穿越时空的类地行星
Terrestrial planets across space and time
摘要：
对宇宙学、星系形成和系外行星的研究现在已经发展到可以尝试对类地行星进行宇宙清查的阶段。通过耦合半解析模型的星系形成的一个配方，涉及到行星的发生，其宿主恒星的质量和金属丰度，我们跟踪人口的类地行星周围的太阳质量（FGK型）和低质量（M矮星）恒星在整个宇宙历史。我们发现，类地行星的平均年龄为$7\pm{}1$ Gyr的FGK主机和$8\pm{}1$ Gyr的M矮星在本地宇宙。我们估计，热彗星已经耗尽了FGK恒星周围的类地行星的人口不超过$\approx 10\%$，只有$\approx 10\%$的类地行星在当前的时代是轨道上的恒星在金属丰度范围内，这样的行星尚未得到证实。本地宇宙中典型的类地行星位于一个以球状体为主的星系中，其总恒星质量与银河系相当。当查看整个可观测宇宙中的行星清单时，我们认为FGK和M恒星周围分别有$\approx 1\times 10^{19}$和$\approx 5\times 10^{20}$类地行星。由于光旅行时间的影响，我们过去光锥上的类地行星的平均年龄只有$1.7\pm 0.2$ Gyr。这些结果将在宇宙可居住性、哥白尼原理和在宇宙学距离上寻找外星智慧的背景下进行讨论。
英文摘要：
The study of cosmology, galaxy formation and exoplanets has now advanced to a stage where a cosmic inventory of terrestrial planets may be attempted. By coupling semi-analytic models of galaxy formation to a recipe that relates the occurrence of planets to the mass and metallicity of their host stars, we trace the population of terrestrial planets around both solar-mass (FGK type) and lower-mass (M dwarf) stars throughout all of cosmic history. We find that the mean age of terrestrial planets in the local Universe is $7\pm{}1$ Gyr for FGK hosts and $8\pm{}1$ Gyr for M dwarfs. We estimate that hot Jupiters have depleted the population of terrestrial planets around FGK stars by no more than $\approx 10\%$, and that only $\approx 10\%$ of the terrestrial planets at the current epoch are orbiting stars in a metallicity range for which such planets have yet to be confirmed. The typical terrestrial planet in the local Universe is located in a spheroid-dominated galaxy with a total stellar mass comparable to that of the Milky Way. When looking at the inventory of planets throughout the whole observable Universe, we argue for a total of $\approx 1\times 10^{19}$ and $\approx 5\times 10^{20}$ terrestrial planets around FGK and M stars, respectively. Due to light travel time effects, the terrestrial planets on our past light cone exhibit a mean age of just $1.7\pm 0.2$ Gyr. These results are discussed in the context of cosmic habitability, the Copernican principle and searches for extraterrestrial intelligence at cosmological distances.
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