations. However, the lines on the milk will gradually spread until they disappear completely - we end up with a uniform cup of latte. The same is true for the lines on the stars, so we cannot observe the swallowing events that happened a long time ago.

That is, although the observed probability in our sample is one in twelve, the actual probability may be much higher. The remaining star systems may also have unstable phenomena, or the ejected planets may just become "wandering earths", or the traces of engulfment may be smoothed over time.

While our findings are interesting, like all scientific discoveries, the questions they raise are more important than the conclusions themselves.

Possible explanations are that planets are ejected very frequently, so at any point in time, there are always some stars that show the "latte art" phenomenon; or the convection process of stars is very slow, so the traces of swallowed planets can be preserved for a long time. However, given our incomplete understanding of the dynamics of multi-body systems and the thermal convection process of stars, these questions require further research to answer.

I hope our research can inspire more people to study planetary systems and their relationships with stars and unravel these mysteries.

Perhaps a more important revelation of our work is that the prosperity of life on Earth depends on a delicate balance, and our cosmic home may be more fragile than we think. As an astronomer, I will continue to explore the mysteries of the universe and look for more clues about our place. At the same time, I also hope that our research can trigger more people to think about the stability of planetary systems and enhance our understanding and cherishment of our cosmic home. Only in this way can we better protect this paradise on which life depends and allow civilization on Earth to continue to prosper and develop.

Author: Ding Yuansen, Associate Professor, Department of Astronomy and Computer Science, Australian National University

Review丨Han Wenbiao, Researcher at Shanghai Astronomical Observatory, Chinese Academy of Sciences