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Writer's pictureYang Huang

The Mystery Reopens: How Do Close-in Planets Avoid Engulfment?

Recently, an international research team composed of scholars from Peking University, University of Chinese Academy of Sciences, National Astronomical Observatory, Tsinghua University, and University of Notre Dame utilizes Gaia astrometric data, SOPHIE high-resolution spectroscopy, and age-kinematics/chemical abundance empirical relations constructed by sample from the LAMOST spectroscopic survey to derive the age for a red clump giant star hosting close-in planets. This provides crucial constraints for understanding the formation and evolution of this unique system. This research was led by Huiling Chen, a Ph.D. student at Peking University, Yang Huang, associate professor at University of Chinese Academy of Sciences, and Researcher Zhang Huawei from Peking University. The paper has been published in "The Astrophysical Journal Letters" (2024, ApJL, 966, L27). Shortly after formal publication, it was highlighted by the American Astronomical Society (AAS) Nova on 10 May 2024 (see Figure 1).

Figure 1: The study was highlighted by American Astronomical Society (AAS) Nova Website.

 

Most of the exoplanet systems discovered to date resemble our solar system, with planets stably orbiting around their host stars. Unfortunately, stars have finite lifespans, including our Sun. In approximately 5 billion years, the Sun will cease its stable main sequence phase, becoming a red giant, expanding outward and engulfing planets in its inner orbits. This expanded radius of the central star will create a 'forbidden zone' for planets in its vicinity.

 

However, in June 2023, a study published in Nature (Hon et al., 2023, Nature, 618, 917) revealed an intriguing planetary system: a planet named Halla follows an exceptionally circular orbit within 0.5 astronomical units of the host star 8 UMi, a red clump giant that should undergo radial expansion. So, why did the radius expansion process of 8 UMi not engulf Halla, despite its close proximity?

 

Hon et al. attributed this to the unique evolutionary process of 8 UMi. They employed a binary merger model to allow 8 UMi to bypass the radial expansion process. Nevertheless, this model requires at least 8.6 billion years to complete the evolution. Therefore, age of this star are key to determining the evolutionary path of 8 UMi.

 

Due to the fact that the binary merger process does not alter the motion of the system within the Milky Way since its inception, nor its abundance of heavy elements, the group has an opportunity to explore the true age of 8 UMi from both kinematic and chemical properties using astrometric data sourced from Gaia DR3, alongside high-resolution spectroscopic observations taken by SOPHIE. Kinematic analysis indicates that 8 UMi is a young thin-disk star (see Figure 2). Additionally, two chemical indicators, [C/N] and [Y/Mg], capable of tracing age through elemental abundance ratios, offer similar estimations for the age of 8 UMi, approximately 3 to 4 billion years (see Figure 3). Both observational properties indicate that 8 UMi is a young star, consistent with the age of single-star evolution and significantly younger than the time required for binary star evolution.

 

In conclusion, the evolutionary time required by the binary model does not agree with the current age exhibited by 8 UMi. This suggests that additional effects, such as tidal interaction, may play a crucial role in the later evolution of stellar-planet systems. Therefore, planets similar to Earth, which are relatively close to their parent stars, may undergo orbital evolutions that are more complex and diverse than previously thought. This also sparks limitless imagination about Earth's potential escape from the engulfment of a future 'expanding sun'.


The full text of the paper is available online at: https://iopscience.iop.org/article/10.3847/2041-8213/ad3bb4

 

 

Figure 2: Kinematic properties of 8 UMi and similar red clump giant phase stars in the Milky Way (sampled from the LAMOST survey; Huang et al., 2020). Different color depths represent varying median ages in each velocity bin.

 

 



Figure 3: Age estimation is conducted based on two 'chemical clocks', [C/N] and [Y/Mg]. The red stars in both figures represent the measured elemental abundance ratios and ages of 8 UMi. In the left figure, background samples are from the LAMOST survey (Huang et al., 2020), with contours representing different densities. In the right figure, the black points represent the sample from Nissen et al. (2020), while the blue solid line represents the linear fit to these samples, with the blue dashed lines indicating the 1-sigma error range of the fit.


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