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XUAN JI/纪 璇

I know the world,
but dew upon the morning grass;
and yet, and yet…
我知这世界,
本如露水般短暂。
然而,然而。

WE ARE MADE OF STAR STUFF/我们是星尘


About Me

I’m a Ph.D. candidate in the Department of Geophysical Sciences at the University of Chicago. I grew up in Shandong, China, and earned a B.S. in Astrophysics from Beijing Normal University in 2020, the same year I joined UChicago’s Ph.D. program.

I study exoplanets, the diverse worlds beyond our solar system. I am interested in how their formation and orbits shape climate and habitability.

I enjoy science outreach. Check out my blog for notes on cutting-edge research and interesting topics. If you have questions, spot mistakes, or just want to chat, email me! Maybe we’ll cross paths at a future event!

RESEARCH

I study exoplanets—planets beyond our solar system. I am broadly interested in how the formation history and orbital evolution of exoplanets shape their potential habitability.

I enjoy developing my own box models to tackle to problems to build physical intuition, but I also run complex models, for example, global climate models (ExoCAM, Isca), N-body simulations (REBOUND) with additional forces to test hypotheses and explore new phenomena.

My long-term goal as a researcher is to identify which types of planetary system architectures are most likely to produce and sustain habitable planets.

Where Are the Atmospheres?

Cosmic Shoreline


My recent work revised the Cosmic Shoreline, the boundary separating atmosphere-retaining and airless worlds, by coupling hydrodynamic escape outputs with Monte Carlo sampling of initial conditions and related processes. I also developed a priority metric for atmospheric detection that integrates Cosmic Shoreline results with observational detectability. This physics-based framework prioritize the targets for atmospheric searching.

  • 🏝️ The Cosmic Shoreline Revisited (Accepted by ApJ, 2025)
  • Orbital Eccentricity as a Driver of Climate

    Inner Edge of Habitabl Zone


    A potentially habitable planet with an ocean at least 10 m deep should have surface temperature variations small enough that its IHZ is set by the mean stellar flux. When assessing habitability for eccentric exoplanets, use the orbit-averaged stellar flux, not the semi-major axis flux, to define the inner edge of the habitable zone.

  • 💧 Inner Edge of Habitable Zone as a Function of Eccentricity (ApJL, 2023)
  • Snowball Bistability

    While snowball bifurcation is a robust nonlinear climate phenomenon on Earth, it is not necessarily robust for exoplanets. The snowball state becomes unstable as orbital eccentricity increases, because winter freezing at the ice bottom is reduced relative to summer melting at the ice top due to ice self-insulation.

  • ☃️ Snowball Bistability Vanishes at Moderate Orbital Eccentricity (PSJ, in review, 2025)
  • Orbital Eccentricity as a Driver of Rocky Planet Formation

    Sweeping Secular Resonance Under construction

    Rotational Rate as a Driver of Climate

    Under construction

    文章

    ARTICLES

    My goal is to make these astronomy articles enjoyable and understandable for everyone, regardless of background. If you have any questions, spot a mistake, or just feel something could be better, please don’t hesitate to let me know! I truly appreciate your feedback and would love to hear your thoughts. :P.
    我的目标是让所有人--无论是否有天文背景--都能读懂并喜欢这些文章,并从中领略天文的魅力。如果你发现任何错误、有什么疑问,或者只是对文章哪里不满意,请一定告诉我!你的反馈对我非常重要~ 此外,我还写了一些科普向的调研小报告,内容略微正式,可能读起来稍微有点无聊 :P

    递归贼好用!
    Recursion is better than Loop! (Sometimes.)

    We have been compiling a list of tips and tricks for performing order of magnitude (OOM) calculations in the OOM class (ASTR 35000). For this homework, I was tasked to write a blog post that briefly summarizes an OOM tip/trick in my own words and demonstrates the application of the tip/trick with a brief back-of-the-envelope calculation. The trick that I choose is recursion.

    低级文明的疯狂梦想:太阳蒸汽机
    Undeveloped civilization has wild dreams: To build a solar steam engine.

    I am a normal citizen living in the Victorian era, and steam engine is the most powerful engine I have ever known. Science is such a good thing! I want to maximize the application of the steam engine to increase the productivity of the whole society and then to improve our lives. Sun seems to be a wonderful heat source!  In my imagination, in order to fully exploit the solar energy, I will build a sphere that completely encompasses the sun (see Dyson sphere.) and can 100% absorb the radiant energy without any reflection at any wavelength.

    利用太阳引力透镜望远镜对系外行星进行成像的调研报告
    An Introduction to the Solar Gravity Lens Mission

    本文对NASA近年来提出的太阳引力透镜望远镜提案展开调研。 遥远天体发出的光束经过太阳时,由于太阳周围引力场的弯曲,光线会发生偏折,聚焦于550AU以外的一点。如果在焦点位置上安置探测器,便可以实现成像。这一想法在1979年被提出后进展缓慢,近年来,借力于行星科学的迅速发展,行星成像这一追求让天文学家重新燃起对这一项目的热情。借助太阳引力透镜望远镜,可以实现对地球大小的行星表面200千米级分辨率的成像,具有当前所有望远镜都无法比拟的优秀性能以及绝对优势。但是这一项目也面临着探测器运送、定位精度、图像处理等技术难题,目前还处在提议、讨论阶段。

    斯隆巡天奇妙夜:20年再出发
    “Wow, what a night”: Know about SDSS

    1998年5月10日的清早,观测者和工程师们把斯隆基金会望远镜指向天赤道,天光第一次射入斯隆无比精巧的灵敏照相机中。
    当黎明破晓,斯隆的观测员丹·朗结束了一晚的观测任务,他要在日志中总结前一晚的工作。他在描述仪器使用细节后,照例要列出一系列新出现的待解决问题。
    此时,丹·朗似乎意犹未尽,忍不住写到:「哇,多么奇妙的一夜啊!」

    Gaia新数据发布,一个崭新的银河系出现!
    Gaia New Data Release Reveals a New the Galaxy!

    4月25日,Gaia 卫星第二次数据发布,天文圈内外震了三震。
    疯狂的天文学家挤爆了 Gaia 发布数据的服务器。
    吃瓜天文爱好者惊叹于刷新三观的赫罗图。
    普通吃瓜群众:哇哦,银河系全景诶,这密密麻麻全是星星啊!
    同志们,满足5K高清屏幕的15m高清大图,点击左下角「阅读原文」下载。
    而作为外围踮着脚尖看不到内场的吃瓜群众,脸上只有:???

    越老的恒星,越穷
    Stars: The Older, The Poorer

    恒星活着的时候,体内拼命制造金属。死了,就把金属扔出来,撒遍周围太空。新一代的恒星崛起的时候,可就不是白手起家了。这些不劳而获的富二代从废墟中继承了爸爸们的金属,一诞生,就富含金属。富三代,显然只会更加富裕。所以,当几代人混居一起的时候,那个兜里金属最少的恒星,就是白手起家的第一代老同志。

    天文学家的愚人节
    Astronomers' April Fool's Day

    观众朋友们,大家好,今天是四月一日,星期天,农历二月十六,欢迎收看CCHM愚人节版特别节目——天文学系列之“探索发现”。
    首先向您预告这次节目的主要内容有:
    利用搜索引擎上的图片,计算出彗星轨道;
    利用太阳系外的行星,重新占星;
    宇宙中发现神秘圆周率信号。

    明早发射!寻找另一个地球的新武器
    A New Weapon to Search for New Earth!

    说到人类的太空移民,地球2.0(Kepler 452b)是不是立马蹦到了你脑中?这位地球的表兄弟刚被发现时,新闻可是铺天盖地,连新闻联播也在讨论着这位远方的表兄。一切都历历在目,而一晃已经三年了,发现地球2.0的开普勒望远镜也早在2013年5月就结束了自己搜寻系外行星的使命,而这5年中还没有任何专业的「行星猎手」上天,说好的寻找星辰大海呢?我们得快些抓紧脚步啊!

    世界上最稀有的珍宝,每颗仅存几秒钟
    The Most Rare Jewelry in the world, which only exists for several seconds

    「摘下星星送给你,拽下月亮送给你~」
    「你去摘贝利珠啊。」
    「???」
    当诸如摘星星送月亮一类的情话已经变成广场舞配乐的歌词,我们怎样才能用更专业的姿势使出浪漫的天文杀?
    星星月亮实在是不够稀奇,摘下它们怎么能显示你独一无二的爱呢?能拿得出手的天文界「珠宝」,首先得稀有,其次得漂亮。因此,我向您推荐——贝利珠。

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    Say Hello.

    Contact Me

    If you have any questions about astronomy, or about me, feel free to reach out! If you spot a mistake in my article, don’t hesitate to point it out. I truly appreciate your feedback and am always happy to connect with new friends and fellow science enthusiasts!

    Email

    xuanji@uchicago.edu

    Address

    Office 441, Hinds
    Department of Geophysical Sciences,
    The University of Chicago.