近期，来自北京大学、中国科学院大学、国家天文台、清华大学以及美国圣母大学的学者组成的国际研究团队，利用Gaia天体测量数据和SOPHIE高分辨率光谱，以及LAMOST等分光样本构建的年龄-运动学/元素丰度标度关系，对一颗拥有近距离行星的红团簇巨星的年龄进行了计算，为理解该系统的形成与演化提供了重要限制。论文第一作者为北京大学天文系博士研究生陈慧玲，通讯作者为中国科学院大学黄样副教授和北京大学张华伟研究员。该研究成果发表于国际专业学术期刊《天体物理学报通讯》（2024, ApJL, 966, L27），正式发表后即被美国天文学会（AAS）Nova网站选为研究亮点，以《谜云再现：不可能存在的行星之谜》（Astronomers Reopen the Mystery of a Planet That Shouldn’t Exist ）为题在网站头条报道（见图1）。
目前所发现的大部分系外行星系统与太阳系一样，行星围绕燃烧的“太阳” 稳定运行。当然，恒星的寿命终有尽头，太阳并不是永恒的。大约50亿年后，太阳将结束稳定的主序阶段，成为一颗红巨星，半径向外急剧扩张并吞噬位于内侧轨道的行星。这部分中心恒星半径扩张所覆盖的范围将成为其附近行星的“禁区”。
然而，2023年6月，《自然》杂志上发表的工作（Hon et al. 2023, Nature, 618, 917）发现了一个特殊的行星系统：在距离经历过半径膨胀过程的主星8 UMi（一颗红团簇星）不到0.5个天文单位的地方，有一颗行星Halla正以极圆的轨道围绕8 UMi运行。那么，为什么8 UMi的半径增长过程没有吞噬如此靠近它的Halla？
Hon等人将其归因于8 UMi的特殊演化过程，他们采用双星并合模型使得8 UMi可以跳过半径增长过程，但这个模型需要至少86亿年来完成演化。因此，年龄测量成为了判断8 UMi演化路径的重要参数。
由于双星并合过程不改变系统自诞生时起在银河系中的运动以及重元素丰度特征，这就为该研究团队基于Gaia DR3提供的天体位置测量数据以及SOPHIE高分辨率观测谱从8 UMi的运动学和化学性质推断其真实年龄提供了契机。运动学分析表明，8 UMi是一颗年轻的薄盘星（见图2）；作为“化学时钟”的元素丰度比，[C/N]和[Y/Mg]，给出的年龄则为30-40亿年（见图3）。两种观测性质皆显示8 UMi是一颗年轻恒星，与该星单星演化的年龄一致，显著年轻于双星演化所需的时间。
综合来看，双星模型所需的演化时间与8 UMi目前呈现的年龄不符，这说明还有更多潜在效应（例如引力潮汐）可能会在恒星-行星系统的后期演化中发挥重要作用。因此，类似地球这样比较接近母星的行星，其最终的轨道演化或许比我们曾经认为的更加复杂多样，也为地球可以逃脱未来“膨胀太阳”的吞噬提供了无限遐想。
文章链接：https://iopscience.iop.org/article/10.3847/2041-8213/ad3bb4
AAS NOVA报道：https://aasnova.org/2024/05/10/astronomers-reopen-the-mystery-of-a-planet-that-shouldnt-exist/
图1：美国天文学会（AAS）Nova网站的专题报道。
Figure 1: New report on the American Astronomical Society (AAS) Nova Website.
图2: 8 UMi及银河系内与其类似的红团簇星阶段恒星（样本来自LAMOST巡天；Huang et al. 2020）的运动学特征图像。不同颜色深度表示不同的平均年龄。
Figure 2: Kinematic properties of 8 UMi and similar red clump giant phase stars in the Milky Way (sample from LAMOST survey; Huang et al. 2020). Different color depths represent varying median ages in each velocity bins.
图3: 从两个“化学时钟”，[C/N]和[Y/Mg]，进行年龄估计。两个图中的红色星代表测量得到的8 UMi元素丰度比及年龄。左图中的背景样本来自LAMOST巡天（Huang et al. 2020），实线圈代表不同密度。右图中黑色点为Nissen et al. (2020)的样本，蓝色实线是对这些样本做线性拟合的结果，蓝色虚线内为拟合的1-sigma误差范围。
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. The black points in the right figure 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.
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 calibration constructed by spectral samples from LAMOST to derived the age for a red clump giant star hosting close-in planets. This provides crucial constraints for understanding the formation and evolution of the system. The first author of the paper is Ph.D. student, Chen Huiling, from the Department of Astronomy at Peking University, with corresponding authors being Associate Professor Huang Yang from the University of Chinese Academy of Sciences and Researcher Zhang Huawei from Peking University. The paper is published in the international professional academic journal "The Astrophysical Journal Letters" (2024, ApJL, 966, L27). Shortly after formal publication, it is selected as a highlight by the American Astronomical Society (AAS) Nova website, with the headline "Astronomers Reopen the Mystery of a Planet That Shouldn’t Exist" (see Figure 1) on the website's front page.
Most of the exoplanetary systems discovered to date resemble our solar system, with planets stably orbiting around their burning "suns." 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 on 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), which has undergone radial expansion. So, why did the radius expansion process of 8 UMi not engulf Halla, which is so close to it?
Hon et al. attributed this to the unique evolutionary process of 8 UMi. They employ 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 becomes an important parameter for 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 perspectives with astrometric data sourced from Gaia DR3, alongside high-resolution spectroscopic observations conducted with SOPHIE. Kinematic analysis indicates that 8 UMi is a young thin-disk star (see Figure 2). Besides, two chemical indicators, [C/N] and [Y/Mg], which are 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 a 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 align with the current age exhibited by 8 UMi. This suggests that there are additional effects, such as tidal interaction, which 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
The AAS NOVA news press: https://aasnova.org/2024/05/10/astronomers-reopen-the-mystery-of-a-planet-that-shouldnt-exist/
