Stars will expand as they get "old", and planets that are too close will be "swallowed up" by them. This has been a long-standing knowledge in astronomy, but a latest discovery by Chinese astronomers has overturned this knowledge. Recently, Professor Qian Shengbang of the School of Physics and Astronomy of Yunnan University led a team to publish a paper in the Proceedings of the National Academy of Sciences of the United States, announcing the discovery of three exoplanets that should have been swallowed by stars but "survived".
In our understanding, planets revolve around stars, and the evolution path of stars is to gradually expand until the end of their lives. Qian Shengbang, a professor at the School of Physics and Astronomy at Yunnan University, said that the academic community generally believes that as a star expands, planets close to it cannot escape the fate of being "swallowed". When stars like the sun exhaust the hydrogen fuel in their cores, they will gradually evolve into red giants, and their radius can expand hundreds of times its original size. In the process, closer planets may be directly engulfed by the expanding star's envelope. At the same time, as the radius of the star increases, the tidal interaction between the star and the planet increases significantly, which may cause the planet's orbit to gradually shrink and eventually fall into the star's interior and be swallowed. Therefore, traditional stellar evolution theory holds that planets that are too close to their host star usually have difficulty surviving the red giant stage.
However, what I did not expect is that the saying "everything has exceptions" also occurs in this common understanding. Qian Shengbang's team first used satellite data to analyze the weak changes in the brightness of the host stars of the three exoplanets and confirmed that they had entered the core helium burning stage.
According to Qian Shengbang, through asteroseismological analysis of the transiting exoplanet survey satellite data, it was confirmed for the first time that their host stars are red cluster giants that evolved after undergoing a violent expansion of the red giant stage. Next, they used the model to trace the past evolutionary expansion range of the three host stars, and compared it with the current star orbits. They found that their orbital scales all fell into the theoretical engulfment zone, but the three planets were not swallowed by the host star and still existed.
Qian Shengbang further explained that the three surviving planets are still orbiting their original host stars and have not changed stars. After their host stars undergo violent expansion in the red giant stage, their outer layers shrink again and enter the red cluster evolution stage characterized by core helium burning. Therefore, these planets are still in their original star systems.
Qian Shengbang said that if only a single surviving planet was discovered in the past, it might be explained by accidental events. But this time, three "lucky ones" were found at once, and the hosts of these three planets are three different stars, which shows that this is not an isolated case, and there may be some common physical mechanism behind it.
This discovery that subverts cognition is refreshing to everyone, and it also brings more new questions. How do planets "survive"? What is the state of survival? During the expansion of the sun, is it possible for the earth to escape the fate of being "swallowed" and "survive"?
These three host stars evolved to the red giant stage, indicating that they experienced at least one expansion and reached the end of their lives. Lin Wenxu, the first author of the paper and a postdoctoral fellow at the School of Physics and Astronomy of Yunnan University, pointed out that the star at this time still has a certain degree of gravity, so that the planet still revolves around it, but the "survival" planet does not mean that it has life.
Lin Wenxu introduced that this so-called survival means that the planet is retained as an independent celestial body, and it does not mean that there is still life on the surface. There are many rogue planets in the universe. They themselves do not orbit any stars. However, among the objects we study, these planets are orbiting a single star. However, this star has entered an advanced stage of evolution. They have expanded and contracted, and they will continue to expand in the future.
If these stars continue to expand, it is unknown whether these three planets can continue to "survive", but their ability to escape this expansion from stars to red giants has broken the traditional understanding.
Why can they "survive"? Lin Wenxu said that the team has several speculations: It is possible that this system has many planets like the solar system. During the expansion process, the inner planets are swallowed up, but during the swallowing process, the envelope material of the star will be excited, and these materials will move away from the star, forming a "regenerating planetary disk" around it. This "regenerating planetary disk" will cause the orbits of the outer planets to gradually shrink. In the process, the middle star has finished expanding and contracted back. But the outer planets still interact in this planetary disk, and finally the orbit slowly shrinks to its current state, which will form such a paradox. There is also a speculation that the various interactions produced by the star during the violent expansion process "pushed" some planets out.
These reasons are just some reasonable speculations and still need to be verified by further observations and theoretical simulations. Lin Wenxu said that this research makes us realize that the final fate of planets is more complicated than we imagined. We cannot make simple judgments based only on the maximum expansion radius of the star, but also need to consider other factors. This provides new observational basis for further understanding the long-term evolution of the solar system, especially the ultimate fate of the earth.
This discovery makes people think more: when the sun expands into a red giant, what will be the fate of these planets, including the earth?
Lin Wenxu explained that these studies can show that some planets may survive this process. Although the reason for this survival is unknown, it provides more possibilities for the future fate of the earth. It may be possible that after the sun turns into a red giant in the future, there are some special effects or laws that allow the earth to survive instead of being swallowed up by the expansion of the sun, but it cannot be directly inferred that the earth will also survive. Even if the earth does not survive in the end, the sun will slowly continue to increase, causing the earth to lose conditions suitable for life before the arrival of the sun's red giant.
However, hope remains. Perhaps in the process of the sun getting "old", humans have found a way to keep the earth away from the sun, create new conditions for the long-term continuation of life, and become a "wandering earth" that is no longer attached to the sun.
