祝伟

《天体物理前沿讲座》

2021年12月7日

我们在宇宙中是唯一的吗？

• 既是一个哲学问题，也是一个科学问题
• 德雷克公式（Drake Equation）

讲座大纲

• 太阳系和行星形成模型

• 系外行星的探测方法

• 系外行星的统计分布

• 总结和展望

Planet formation in one picture

Planet formation to scales

原行星盘存在的时间

Image credit: C. Mordasini

(~1,000 km)

星子

原行星

（行星胚胎）

岩石行星

气态巨行星

失控气体吸积

大碰撞

• Terrestrial planets: gas wt%<<1%
• Gaseous planets: gas wt%>~10%
• Inner small planets & outer giant planets;
• One system having planets of very diverse properties(!);
• Dynamically cold (small eccentricity & mutual inclination);
• ...

Planets known before 1995\(^*\)

* Ignoring the pulsar planets.

公转轨道半长轴（日地距离）

Exoplanet Discoveries

Image from NASA Exoplanet Archive

视向速度法

凌星法

微引力透镜法

直接成像法

Planets known of today

Known planets around Sun-like stars, data from NASA Exoplanet Archive, figure adapted from Zhu & Dong (2021)

如何探测系外行星？

How to detect planets around other stars?

How does solar system look like from outside?

'Pale Blue Dot' by Voyager 1 from 40 AU

旅行者1号“暗淡蓝点”照片

Can we see planets directly?

• 年轻、距离主星较远的行星
• 至今共探测到54颗系外行星
• Star light must be suppressed
  • atmosphere turbulence \( \rightarrow \) Adaptive optics (AO) 自适应光学

HR 8799: planet brightness ~\(10^{-5}\) star

系外行星直接成像

间接测量法：视向速度法（Radial velocity）

Image credit: ESO

Radial velocity (RV) method

自1990年代起，电荷耦合器件CCD和计算机的使用极大地提高了天文观测的精度和效率。

第一颗类太阳恒星周围的行星：飞马座51b

热木星（hot Jupiters)

如何形成热木星？

Image from Dawson & Johnson (2018)

间接测量法：凌星法（Transit）

Image credit: LCOGT

Transit 凌星法

Kepler mission (2009-2013)

K2 mission (2014-2019)

• \(10^5\) target stars & 4-yr observations, how many Earth-like planets do we expect to detect?
• 0 Earth-like planets, but 1000s of exoplanets!

天鹅座和天琴座区域约100平方度

Know thy star, know thy planet

Chen & Kipping (2017)

Abundant super Earths unpredicted by theoretical models

Mordasini et al. (2009) (see also Ida & Lin 2004)

凌星时刻变化（Transit Timing Variation）

• 行星之间的相互作用导致凌星到达的时刻偏离严格的周期性。

Holman & Murray (2005), Agol et al. (2005), Holman et al. (2010)

系外行星的质量-半径关系

Figure from Hadden & Lithwick (2017)

(see also Wu & Lithwick 2011, Hadden & Lithwick 2014)

Photo-evaporation valley

Fulton et al. (2017)

Image credit: NASA

Kepler's discoveries

• Unpredicted but abundant super Earths.
• Atmosphere fraction of a few % \(\longrightarrow\) Early formation.
• Typically super Earths & ~3 per system \(\longrightarrow\) Kepler planetary systems are massive.

Super Earths

Paczynski (1986); Mao & Paczynski (1991)

\(t_{\rm E} \sim 30{\rm days} \left(\frac{M_{\rm L}}{M_\odot}\right)^{1/2} \)

Figure from Zhu & Dong (2021)

Microlensing probes cold planets

行星微引力透镜事件的光变曲线

Animations from B. Scott Gaudi

Q：微引力透镜和凌星的区别是什么？

地面微引力透镜巡天观测: KMTNet

• 3台1.6米口径望远镜，配备大视场；
• 监测银河系中心方向~10\(^8\)颗恒星的亮度变化；
• 探测效率：每年~3000例微引力透镜事件，从中发现>30颗 系外行星；
• 探测极限: planet-to-star mass ratio \(q\sim10^{-5}\).
  • ref: Earth/Sun \(q=3\times10^{-6}\).

Credit to: KASI, Gould et al. (2020)

Exoplanet statistics from microlensing

• Cold Neptunes are more abundant than cold Jupiters.
• Cold Neptunes are perhaps the most abundant?

Figures from Suzuki et al. (2016)

(see also Gould et al. 2010, Cassan et al. 2012, Clanton & Gaudi 2014, Udalski et al. 2018, Jung et al. 2019)

Microlensing@Tsinghua

• Real-time alert & follow-up observations (e.g. TAP), data reduction;
• Planet search, light curve modeling;

Yee, Zang, et al. (2021)

Microlensing from space

• Higher resolution：Individual stars resolved.
• Deeper observations：
  • More stars \(\rightarrow\) More microlensing events
  • Smaller stars \(\rightarrow\) Lower-mass planets

Images from Pietrukowicz et al. (2019)

Microlensing from space

• 中国空间站望远镜（CSST）
• 美国Roman Telescope
• 欧洲Euclid

Transit (ground)

Transit (space)

Radial velocity

Microlensing

Imaging

Figure from Zhu & Dong (2021)

系外行星普查 (Exo-)Planet Demographics

• Terrestrial planets: gas wt%<<1%, Gaseous planets: gas wt%>~10%
• Inner small planets & outer giant planets;
• One system having planets of very diverse properties(!);
• Dynamically cold (small eccentricity & mutual inclination);
• ...

Solar system vs. extra-solar systems

系外行星统计和系统构型

Exoplanet statistics & system architecture

Figure from Zhu & Dong (2021)

系外行星普查（Exoplanet demographics)

Transit (ground)

Transit (space)

Radial velocity

Microlensing

Imaging

Figure adapted from Zhu & Dong (2021)

行星多重性（multiplicity）: 1+1>2

• Unpredicted but abundant super Earths.
• Atmosphere fraction of a few % \(\longrightarrow\) Early formation.
• Typically super Earths & ~3 per system \(\longrightarrow\) Kepler planetary systems are massive.

Super Earths

Kepler systems vs. Solar system

Forming Kepler planetary systems

Image credit: P. Armitage

• 行星迁移模型（Disk-driven planet migration）

• 卵石吸积模型（Pebble accretion model）

• 1/3的拥有超级地球的行星系统中同时也存在冷木星。
  • >50%, if [Fe/H]>0.

• 拥有冷木星的行星系统几乎都存在超级地球！

Zhu & Wu, 2018, AJ, 156, 92

(see also Bryan et al. 2019, Herman, Zhu, & Wu 2019)

内行星系统和外行星系统的强相关性Inner-outer correlation

\( P({\rm SE}|{\rm CJ}) \cdot P({\rm CJ}) = P({\rm CJ}|{\rm SE}) \cdot P({\rm SE}) \) \( \rightarrow P({\rm SE}|{\rm CJ})=100\% \)

类太阳系行星系统的普遍性

Zhu & Wu, 2018, AJ, 156, 92

• 太阳系不存在超级地球 (70%).
• 太阳系存在冷木星 (10%).

(see also Batygin & Laughlin 2015)

\( P({\rm CJ,~no~SE}) = P({\rm CJ}) \cdot [1-P({\rm SE}|{\rm CJ})] \)

• Terrestrial planets: gas wt%<<1%, Gaseous planets: gas wt%>~10%
• Inner small planets & outer giant planets;
• One system having planets of very diverse properties(!);
• Dynamically cold (small eccentricity & mutual inclination);
• ...

Solar system vs. extra-solar systems

总结

• 基于太阳系的行星形成模型无法解释探测到的系外行星
  • Close-in massive planets (hot Jupiters, super Earths);
  • Abundant planets with ~1%wt atmosphere;
  • Dynamically hot orbits.
• 系外行星探测和统计
  • 单一方法的工作原理、成就和探测极限
    • RV finds relatively massive planets;
    • Transit finds close-in planets;
    • Microlensing detects distant planets;
  • 行星多重性提供更丰富的信息
    • The strong inner-outer correlation

• Planet mass-radius relation \(\rightarrow\) constraint on composition;
• Exoplanet atmosphere observations.

Future directions:

Exoplanet atmosphere characterization

Future directions:

Observing planet formation

Figures from Andrews (2020)

Atacama Large Millimeter/submillimeter Array (ALMA)

系外行星研究@中国

• 问题：第一颗利用中国自主设备探测到的太阳系外行星发现于哪一年？

系外行星研究@中国

中国太空站望远镜CSST、地球2.0凌星项目（ET）、近邻宜居行星巡天计划（CHES）、紫瞳、觅音、天邻

总结 & 展望

• 基于太阳系的行星形成模型无法解释探测到的系外行星
  • Close-in massive planets (hot Jupiters, super Earths);
  • Abundant planets with ~1%wt atmosphere;
  • Dynamically hot orbits.
• 系外行星探测和统计
  • 单一方法的工作原理、成就和探测极限
    • RV finds relatively massive planets;
    • Transit finds close-in planets;
    • Microlensing detects distant planets;
  • 行星多重性提供更丰富的信息
    • The strong inner-outer correlation
• 展望：行星大气研究，原行星盘研究，中国未来的系外行星空间探测项目

• Radial velocity

How to detect planets indirectly?

• Transit

• Astrometry

• Microlensing

First detection: 1989/1995

# of detections: ~700

First detection: 2000

# of detections: >4000

# of detections: 0

(Gaia ~2024)

First detection: 2003

# of detections: ~50

Kepler systems are dynamically hot: large eccentricities

• Fewer-planet systems are dynamically hotter.

• Each system has on average 3 planets (average multiplicity).

• Dynamical evolution may have reshaped the architecture.

Zhu et al., 2018

(see also He et al. 2020)

Kepler systems are dynamically hot

Implications to future observations

• All-sky survey, bright stars
• 27 days per sector
• Looking for close-in planets around nearby stars

Transiting Exoplanet Survey Satellite

Huang et al., (2018) (see also Gandolfi et al. 2018)

Another similar system: HD 86226 (TOI-652.01)

Image credit: NASA/TESS

• \(\sim10^4\) planet detections, \(\sim10^3\) multi-planet systems.

• Gaia+TESS+LAMOST: 100s of TESS-Gaia systems \(\rightarrow\) refining the inner-outer correlation.

(Gaia+RV: Better characterization of Gaia multi-planet systems.)

Perryman et al. (2014)

Gaia (& future mission) astrometry