天文学前沿探秘 - 系外行星遨游纪实

Frontiers of Exoplanet Astronomy

Joshua Winn

普林斯顿大学天体物理系终身教授

(1)美国宇航局开普勒任务科学家顾问

(2)凌日系外行星巡天卫星任务联合调查员

(3)英国剑桥大学Fulbright学者，经济学人科学版主笔

(4)曾获哈佛大学史密森天体物理学中心N.S.F.和哈勃博士后奖学金

(5)学术论文总引用次数逾40,000次，h指数81，i10指数248

课题背景
系外行星，即太阳系外行星，是围绕太阳以外的恒星运行的行星。系外行星天文学是现今最令人兴奋和快速发展的科学领域之一，最近还获得了2019年诺贝尔物理学奖的认可。我们现在知道，大多数类太阳恒星都有自己的行星系统。通过探测和研究系外行星，天文学家希望更多地了解行星的来源，并最终在宇宙其他地方是否有生命这个古老的问题上取得进展。

课题内容
本课程的学生将接触到系外行星科学研究的前沿知识。他们将了解恒星和行星的观测特性，以及天文学家用来探测系外行星的方法。学生还将学习控制行星轨道、温度和大气的基本物理，以及它们的母恒星的基本属性。他们将熟悉NASA凌日系外行星勘测卫星TESS，这颗太空望远镜目前正在寻找距离地球最近、最亮的恒星周围的系外行星。学生们将利用这次正在进行的太空任务的数据进行课程研究项目。

适合人群
 对天文学，物理学感兴趣的高中生、本科生
 修读数学，物理，工程等专业，以及未来希望在天体物理以及相关研究领域从业或是从教的学生
 具备一定的数学、物理以及编程基础的学生优先

教授介绍

Joshua Winn

普林斯顿大学天体物理系终身教授

(1)美国宇航局开普勒任务科学家顾问

(2)凌日系外行星巡天卫星任务联合调查员

(3)英国剑桥大学Fulbright学者，经济学人科学版主笔

(4)曾获哈佛大学史密森天体物理学中心N.S.F.和哈勃博士后奖学金

(5)学术论文总引用次数逾40,000次，h指数81，i10指数248

任职学校
普林斯顿大学（Princeton University），简称普林斯顿，位于美国新泽西州的普林斯顿市，是世界著名的私立研究型大学，也是常春藤盟校成员。普林斯顿大学成立于1746年，前身是“新泽西学院（College of New Jersey）”，是九所在美国革命前成立的殖民地学院之一，同时也是美国第四古老的高等教育机构。学校在1747年移至纽瓦克，最终在1756年搬到了现在的普林斯顿市，并于1896年正式更名为“普林斯顿大学”。普林斯顿大学拥有的世界领先水平的学科包括数学、物理学、经济学、生物科学、计算机科学、地球科学、心理学、政治学、社会学、工程学、历史学、古典学、艺术史学、英语文学、哲学、比较文学、东亚研究与汉学等。在学术以外的领域，截止2018年，普林斯顿大学共培养出了2位美国总统、12位美国最高法院法官、1位世界首富（亚马逊公司创始人及现任董事长兼CEO杰佛瑞˙贝佐斯）以及200余名罗德奖学金得主。

课程安排与收获
• 8周在线小组科研（总课时78小时）
• 网申推荐信
• 学术评估报告
• 项目成绩单
• 论文成果

课纲

Frontiers of Exoplanet Astronomy

Instructor: Professor Joshua N. Winn, Princeton University

An exoplanet, or extrasolar planet, is a planet that orbits around a star other than the Sun. Exoplanet astronomy is one of the most exciting and rapidly advancing areas of science, and was recently recognized with the 2019 Nobel Prize in Physics. We now know that most Sun-like stars have their own systems of planets. By detecting and studying exoplanets, astronomers hope to learn more about where planets come from, and — eventually — to make progress on the age- old question of whether there is life anywhere else in the universe.

Despite the timeless nature of these questions, exoplanet astronomy only began in the mid- 1990s. The reason for the late start is that exoplanets are extremely difficult for astronomers to detect. Planets are much smaller than stars, and do not emit light of their own. Even with one of the world’s best telescopes, an exoplanet is easily lost in the glare of the star that it orbits.

Astronomers have pursued many methods to detect exoplanets. One approach is to design a sophisticated astronomical camera that makes reliable images with extremely high contrast levels. This has worked in a few cases, but is mostly a task for the future — and even when it does work, the planet appears as an unresolved point of light.  Most of our knowledge about exoplanets comes from more indirect methods, based on monitoring the properties of the star. For example, variations in a star’s spectrum of radiation can reveal that the star is being pulled around by the gravity of a planet. Variations in a star’s brightness can be caused by miniature eclipses, when a planet passes in front of a star.

Students in this course will travel to the frontier of research in exoplanetary science. They will learn about the observed properties of stars and planets, and the methods astronomers use to detect exoplanets. Students will understand the basic physics governing the orbits, temperatures, and atmospheres of planets, along with the essential properties of their parent stars. They will become familiar with the NASA Transiting Exoplanet Survey Satellite (TESS), a space telescope that is currently searching for exoplanets around the nearest and brightest stars in the sky. They will conduct research projects using the data from this ongoing space mission.

Prerequisites:

•         Mathematics: algebra, trigonometry, logarithms, polar coordinates, derivatives, vectors.

•         Physics: Newton’s laws of motion and gravity, energy, angular momentum, ideal gas law.

•         Computing: ability to write simple programs to read data, plot data, and perform mathematical operations.  Access to a Python computing environment.  For those with no experience, a major time investment will be required to get up to speed.

Lecture topics:

1.      Foundational astronomy.

a.       Overview of the universe and orders of magnitude.

b.      Telescopes.

c.       Measuring distances, fluxes, and spectra of celestial objects.

d.      Ideal gas law and Maxwell-Boltzmann distribution.

e.       Ideal blackbody and Planck function: Wien’s law and Stefan-Boltzmann law.

f.       Kepler’s laws of planetary motion.

2.      Understanding the solar system.

a.       Overview of the solar system: geometry and composition.

b.      Radiative equilibrium: global and local.

c.       Planet formation theory.

3.      Exoplanets.

a.       The astrometric method.

b.      The Doppler method.

c.       The transit method.

d.      Overview of current discoveries.

e.       The Kepler and TESS missions.

f.       Open research questions and future plans.

Student expectations:

•         Attendance and active participation in all class meetings.

•         Reading required articles and asking questions about them.

•         Weekly problem sets for the first 4 weeks.

•         Daily or near-daily contributions to the chosen research project.

•         Substantial work on the final paper.

Approximate schedule for each session:

Research projects (subject to change depending on student ability and interest):

1.      Searching for circumbinary planets.

Background:  Planets that orbit around two stars, rather than one, are called “circumbinary” planets.  They appear often in science fiction, with evocative scenes of alien double sunsets.  The first discovery of a circumbinary planet orbiting a pair of Sun- like stars was announced less than 10 years ago, based on data from the Kepler spacecraft (Doyle et al. 2011).  Since then, about a dozen more circumbinary planets have been found. They necessarily have wide orbits in order to remain stable, but otherwise they appear to be just as common as planets around single stars. But, they are more difficult to detect, because of the long periods and irregular timing of the transits.  Almost all of the known systems were detected by visual inspection of the light curves of eclipsing binary stars.  Therefore, it is possible that new circumbinary planets can be discovered by inspecting a large number of TESS light curves.

Project: Students will download and analyze TESS observations of eclipsing binary stars. They will process the data to remove variation due to scattered light in the cameras and other instrumental artifacts. Then, they will look for evidence of any circumbinary planets, by searching for transient dimming events. The TESS database includes thousands of bright eclipsing binaries. Students can examine as many cases as time permits.

Relevant articles:

Doyle, L., et al., (2011), “Kepler -16:  A  transit in g circumbi nar y plan et”

 Kostov,  V.,  et  al.  (2020),  “TO I-1338:  TES S ’s  First  Transit ing C ircumbi na r y P lanet”

2.      Orbital decay of hot Jupiters.

Background: According to the theory of tidal interactions, the orbits of most hot Jupiters should be steadily shrinking, ultimately leading to the fiery destruction of the planet as it is immersed within the star. But, the theory does not specify the timescale over which this process should occur. For one particular planet, named WASP-12b, observations spanning more than a decade have revealed that the orbital period is decreasing with time. This may be the first discovery of tidal orbital decay. If the orbit continues to shrink at the current rate, the planet will be engulfed by the star within a few million years.

Project: Students will download the TESS data for WASP-12, process the data to construct light curves, and measure the times of transits. They will compare the newly measured times with previous data, to confirm or refute the previous claims of orbital decay. Time permitting, they can also download the data for other hot Jupiters and perform similar transit-timing studies.

Relevant articles:

 Yee, S ., W inn, J ., Knutson, H., et al. (2020),  “The Orbit of W ASP -12 is Deca yin g”

 P atra,  K.,  W inn,  J .,  Holman,  M.,  et  al.  (2020),  “Th e C onti nuing S ear ch  for  Evidence of

 Tidal  Orbit al  Dec a y of  H ot  J upiters”

3.      An enigmatic eccentric exoplanet.

Background: A star named HD 17156 has a gas giant planet with an orbital period of 21 days, and an unusually high orbital eccentricity of 0.68.  It is a good example of an “eccentric exoplanet.” Prior to the discovery of exoplanets, it had been predicted that circular orbits would be nearly universal.  The origin of the high eccentricity of

HD 17156b and similar planets is still unclear.

Project: Students will use new TESS observations of HD 17156 to refine our knowledge of the size, orbital period, and perhaps other parameters of this puzzling planet. They will download the data, produce a time series, and fit a parameterized model to the data. Time permitting, they will combine the TESS data with Doppler observations to determine the orbital eccentricity. They can use the results to estimate the surface temperature and other key properties of the planet, and discuss the prospects for detecting general relativistic precession of the orbit.

Relevant articles:

Barbieri, M, et  al.  (2007 ) ,  “HD 17156b: a tr ansit ing plan et wit h a 21.2 -day period and an

 ecc entric orbit”

 Nutz man,  P.,  et  al.  (2011),  “P recise  Esti mates  of  the P h ysic al  P aramete rs  f or the  Exoplanet System HD 17156 Enabled by Hubble Space Telescope Fine Guidance Sensor

 Transit  and  Asteros eism ic  Observ ati ons”

Additional resources and introductory readings (* indicates mandatory reading)

Openstax Astronomy, a free online introductory astronomy textbook.

https://openstax.org/books/astronomy/pages/1-introduction

Winn, J.N., "Who Really Discovered the First Exoplanet?", Scientific American Blogs, November 2019

• Winn, J.N., "Shadows of Other Worlds", Scientific American, 318, 32-39 (2018)

• Ricker, G.R., Winn, J.N., Vanderspek, R., et al., "Transiting Exoplanet Survey Satellite", J. Astron. Telesc. Instrum. Syst., Volume 1, id. 014003 (2015)

• Winn, J.N., "Exoplanet Transits and Occultations," in Exoplanets, ed. Seager, S., University of Arizona Press, Tucson (2011)

 W inn, J .N.  & Fabr yc k y,  D., “Oc curr ence  and  Ar c hit ecture of Ex oplanetar y S ystems,”  Annual  Reviews in Astronomy & Astrophysics, 53, 409 (2015)

Winn, J.N., "Planet Occurrence Rates from Doppler and transit surveys," in Handbook of Exoplanets, eds. Deeg H., Belmonte J., Springer (2018)

Lightkurve, a Python software package for accessing and plotting data from the NASA Kepler and Transiting Exoplanet Survey Satellite missions: https://docs.lightkurve.org/

Exoplanet, a Python software package for probabilistic modeling of transit and radial-velocity observations of exoplanets and other astronomical time series: https://docs.exoplanet.codes/en/stable/

Rebound, a Python software package that can integrate the motion of particles under the influence of gravity: https://rebound.readthedocs.io/en/latest/

Data archive for Transiting Exoplanet Survey Satellite: https://archive.stsci.edu/tess/