1. 中国科学院紫金山天文台 南京 210023) ;2. 中国科学院行星科学重点实验室 南京 210023;3. 中国科学技术大学天文与空间科学学院 合肥 230026;
1. Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210023;2. CAS Key Laboratory of Planetary Sciences, Nanjing 210023;3. School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026;
M型恒星(M dwarf)是主序星中质量较小的恒星, 也是银河系中数量最多的恒星类型, 在其周围形成的行星通常距离主星较近, 宜居带也比F、G、K型恒星更靠近主星, 更有利于发现系外宜居行星.研究表明, M型恒星周围平均存在2.5颗小质量行星, 约为\lk F、G、K型恒星的3.5倍, 但M型恒星周围巨行星的出现率(occurrence rate)则比F、G、K型小一个量级.基于M型恒星周围发现的401颗行星的参数开展了统计研究, 发现质量越大的行星平均轨道半长径越大.类地行星约占行星总数的74%, 且轨道半长径均小于1 au, 其中28颗行星具有潜在宜居性.根据行星质量-半径关系, 在质量等于4倍地球质量(M⊕)处存在一拐点，除少数几颗行星外，大部分小于该质量的行星可能都是由约65%的硅酸盐和约35%的铁组成，大于该质量的行星半径则随质量增加而迅速增大.约60%的M型恒星周围的行星位于多行星系统且轨道分布紧密，相邻行星轨道在3: 2、5: 3及2 : 1等平运动共振位置处存在峰值.M型恒星的多行星系统形成与演化等问题对现今的行星形成理论提出了新挑战.
M dwarf is not only one of the lowest mass star in the main sequence stars, but also the most numerous stars in the Milky Way. Planets and habitable zones around M dwarfs are usually closer to their host star than F, G, K-type stars, so it is more convenient for us to discover new planets and observe habitable planets. On average, there are 2.5 low-mass planets orbiting M dwarf, which is about 3.5 times that of F, G, K star, while the occurrence rate of giant planets, which greatly depends on the metallicity of their host stars, is one order of magnitude lower than that of F, G, K star. Herein we apply a set of selection criteria to collect 401 planets around M dwarfs to investigate the property of planets. Statistical analysis shows that terrestrial planets,whose semi-major axes are less than 1 au, account for about 74% of the total number of planets. About twenty-eight Earth-sized planets orbit within the habitable zone of their stars, which means there might be liquid water and atmosphere on planet surface. In addition, the average distance between planets and their host stars tends to increase with the masses of planets. According to the planetary mass-radius relationship, there is a turning point at 4M⊕. With few exceptions, most planets with mass < 4M⊕ may be composed of 65% silicate and 35% iron, otherwise the radius of planet grows rapidly with the increase of mass because of a substantial gaseous envelope. Approximately 60% planets around M dwarfs are in tightly packed planetary systems, whose orbital configuration are observed to be trapped into 3 : 2, 5 : 3 and 2 : 1 mean motion resonances. To explain the formation of compact system, several scenarios have been proposed, for instance inside-out formation and pebble-driven planet formation, even though the initial position of planetary embryos remain ambiguous. The formation of giant planets around low-mass stars also challenges the existing planet formation theory. Fortunately, high precision and resolution observation of space and groundbased telescopes are unveiling the structure of protoplanetary disks gradually, which offers the crucial initial conditions of planet formation, as well as the observation of planetary atmospheric composition.
潘梦睿,季江徽,王素. M型恒星周围系外行星统计研究[J].天文学报,2021,62(4):38. PAN Meng-rui, JI Jiang-hui, WANG Su. The Statistical Investigation of Exoplanets around M Dwarfs[J]. Acta Astronomica Sinica,2021,62(4):38.复制