Noname manuscript No.
(will be inserted by the editor)
ET White Paper: To Find the First Earth 2.0
Edited by :
Jian Ge, Hui Zhang, Hongping Deng, Kevin Willis,
& Beverly Ge
Contributing Authors:
Jian Ge
1
, Hui Zhang
1
, Weicheng Zang
2
, Hongping
Deng
1
, Shude Mao
2
,
17
, Ji-Wei Xie
3
, Hui-Gen Liu
3
, Ji-Lin Zhou
3
,
Kevin Willis
20
, Chelsea Huang
26
, Steve B. Howell
41
, Fabo Feng
5
,
Jiapeng Zhu
1
, Xinyu Yao
1
, Beibei Liu
8
, Masataka Aizawa
5
, Wei
Zhu
2
, Ya-Ping Li
1
, Bo Ma
4
, Quanzhi Ye
11
,
12
, Jie Yu
6
, Maosheng
Xiang
7
,
17
, Cong Yu
4
, Shangfei Liu
4
, Ming Yang
3
, Mu-Tian Wang
3
,
Xian Shi
1
, Tong Fang
1
, Weikai Zong
28
, Jinzhong Liu
13
, Yu Zhang
13
,
Liyun Zhang
16
, Kareem El-Badry
36
, Rongfeng Shen
4
, Pak-Hin
Thomas Tam
4
, Zhecheng Hu
4
, Yanlv Yang
4
, Yuan-Chuan Zou
14
,
Jia-Li Wu
14
, Wei-Hua Lei
14
, Jun-Jie Wei
15
, Xue-Feng Wu
15
,
Tian-Rui Sun
15
, Fa-Yin Wang
3
, Bin-Bin Zhang
3
, Dong Xu
17
,
Yuan-Pei Yang
18
, Wen-Xiong Li
19
, Dan-Feng Xiang
2
, Xiaofeng
Wang
2
, Tinggui Wang
9
, Bing Zhang
43
, Peng Jia
40
, Haibo Yuan
28
,
Jinghua Zhang
17
, Sharon Xuesong Wang
2
, Tianjun Gan
2
, Wei
Wang
14
, Yinan Zhao
24
, Yujuan Liu
14
Yonghe Chen
21
, Chuanxin
Wei
21
, Yanwu Kang
21
, Baoyu Yang
21
, Chao Qi
21
, Xiaohua Liu
21
,
Quan Zhang
21
, Yuji Zhu
21
, Dan Zhou
1
, Congcong Zhang
1
, Yong
Yu
1
, Yongshuai Zhang
1
, Yan Li
1
, Zhenghong Tang
1
, Chaoyan
Wang
1
, Fengtao Wang
22
, Wei Li
22
, Pengfei Cheng
22
, Chao Shen
22
,
Baopeng Li
22
, Yue Pan
22
, Sen Yang
22
, Wei Gao
22
, Zongxi Song
22
,
Jian Wang
9
, Hongfei Zhang
9
, Cheng Chen
9
, Hui Wang
9
, Jun
Zhang
9
, Zhiyue Wang
9
, Feng Zeng
9
, Zhenhao Zheng
9
, Jie Zhu
9
,
Yingfan Guo
9
, Yihao Zhang
9
, Yudong Li
44
, Lin Wen
44
, Jie Feng
44
,
Wen Chen
23
, Kun Chen
23
, Xingbo Han
23
, Yingquan Yang
23
, Haoyu
Wang
23
, Xuliang Duan
23
, Jiangjiang Huang
23
, Hong Liang
23
,
Shaolan Bi
28
, Ning Gai
30
, Zhishuai Ge
46
, Zhao Guo
29
, Yang
Huang
18
, Gang Li
39
, Haining Li
17
, Tanda Li
28
, Yuxi (Lucy) Lu
37
,
38
,
Hans-Walter Rix
7
, Jianrong Shi
17
, Fen Song
31
, Yanke Tang
30
,
Yuan-Sen Ting
26
,
27
, Tao Wu
63
,
64
,
65
,
66
, Yaqian Wu
17
, Taozhi Yang
47
,
Qing-Zhu Yin
45
, Andrew Gould
7
,
32
, Chung-Uk Lee
33
, Subo Dong
34
,
Jennifer C. Yee
34
, Yossi Shvartzvald
35
, Hongjing Yang
2
, Renkun
Kuang
2
, Jiyuan Zhang
2
,Shilong Liao
1
, Zhaoxiang Qi
1
, Jun Yang
44
,
Ruisheng Zhang
3
, Chen Jiang
6
, Jian-Wen Ou
48
, Yaguang Li
49
,
54
,
Paul Beck
50
, Timothy R. Bedding
49
,
54
, Tiago L. Campante
51
,
52
,
William J. Chaplin
53
,
54
,
55
, Jørgen Christensen-Dalsgaard
54
, Rafael
A. Garc ́ıa
56
, Patrick Gaulme
6
, Laurent Gizon
6
,
57
,
58
, Saskia
Hekker
59
,
60
, Daniel Huber
61
, Shourya Khanna
62
, Yan Li
63
,
64
,
65
,
66
,
Savita Mathur
67
,
68
, Andrea Miglio
53
,
70
,
71
, Benoˆıt Mosser
72
, J. M.
Joel Ong
61
,
73
,
ˆ
Angela R. G. Santos
51
, Dennis Stello
49
,
54
,
74
, Dominic
M. Bowman
75
, Mariel Lares-Martiz
69
, Simon Murphy
76
, Jia-Shu
Niu
40
, Xiao-Yu Ma
28
, L ́aszl ́o Moln ́ar
78
,
79
, Jian-Ning, Fu
28
, Peter De
Cat
77
, Jie Su
63
,
64
,
65
, and the ET consortium
June 15, 2022
arXiv:2206.06693v1 [astro-ph.IM] 14 Jun 2022
2
Ge et al.
Institutes:
1. Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai, China
jge@shao.ac.cn, zhangh@shao.ac.cn
2. Tsinghua University, Beijing, China
3. Nanjing University, Nanjing, China
4. Sun Yat-Sen University, Zhuhai, China
5. Tsung-Dao Lee Institute, Shanghai Jiao Tong University, Shanghai, China
6. Max Planck Institute for Solar System Research, G ̈ottingen, Germany
7. Max-Planck-Institute f ̈ur Astronomie, Heidelberg, Germany
8. Zhejiang University, Hangzhou, China
9. University of Science and Technology of China, Hefei, China
10. Taiyuan University of Technology, Taiyuan, China
11. University of Maryland, College Park, MD, USA
12. Boston University, Boston, MA, USA
13. Xinjiang Astronomical Observatory, Chinese Academy of Sciences, Xinjiang, China
14. Huazhong University of Science and Technology, Wuhan, China
15. Purple Mountain Observatories, Chinese Academy of Sciences, Nanjing, China
16. Guizhou University, Guiyang, China
17. National Astronomical Observatories, Chinese Academy of Sciences, Beijing, China
18. Yunnan University, Kunming, China
19. Tel Aviv University, Tel Aviv, Israel
20. Science Talent Training Center, Gainesville, USA
21. Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai, China
22. Xi’An Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi’an, China
23. Innovation Academy for Microsatellites, Chinese Academy of Sciences, Shanghai, China
24. Department of Astronomy of the University of Geneva, Versoix, Switzerland
25. University of Southern Queensland, QSL, Australia
26. Research School of Astronomy & Astrophysics, Australian National University, ACT, Australia
27. Research School of Computer Science, Australian National University, ACT, Australia
28. Beijing Normal University, Beijing, China
29. University of Cambridge, Cambridge, UK
30. Dezhou University, Dezhou, China
31. Jimei University, Xiamen, China
32. Ohio State University, Columbus, OH, USA
33. Korea Astronomy and Space Science Institute, Daejon, Republic of Korea
34. Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing, China
35. Center for Astrophysics
|
Harvard & Smithsonian, Cambridge, MA, USA
36. Weizmann Institute of Science, Rehovot, Israel
37. Columbia University, New York, NY, USA
38. American Museum of Natural History, Central Park West, Manhattan, NY, USA
39. IRAP, Universit ́e de Toulouse, CNRS, CNES, UPS, Toulouse, France
40. Shanxi University, Taiyuan 030006, China
41. NASA Ames Research Center, Moffett Field, CA, USA
42. University of Nevada, Las Vegas, NV 89118, USA
43. Xinjiang Technical Institute of Physics and Chemistry, CAS, Urumqi, China
44. School of Physics, Peking University, Beijing, China
45. University of California, Davis, USA
46. Beijing Planetarium, Beijing Academy of Science and Technology, Beijing, China
47. Xi’an Jiaotong University, Xi’an, China
48. Shaoguan University, 512005 Shaoguan, China
49. Sydney Institute for Astronomy (SIfA), School of Physics, University of Sydney, NSW 2006, Australia
50. Institut f ̈ur Physik, Karl-Franzens Universit ̈at Graz, Graz, Austria
51. Instituto de Astrof ́ısica e Ciˆencias do Espa ̧co, Universidade do Porto, Porto, Portugal
52. Departamento de F ́ısica e Astronomia, Faculdade de Ciˆencias da Universidade do Porto, Porto, Portugal
ET White Paper: To Find the First Earth 2.0
3
53. School of Physics and Astronomy, University of Birmingham, Birmingham, UK
54. Stellar Astrophysics Centre (SAC), Department of Physics and Astronomy, Aarhus University, Aarhus,
Denmark
55. Kavli Institute for Theoretical Physics, University of California, Santa Barbara, CA, USA
56. AIM, CEA, CNRS, Universit ́e Paris-Saclay, Universit ́e de Paris, Sorbonne Paris Cit ́e, France
57. Institut f ̈ur Astrophysik, Georg-August-Universit ̈at G ̈ottingen, G ̈ottingen, Germany
58. Center for Space Science, NYUAD Institute, New York University Abu Dhabi, Abu Dhabi, UAE
59. Landessternwarte K ̈onigstuhl (LSW), Heidelberg University, K ̈onigstuhl 12, 69117 Heidelberg, Germany
60. Heidelberg Institute for Theoretical Studies (HITS) gGmbH, Heidelberg, Germany
61. Institute for Astronomy, University of Hawai‘i, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
62. INAF - Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese (TO), Italy
63. Yunnan Observatories, Chinese Academy of Sciences, Kunming, China
64. Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kun-
ming, China
65. Center for Astronomical Mega-Science, Chinese Academy of Sciences, Beijing, China
66. University of Chinese Academy of Sciences, Beijing 100049, China
67. Instituto de Astrof ́ısica de Canarias (IAC), 38205, La Laguna, Tenerife, Spain
68. Universidad de La Laguna (ULL), Departamento de Astrof ́ısica, 38206, La Laguna, Tenerife, Spain
69. Instituto de Astrof ́ısica de Andaluc ́ıa (IAA-CSIC), 18008, Granada, Spain
70. Dipartimento di Fisica e Astronomia Augusto Righi, Universit`a degli Studi di Bologna, Bologna, Italy
71. INAF-Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Bologna, Italy
72. LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universit ́e, Universit ́e Paris
Diderot, 92195, Meudon, France
73. Department of Astronomy, Yale University, 52 Hillhouse Ave., New Haven, CT 06511, USA
74. School of Physics, University of New South Wales, NSW 2052, Australia
75. Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium
76. Centre for Astrophysics, University of Southern Queensland, Toowoomba, QLD 4350 Australia
77. Royal Observatory of Belgium, Ringlaan 3, B-1180 Brussel, Belgium
78. Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, E ̈otv ̈os Lor ́and Research Net-
work (ELKH), Konkoly Thege Mikl ́os ́ut 15-17, H-1121 Budapest, Hungary
79. ELTE E ̈otv ̈os Lor ́and University, Institute of Physics, 1117, P ́azm ́any P ́eter s ́et ́any 1/A, Budapest, Hun-
gary
4
Ge et al.
Acknowledgment
of
Reviewers
The science and technologies of the ET project have been reviewed by many science and technical experts.
These experts provided candid and critical comments, which helped the ET team pinpoint the scientific
objectives and improve the systematic design of ET. The ET team is very grateful for the invaluable contri-
butions from all reviewers listed below (in alphabetical order by last name):
Prof. Qi An,
University of Science and Technology of China
;
Prof. Zhiming Cai,
Innovation Academy for Microsatellites, CAS (Chinese Academy of Sciences)
;
Prof. Jing Chang,
National Astronomical Observatories, CAS;
Prof. Xiangqun Cui,
Nanjing Institute of Astronomical Optics & Technology, CAS;
Prof. Mingde Ding,
Nanjing University;
Prof. Jiangpei Dou,
Nanjing Institute of Astronomical Optics & Technology, CAS;
Prof. Cheng Fang,
Nanjing University;
Prof. Quanlin Fan,
Technology and Engineering Center for Space Utilization, CAS;
Prof. Xuewu Fan,
Xi’An Institute of Optics and Precision Mechanics, CAS;
Prof. Huixing Gong,
Shanghai Institute of Technical Physics, CAS;
Prof. Yidong Gu,
Technology and Engineering Center for Space Utilization, CAS;
Prof. Baozhu Guo,
China Aerospace Science and Technology Corporation;
Prof. Luis C. Ho,
Peking University;
Prof. Xiaoxia Huang,
Beijing Institute of Remote Sensing Information;
Prof. Haiying Hu,
Innovation Academy for Microsatellites, CAS;
Prof. Aiming Jiang,
National Astronomical Observatories, CAS;
Prof. Guang Jin,
Changchun Institute of Optics, Fine Mechanics and Physics, CAS;
Prof. Jing Li,
Technology and Engineering Center for Space Utilization, CAS;
Prof. Douglas. C. Lin,
University of Santa Cruz / Tsinghua University;
Prof. Huawang Li,
Innovation Academy for Microsatellites, CAS;
Prof. Xinfeng Li,
Technology and Engineering Center for Space Utilization, CAS;
Prof. Jie Lv,
Technology and Engineering Center for Space Utilization, CAS;
Prof. Jianwei Pan,
University of Science and Technology of China;
Prof. Yuntian Pei,
Shanghai Institute of Technical Physics, CAS;
Prof. Zhiqiang Shen,
Shanghai Astronomical Observatory, CASs;
Prof. Jianzhong Shi,
Aerospace Systems Division;
Prof. Shengcai Shi,
Purple Mountain Observatories, CAS;
Prof. Huixian Sun,
Technology and Engineering Center for Space Utilization, CAS;
Prof. Chi Wang,
Technology and Engineering Center for Space Utilization, CAS;
Prof. Jianyu Wang,
Shanghai Institute of Technical Physics, CAS;
Prof. Desheng Wen,
Xi’An Institute of Optics and Precision Mechanics, CAS;
Prof. Ji Wu,
Technology and Engineering Center for Space Utilization, CAS;
Prof. Xiangping Wu,
National Astronomical Observatories/Shanghai Astronomical Observatory, CAS;
Prof. Yanqing Wu,
Toronto University;
Prof. Weiming Xiong,
Technology and Engineering Center for Space Utilization, CAS;
Prof. Genqing Yang,
Shanghai Institute of Microsystem and Information Technology, CAS;
Prof. Jianfeng Yang,
Xi’An Institute of Optics and Precision Mechanics, CAS;
Prof. Wengang Yang,
Xi’An Institute of Optics and Precision Mechanics, CAS;
Prof. Shuhua Ye,
Shanghai Astronomical Observatory, CAS;
Prof. Jinpei Yu,
Innovation Academy for Microsatellites, CAS;
Prof. Shuangnan Zhang,
Institute of High Energy Physics, CAS;
Prof. Yonghe Zhang,
Innovation Academy for Microsatellites, CAS;
Prof. Yongwei Zhang,
China Spacesat Co. Ltd.;
Prof. Jianhua Zheng,
Technology and Engineering Center for Space Utilization, CAS;
Prof. Zi Zhu,
Nanjing University.
Contents
1 Executive Summary
7
2 Scientific Goals
9
2.1
Planetary Science
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
2.1.1
Earth 2.0s and Terrestrial Planets . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
2.1.2
Mass Functions for Free-Floating Planets and Long-Period Planets . . . . . . . . . . .
14
2.1.3
Super-Earths and sub-Neptunes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18
2.1.4
Cold Giant Planets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
2.1.5
Correlations Between Exoplanets and Their Host Stars . . . . . . . . . . . . . . . . . .
21
2.1.6
Multi-Planet Exoplanetary System . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23
2.1.7
Exomoons, Exorings & Exocomets . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25
2.1.8
Circumbinary Planets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27
2.1.9
Transiting Planets around White Dwarfs . . . . . . . . . . . . . . . . . . . . . . . . . .
29
2.1.10 The Solar System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
31
2.1.11 Planetary Science Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
34
2.2
Stellar and Milky Way Science
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
35
2.2.1
Asteroseismology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
35
2.2.2
Stellar Age and Galactic Archaeology . . . . . . . . . . . . . . . . . . . . . . . . . . .
41
2.2.3
Binary and Multiple Stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
47
2.2.4
Black Holes with Visible Companions . . . . . . . . . . . . . . . . . . . . . . . . . . .
50
2.3
Time-domain Sciences
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
52
2.3.1
High-Energy Transients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
52
2.3.2
Supernovae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
55
3 Payload
58
3.1 Basic Instrument Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
58
3.2 Telescope Optical Units (TOU)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
59
3.2.1
Transit Telescope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
59
3.2.2
Microlensing Telescope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
63
3.3 Focal Plane Array Assembly (FPAA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
65
3.4 Front End Electronics (FEE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
66
3.4.1
Composition and Main Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
66
3.4.2
System Software Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
66
3.4.3
Camera Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
67
3.5 Thermal Subsystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
68
3.6 On-board Data Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
68
4 Mission Design
70
4.1 Mission Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
70
4.1.1
Proposed Mission Profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
70
4.1.2
Mission Orbit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
71
4.1.3
Pointing Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
71
4.1.4
Orbit Environment Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
72
4.2 Key Design Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
72
4.2.1
Pointing Stability Requirement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
72
4.3 Spacecraft Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
74
4.3.1
Spacecraft Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
74
4.3.2
Spacecraft Budgets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
77
4.4 Technology Readiness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
77
5
6
Ge et al.
5 Survey Simulations and Yield Predictions
78
5.1 End-to-End Photometry Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
78
5.1.1
Simulator Design and Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . .
78
5.1.2
Testing and Validation of Simulator Output
. . . . . . . . . . . . . . . . . . . . . . .
79
5.2 Transiting Exoplanet Yield Predictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
81
5.2.1
The Input Stellar Population . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
82
5.2.2
The Planet Population . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
84
5.2.3
Transit Signal Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
85
5.2.4
Sanity Check with
Kepler
’s Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
86
5.2.5
The Planet Yield of ET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
88
5.3 Microlensing Exoplanet Yield Estimates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
91
6 Target Selection and Follow-up Observations
93
6.1 ET Target Selection and Input Catalogue . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
93
6.1.1
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
94
6.1.2
Building ET’s Input Catalog . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
94
6.1.3
Building ET’s Target List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
96
6.1.4
Prospect of Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
97
6.2 Ground and Space-based Follow-up Programs . . . . . . . . . . . . . . . . . . . . . . . . . . .
97
6.2.1
Ground-based Photometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
97
6.2.2
High-Resolution Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
98
6.2.3
Reconnaissance Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
98
6.2.4
Precise RVs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
99
6.2.5
Planetary Atmosphere Characterization . . . . . . . . . . . . . . . . . . . . . . . . . .
99
7 Summary
100
ET White Paper: To Find the First Earth 2.0
7
1 Executive Summary
The Earth 2.0 (ET) mission is a space telescope being developed in China to detect thousands of terrestrial
exoplanets over a wide range of orbital periods and in interstellar space including Earth 2.0s, which are
habitable Earth-like planets (0.8-1.25 Earth radius) orbiting solar-type stars. The long-baseline precision
photometry enabled by the first space-based ultra-high precision CMOS photometer combined with a mi-
crolensing telescope allows the determination of the occurrence rate of Earth 2.0s, for the first time, as well
as cold and free-floating low-mass planets.
Science Goals
1. The ET mission will explore the diversity of Earth-sized planet populations with different orbital periods
including close-in sub-Earths, terrestrial-like planets in habitable zones, cold planets, and free-floating
planets, and will accurately determine the occurrence rates of these small/low-mass planets. ET will
also study correlations between Earth-sized planets with stellar properties (e.g., mass, multiplicity) and
Galactic environments (e.g., thin disks, thick disks, bulge, and halo). These studies will address three
key questions: 1) How common are habitable Earth-like planets orbiting around solar-type stars? 2) How
do Earth-like planets form and evolve? 3) What is the mass function and likely origin of free-floating
low-mass planets?
2. The ET mission will significantly expand known planet populations, especially for cold planets with
orbital periods up to 4 years. Observations of diverse multiple systems consisting of super-Earths, sub-
Neptunes, and gas giants will shed further light on the coevolution of planetary worlds. In addition, there
will be a considerable gain in the observation of rare populations such as ultra-short period planets,
circumbinary planets, and metal-rich terrestrial planets which will allow for the development of coherent
formation theories. The detection of more exotic objects such as exomoons, exorings, and exocomets
is attainable with ET allowing for our Solar System to be put into context via censuses for extrasolar
systems.
3. ET survey data will also facilitate studies in fields such as asteroseismology, Galactic archeology, time-
domain sciences, Solar System objects, and black holes in binaries. Asteroseismology of bright host stars
with planet candidates will provide accurate measurements of stellar parameters such as mass, radius, and
age. Asteroseismology of different types of stars at different evolution stages will provide a comprehensive
understanding of stellar interiors and evolution. ET will enable the rigorous archaeology of the Milky
Way by providing state-of-the-art age dating for a substantial set of the oldest stars in our galaxy. ET
will also serendipitously observe tens of thousands of Solar System objects over its lifetime, providing
uninterrupted monitoring of bodies from the inner Solar System region to the trans-Neptunian region.
This unique dataset will enhance our understanding of the dynamical and physical state of Solar System
bodies.To get an idea of the breadth of ancillary science possible with a space telescope such as ET, see
Howell (2020).
Payload
ET consists of seven 30 cm telescopes, of which six are transit telescopes and one is a microlensing
telescope. Each telescope will be equipped with a mosaic of four 9K
×
9K CMOS detectors with a pixel size
of 10
μ
m. The average detector readout noise is 4 e
−
/pix and the readout time is 1
.
5 s. An electronic rolling
shutter is used for exposure control to avoid image smear.
Each transit telescope consists of eight transmissive lenses with a field of view (FOV) of 500 deg
2
.
The spectral range is 450-900 nm. The telescope works at a focal ratio of 1.57 which offers a pixel scale
of 4.38 arcsec/pixel. The planned individual exposure time is 10 s. The image diameter for 90% of the
encircled energy is within 3.8 pixels for the entire FOV. The telescope’s overall transmission exceeds 76%.
The telescope operates at a temperature of about
−
30
°
C with a stability of
±
0
.
3
°
C to minimize image drifts
and size changes while the detector is passively cooled to
−
40
°
C with its temperature stability maintained
within
±
0
.
1
°
C. The focus can be precisely adjusted by the temperature control of the telescope. A sunshield
is used to block the Sun from hitting the telescopes and provide a stable thermal environment for all science
payloads. With an additional hood on top of each telescope and optical baffles, the telescope’s scattered light
level is maintained at less than 3 e
−
/pix/s. The telescope hood is also used to radiatively cool the CMOS
detectors.
8
Ge et al.
The microlensing telescope is based on a Schmidt-Mann catadioptric system, providing a 4 deg
2
FOV
with a spectral range of 700-900 nm. Its focal ratio is f/17.2 with a plate scale of 0.4 arcsec/pix. It will be
operated at the diffraction limit with FWHM less than 0.85 arcsec. The focus can be precisely adjusted by
the temperature control of the telescope. The planned individual exposure time is 10 min.
ET will carry out a four-year high precision and uninterrupted photometric monitoring of about 1.2
million FGKM dwarfs (Gaia magnitude G
<
16) in the direction that encompasses the original
Kepler
field
to obtain light curves of these stars for detecting planetary transits. Planetary radii and orbital periods
can be determined from the planetary transits. Light curves of relatively bright stars (G
<
14) will also be
used in asteroseismology analyses to derive accurate stellar parameters such as masses, radii, and ages. Most
bright stars which are found to have planet candidates can be followed up with ground-based telescopes to
determine the masses and densities of these planet candidates while some highly valuable targets will be
observed with ground-based and space-based telescopes to measure atmospheric compositions.
ET will also conduct a four-year high precision photometric monitoring of over 30 million stars (
I
≤
20
.
6
mag) in the direction of the Galactic bulge to detect planetary microlensing events. The same field will be
simultaneously monitored by three ground-based KMTNet telescopes. The combined data will be used to
measure the masses of hundreds of cold planets including free-floating planets.
Mission Design
The ET space telescope craft will be comprised of a spacecraft platform and scientific payloads. The
spacecraft is designed to provide an extremely stable platform for high-precision photometry measurements.
ET will operate at the L2 halo orbit to ensure a good external heat flow and light environment, with transit
telescopes always pointing toward the
Kepler
field. Every season, the spacecraft will be rotated by 90 degrees
around the optical axis of the transit telescopes to keep the payload in the shade of the sunshield while the
Sun powers the solar array of the satellite. The microlensing telescope will observe the Galactic bulge from
March 21 to September 21 every year. The satellite will be built with an in-orbit lifetime of at least 4 years.
The ET spacecraft will adopt inertial orientation three-axis stabilization, with a telescope pointing accuracy
of better than 1.5 arcsec. The telescope pointing will be measured by star trackers, optical gyroscopes, and
fine guidance sensors while its stability will be controlled and maintained by reaction wheels and thrusters.
This will keep the telescope’s high-frequency jitters (up to 10 Hz) within 0.15 arcsec. By carefully controlling
the temperatures of the scientific payloads and their mounting optical bench while thermally isolating the
payloads from the spacecraft platform, the telescope’s long-term thermal drift will be controlled to within
0.4 arcsec. X-band will be used for the telemetry, tracking, and command of the spacecraft, as well as science
data downloads. A daily scientific data volume of roughly 169 Gb will be downloaded at a rate of 20 Mbps
via phased array antennas. The entire spacecraft, including payloads, will weigh
∼
3.2 tons and the long-term
power consumption is
∼
1500 watts. The ET mission will enter its preliminary design phase after the project’s
down-selection this June. The team aims to launch the mission from the Xichang launch site by a CZ-3B
rocket at the end of 2026.
ET Performance
ET photometry simulations show that the current design can reach a photometric precision of 34 ppm
for a G=13.4 solar-type star with a 6.5-hour integration of the target. This simulation includes photon
and instrument noise and has produced consistent results with
Kepler
on-sky measurements when
Kepler
’s
instrument parameters are used. ET survey simulations show that the ET transit survey will be able to
detect
∼
29
,
000 new planets, including
∼
4
,
900 Earth-sized planets and 10-20 Earth 2.0s assuming an Earth
2.0 occurrence rate of 10%. The ET microlensing survey will be able to detect
∼
400 bound planets and 600
free-floating planets;
∼
300 of these planets will have mass measurements.
The ET mission consortium consists of more than 300 scientists and engineers from over 40 institutions
in China and abroad. The ET technical team has built many hardware and software components for space
missions and will be building many of the components for ET. These include but are not limited to DAMPE,
QUESS, TAIJI-1, SVOM, ASO-S, and EP.
ET White Paper: To Find the First Earth 2.0
9
2 Scientific Goals
2.1
Planetary Science
2.1.1 Earth 2.0s and Terrestrial Planets
Authors:
Jian Ge
1
, Hui Zhang
1
, Hongping Deng
1
, Xinyu Yao
1
, Jiapeng Zhu
1
, Tong Fang
1
, Ji-Wei Xie
2
, Ji-Lin Zhou
2
,
Hui-Gen Liu
2
, Steve B. Howell
3
& Jun Yan
4
1.
Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, China
2.
Department of Astronomy, Nanjing University, Nanjing 210030, China
3.
NASA Ames Research Center, Moffett Field, CA 94035, US
4.
Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing, China
2.1.1.1 Booming of exoplanets and the elusive Earth 2.0
“Are we alone in the universe?” This fundamental question is as old as humankind itself. Humanity has
wondered whether there is life elsewhere in the universe and, if so, whether it exists on a second Earth. The
discovery of the first extrasolar planet (also called an exoplanet) around a Sun-like star, 51 Pegasi (Mayor &
Queloz, 1995), challenged our Solar System’s uniqueness and opened up a new field of astronomy, the study
of extrasolar planets, which has expanded exponentially in recent years. Mayor and Queloz were awarded
the Nobel Prize in Physics in 2019 for this major achievement. The subsequent hunt for exoplanets has
dramatically changed our understanding of extrasolar planetary worlds and Earth-like planets.
Figure 2.1:
The transit method——precision measurements of the dimming of a star’s light curve caused by a planet passing
in front of the star are used to detect exoplanets (credit Wikimedia).
.
During the early phase of planet-hunting, the radial velocity (RV) method used to discover 51 Peg b
was the most efficient and powerful tool for finding massive Jupiter-like planets (giant planets). Continuing
improvements in RV precision have allowed the detection of super-Earth mass planets in recent years (e.g.,
Mayor et al., 2009). By the 2010s, hundreds of giant planets had been detected. Through statistical measures,
these discoveries helped unveil the following characteristics of giant planets: 1) Jupiter-like and Saturn-like
giant planets appear to be very common (occurrence rate about 10%), especially around solar-type stars
with high metallicity; 2) there are other kinds of Jupiter mass planets with unknown origin, such as hot
Jupiters and warm Jupiters, in addition to cold Jupiter-like giant planets with long-period orbits; 3) most
giant planets likely form via the core accretion processes (one of the two leading theories of planet formation
Ida & Lin, 2004, 2005a).
The dominance of the RV method for detecting planets has declined since the launch of NASA’s
Kepler
satellite in 2009. With its 0.95-m space telescope,
Kepler
achieved an ultra-high photometric precision of
10
Ge et al.
Figure 2.2:
The number of confirmed exoplanets with various detection methods over the years. Due to the discovery of a
large number of transiting exoplanets by
Kepler
in 2014 and 2016, the number of known exoplanets doubled. This also resulted
in the transit method becoming the primary method for detecting exoplanets, which is still the most promising way to search
for Earth 2.0s (credit exoplanetarchive.ipac.caltech.edu).
.
around 30 ppm (parts per million, Gilliland et al., 2011) using the transit method (Figure 2.1) and quickly
identified over 4000 new exoplanets and planet candidates (see Figure 2.2, e.g., Borucki et al., 2010b; Howard
et al., 2012). This led to the discovery of two new types of planets that were difficult to detect from ground-
based transit observations: planets with sizes between that of Earth and Neptune, also known as super-Earths
(1
.
25
R
⊕
< R
p
≤
2
R
⊕
), and sub-Neptunes (2
R
⊕
< R
p
≤
4
R
⊕
) (see Figure 2.3). Many of these planets would
orbit within the orbit of Mercury in our Solar System (0.4 AU, where 1 AU is the average distance between
the Sun and Earth). These planets are more common than giant planets - about one-third of the stars
in the Milky Way host such planets (Zhu et al., 2018) - but they are absent in our Solar System. This
observation has challenged the traditional planet formation theory and inspired entirely new theoretical
models. Observations and theories suggest that most of these planets are born early with atmospheres rich
in hydrogen and helium (e.g., Wu & Lithwick, 2013; Owen & Wu, 2017; Fulton et al., 2017) inherited from
the gaseous protoplanetary disks, extended flattened structures around young stars, and are thus considered
the Generation I planets. Although super-Earths are ubiquitous in the universe, their formation processes
and surface environments appear very different from those of our Earth. Unlike super-Earths, Earth-sized
planets likely form from collisional debris after gaseous disks have been dissipated and are likely Generation
II planets in the planetary system, thus resembling Earth in origin (see, e.g., Qian & Wu, 2021; Morbidelli
et al., 2012; Fang & Deng, 2020; Liu et al., 2022). Therefore, it is speculated that super-Earths are unlikely
to harbor life.
The
Kepler
space mission was designed to detect Earth 2.0s — Earth-sized terrestrial planets (0
.
8
R
⊕
<
R
p
≤
1
.
25
R
⊕
) in the habitable zone of a solar-type star (F8V-K2V, Figure 2.3), and measure their occurrence
rate (Borucki et al., 2010a). However, the mission failed to detect them, although
Kepler
successfully detected
over 2000 new planets and identified two major planet populations of close-in super-Earths and sub-Neptunes.
This was primarily due to: 1) the failure of the two reaction wheels in the fourth year of its operation (see
Howell et al., 2014); 2) the unexpectedly high stellar noise of solar-type stars (Gilliland et al., 2011); and
ET White Paper: To Find the First Earth 2.0
11
Figure 2.3:
The distribution of the radii and orbital periods of the small planets discovered by the
Kepler
mission. Most
Kepler
planets are the so-called “super-Earths” and “sub-Neptunes”, which orbit their host stars within the orbit of Mercury
and are 1 to 4 Earth radii in size. A few sub-Earths (roughly Earth-size) lie close to their parent stars, and none of them are
close to Earth 2.0 (long-period Earth-sized planet in the habitable zone of a solar-type star, as denoted by the green box).
3) the high readout noise of detectors (on average 86 e
-
, Gilliland et al., 2011). Specifically, the stellar noise
levels for solar-type stars is about 50% higher than that of our Sun. The high readout noise of the
Kepler
CCD detectors led to high photometric errors for solar-type stars fainter than the 12th magnitude exceeding
34 ppm; smaller errors are necessary for detecting Earth 2.0s. As a result, the
Kepler
field only contains a
small quantity of quiet solar-type stars having a stellar noise of less than 17 ppm in their fields for detecting
Earth 2.0s. Higher stellar noises could have been compensated for by monitoring solar-type stars over a
longer observation period (
∼
8 years) than the originally planned mission lifetime (3.5 years) to increase
signal-to-noise (SN) ratios until they were high enough for detecting Earth 2.0s. However, the failure of the
reaction wheels further impeded
Kepler
’s ability to achieve this potential S/N gain. At the conclusion of its
primary mission in 2014,
Kepler
did not detect any Earth 2.0s as planned.
Ten Earth-sized planets have been detected in the ‘habitable zone’ around M dwarf stars, i.e., TRAPPIST
1 (Gillon et al., 2017), Teegarden’s star (Zechmeister et al., 2019), Gliese 1061 (Dreizler et al., 2020), Kepler
1649 (Vanderburg et al., 2020), TOI 700 (Gilbert et al., 2020), and Kepler 186 (Quintana et al., 2014),
as illustrated in Figure 2.4. A habitable zone around a main-sequence star is defined as the orbital region
at which liquid water could persist on the planet’s surface for a long time. This definition is based on the
fact that liquid water is necessary for all types of life on Earth. The inner edge of the habitable zone is
determined based on runaway greenhouse or moist greenhouse, and the outer edge of the habitable zone is
defined based on CO
2
condensation or maximum CO
2
greenhouse effect (Kasting, 1988; Kasting et al., 1993;
Pierrehumbert, 2010). In this regard, solar-like stars have wider habitable zones than M dwarfs, as shown
in Figure 2.4. However, the Earth-sized planets in the habitable zone of M dwarfs may not be habitable
(see, e.g., Stevenson, 2019; Tarter et al., 2007). First, the habitable zone of M dwarfs lies close to the star
resulting in a strong tidal force on planets which may tidally lock the planet, forming peculiar atmosphere
circulations (Yang et al., 2013, 2014). Second, M dwarfs are more variable than solar-type stars with intense
flares, releasing more X ray and ultraviolet radiation that may deprive the planet of its atmosphere. Last
but not least, in the extended formation stage, young M dwarfs may have already evaporated all water in
the habitable zones.
12
Ge et al.
Figure 2.4:
Illustration of all detected Earth-sized (0
.
8
R
⊕
< R
p
≤
1
.
25
R
⊕
) planets in habitable zones (blue shaded region)
and the expected 2.0 detections (green spheres) by the ET transit telescope. The comparatively active M dwarfs, hosting all
current Earth-sized planets, may spoil the habitability of these planets.
Habitable Earth-like planets around solar-type stars, i.e., Earth 2.0s, are likely the most favorable places
to search for extraterrestrial life due to their potentially having physical, chemical, and potentially biological
environments similar to Earth. Therefore, it is necessary to identify Earth 2.0s first before extraterrestrial life
can be possibly detected. Most current space missions for exoplanets do not cover this key area. For instance,
NASA’s Transiting Exoplanet Survey Satellite (
TESS
) mission, which is currently in operation in space, has
been very successful in detecting thousands of short-period planet candidates including Earth-mass planets
(e.g., Kunimoto et al., 2022; Guerrero et al., 2021 and references therein). However,
TESS
is highly unlikely
to detect any Earth 2.0s as this mission was not designed to monitor the same sky region for multiple years
in order to detect Earth 2.0s with a long orbital period. Additionally, due to the
TESS
telescope size, it
does not reach sufficient photometric precision to detect an Earth orbiting a G star. Similarly, the ESA’s
CHaracterising ExOPLanet Satellite (CHEOPS) transit space mission is dedicated to studying bright, nearby
stars that are already known to host exoplanets, not to search for planet candidates (Benz et al., 2021; Lendl
et al., 2020). The
Plato
2.0 (PLAnetary Transits and Oscillation of stars) mission focuses on transit planets
around bright stars (4-11 mag) and bulk planet characterization with mass determination from follow-up
RV studies (Rauer et al., 2014). Although
Plato
has some chance of detecting Earth 2.0s, its short staring
time even in the long-pointing mode (2-3 years) limits its ability to robustly detect Earth 2.0s.
This research has been prioritized in multiple programs by different funding agencies. For instance, the
US National Academy of Sciences recently announced the study of Habitable Worlds as one of the three
highest priority areas for future astronomy in the Decadal Survey on Astronomy and Astrophysics (National
Acad Sciences, 2021). In 2018, the committee on exoplanet science strategy formed by the National Aeronau-
tics and Space Administration (NASA) announced that searching for habitable planets and understanding
the formation and evolution of planetary systems along with the diversity of planetary system architectures,
planetary compositions, and planetary environments were the two main goals of NASA’s exploration in the
next 20 years (National Acad Sciences, 2018). In addition, the next generation of flagship space telescopes of
NASA and the European Space Agency (ESA) (e.g., the James Webb Space Telescope (JWST), the Nancy
Grace Roman Space Telescope (ROMAN), the Large UV/Optical/IR Surveyor (LUVOIR), ARIEL, etc. and
the next generation ground-based 30-meter class telescopes (e.g., the Thirty Meter Telescope (TMT), the Eu-
ropean Extremely Large Telescope (E-ELT), the Giant Magellan Telescope (GMT), etc.) have asserted that
ET White Paper: To Find the First Earth 2.0
13
the study of the atmospheric compositions, the internal structures of terrestrial planets, and the exploration
of these planets’ habitability are among their key scientific goals.
2.1.1.2 Survey for Earth 2.0s and a census for terrestrial planets
Although Earth 2.0s have not yet been detected, we are confident that they do exist since terrestrial-size
planets are already known to exist across a wide range of orbital periods around solar-type stars from close-in
orbits all the way out to extremely cold orbits beyond the snowline (where water turns into ice), and even
in interstellar space as free-floating terrestrial mass planets. In fact,
Kepler
has identified over 300 Earth-
sized planets with short-period orbits (most of them shorter than 30 days) around some quiet bright stars.
These Earth-sized planets (also called “sub-Earths”) and specifically the ones that orbit in their host stars’
habitable zones are the Earth 2.0s that we are looking for.
On the other hand, our current knowledge about exo-terrestrial-planets is limited to
Kepler
’s close-in
samples. We know very little about: 1) the occurrence rate of long-period (from several to hundreds of years)
terrestrial planets, 2) how terrestrial planets are formed and how mobile these small planets are in their host
systems, and 3) the population of free-floating Earths and how they get exiled. To fully understand their
formation and evolution, a census for exo-terrestrial planets is crucial. Their small size and low mass pose
significant challenges to transit and RV searches, especially so for those with long-period orbits. Microlensing
is currently the only method for studying low-mass, long-period and free-floating planets. However, current
microlensing planetary studies are limited by small number statistics and a lack of mass measurements (see
section 2.1.2). The current largest statistical sample of microlensing planets contains only 23 planets, with a
minimum mass of about 10
M
⊕
, assuming a host mass of 0
.
5
M
(Suzuki et al., 2016). In addition, for most
microlensing planetary events, ground-based light-curve analyses only yield the planet-to-host mass ratio
leaving the actual masses of the host star and the planet unknown. Thus, determining the mass function
for long period and free-floating Earths from a large statistical sample offers a unique chance to reveal their
origin. Microlensing appears to be the most favorable approach for accomplishing this task at the present
time.
Today, the exoplanet field is approaching a critical point: the entire astronomical community urgently
needs to find Earth 2.0s and collect a large sample of terrestrial planets in various orbits. Fortunately, recent
technological advances in space planet detection can readily support the achievement of these scientific goals.
Figure 2.5: Predicted ET planets in comparison with
Kepler
’s discoveries (see section 5). For sub- and
super-Earth, ET will increase the sample size by a factor of
∼
10 in the near future (before 2030s).