About me

I am a fifth-year PhD student and NASA FINESST Fellow at Caltech.

My research interests include exoplanet and brown dwarf atmospheres, orbital architectures, and substellar binarity. In my PhD, I focus on measuring the atmospheric compositions, spins, and radial velocities of giant planets and brown dwarf companions with high- and medium- resolution spectroscopy (R~2,000-100,000). I have studied atmospheres from 700 to 2500 K, from the late T dwarfs up to late M dwarf stars, using both atmospheric retrievals and forward models. Second, I use optical interferometry and high-resolution spectroscopy to discover tight brown dwarf binaries around stars.

Before starting my PhD, I pursued a MPhil at the University of Cambridge (2020) as a Downing Scholar. There, I worked with Mark Wyatt on calculating the 3-D orbits and dynamical masses of long-period giant planets discovered from radial velocitiy. I made the first mutual inclination measurement between a transiting super-Earth and outer cold Jupiter in the π Men system.

I received a BA in Physics from Pomona College (2019), where I spent 4 enriching years learning about the universe and widening my worldview through a philosophical reading group I organized.

View my CV for more information.

Research Projects

Substellar binarity: a case study with Gliese 229 B

I'm excited to announce our discovery that the first brown dwarf companion, Gliese 229 B, is in fact a tight brown dwarf binary! The two brown dwarfs orbit each other every 12 days, as they orbit a M1V star every 250 years. The discovery resolves the apparent conflict between the high dynamical mass of Gliese 229 B and substellar evolutionary models, which could not explain the low luminosity of the object as a single source. Here's the press release story, and the paper published in Nature I also wrote a more accessible version of the paper with Rebecca Oppenheimer, which is available as a Research Briefing

High-resolution spectroscopy with KPIC

Advised by Dimitri Mawet, Heather Knutson, Jason Wang, and Jean-Baptiste Ruffio

I analyze KPIC high-resolution spectra (R~35,000) of directly imaged giant planets and brown dwarfs companions to measure their atmospheric compositions, rotation rates, and radial velocities. I use atmospheric retrievals for this work, specifically with the radiative transfer code petitRADTRANS (Mollière+2019). In the Paper I on benchmark brown dwarf companion HD 4747 B, I demonstrated that high-resolution spectra yielded consistent results independent of assumptions on clouds, while low-resolution data fail to robustly constrain the brown dwarf's abundances.

In Paper II on late-M dwarf companion HIP 55507 B, I detect 13CO and H218O isotopologues and validated the companion's 12C/13C and 16O/18O ratios are consistent with those of its K6 primary star, which I also measure using KPIC data.

Paper III: I uniformly measured the C and O abundances, spins, and RVs for eight young, planetary-mass companions (~10-30 MJup, ~50-360 AU). We find that these companions have compositions consistent with solar and their host star compositions, which implies that they are consistent with forming via gravitational instabiliy. These results are also consistent with core accretion outside the CO snowline (i.e. in situ) but would require rapid assemby of cores in massive disks.

In addition to the science, I am also heavily involved in the instrument commissioning and data pipeline for KPIC. In fact, my first ever research project (as a first year undergrad) was to build a prototype of the fiber injection system for KPIC (Mawet, Ruane, Xuan et al. 2017). The success of this work laid the foundation for KPIC.

After starting my PhD, I worked with Yinzi Xin on demonstrating speckle nulling through single-mode fibers in KPIC. I'm also involved in the science planning and data analysis for the VFN mode on KPIC, led by Dan Echeverri. VFN is targeting young planets and brown dwarfs orbiting between 1-10 AU (~1 λ/D) from their host stars. Papers on these topics have been published in JATIS. In addition, I worked on streamlining the daytime calibration procedure for KPIC and various other engineering tasks as a Keck Visiting Scholar in 2022.

Super-stellar metallicities and detection of CO2, CH4, H2S in the HR 8799 planets

Collaboration with Dr. Jean-Baptiste Ruffio, Dr. Chas Beichman; part of GTO 1188 (PI: Hodapp)

I am leading the atmospheric retrieval efforts for the 3-5 micron JWST/NIRSpec data (R~2700) of HR 8799 c, d, e (Ruffio, Xuan et al. in prep) and HR 8799 b (Xuan et al. in prep). These iconic planets orbit their host star between 16-71 AU, and have posed challenges to planet formation theories. From the JWST data, we obtain new detections of multiple molecular species in the planet atmospheres, and tightly constrain the C/O and metallicity. These results demonstrate the unprecedented sensitivity of JWST/NIRSpec IFU in studying exoplanet atmospheres, which is feasible for a vast majority of known directly imaged planets (Ruffio et al. 2023).

JWST 1-14 micron spectroscopy of the first T dwarf companion

PI: Xuan

I am leading a JWST Cycle 2 GO program to acquire an extensive spectrum for Gl 229 B, the first T dwarf discovered. We will be observing Gl 229 B from 1-5 micron with NIRSpec (R~2700), and 5-14 micron with MIRI LRS (R~50-140). The MIRI spectrum has arrived, and my paper on it has been submitted! Stay tuned.

JWST/NIRSpec R~3000 spectroscopy of AF Lep b, the lowest-mass directly imaged planet

PI: Xuan, Co-PIs: Jean-Baptiste Ruffio, Yapeng Zhang

I'm very excited about our JWST Cycle 3 GO program to observe AF Lep b, a 3 MJup giant planet orbiting at 8 AU from its host star. AF Lep b is the closest Jupiter analog that has ever been imaged, and we will collect 3-5 micron spectra at R~3000 to characterize the planet's atmosphere in detail. Situated at the peak of the RV giant planet occurrence rate (Fulton et al. 2021), AF Lep b will serve as a bridge to link the hot Jupiter population with widely separated imaged planets (~10-100 AU), which both represent rarer outcomes (~1%) of planet formation.

Direct measurements of orbital mutual inclinations in exoplanet systems

Advised by Mark Wyatt

I'm also interested in combining different methods (imaging, radial velocity, astrometry) to measure 3D orbital architectures and dynamical masses of exoplanets.

I found that the inner and outer planets in π Men and HAT-P-11 are highly misaligned by ~50 deg in Xuan & Wyatt 2020. I also measured planet-debris disk mutual inclinations in two other planetary systems in Xuan et al. 2020b.

Characterizing the Keck/NIRC2 vortex coronagraph for high-contrast imaging

Advised by Dimitri Mawet, Garreth Ruane, and Henry Ngo

In the summer of 2017, I wrote software to automate the data processing workflow for the Keck/NIRC2 vortex coronagraph, and helped set up a Mongo database and website to display the data products. These tools are still in use by the group.

With the pipeline, I re-reduced archival data of more than 300 stars, and wrote a paper characterizing the instrument's performance. This paper remains the standard citation for work involving Keck/NIRC2 high-contrast science, and is still used today for simulations of future Keck instruments such as Keck/SCALES.

Selected Publications

Summary: 36 refereed publications. 9 first author, 8 second author. 845 citations (Oct 24, 2024). The list below only includes 1st, 2nd, and 3rd author papers.

  1. Xuan, J.W., Mérand, A., Thompson, W., et al. (2024) The cool brown dwarf Gliese 229 B is a close binary. Nature, 634, 1070–1074

  2. Xuan, J.W., Hsu, D., Finnerty, L., et al. (2024b) Are these planets or brown dwarfs? Broadly solar compositions from high-resolution atmospheric retrievals of ~10-30 MJup. ApJ, 962, 10

  3. Xuan, J.W., Wang, J., Finnerty, L., et al. (2024a) Validation of elemental and isotopic abundances in late-M spectral types with the benchmark HIP 55507 AB system. ApJ, 962, 10

  4. Xuan, J.W., Wang, J., Ruffio, J.-B., et al. (2022) A Clear View of a Cloudy Brown Dwarf Companion from High-resolution Spectroscopy. ApJ, 937, 54

  5. Xuan, J.W. & Wyatt, M.C. (2020) Evidence for a high mutual inclination between the cold Jupiter and transiting super Earth orbiting π Men. MNRAS, 497, 2096

  6. Xuan, J.W., Kennedy, G.M., Wyatt, M.C., Yelverton, B. (2020) Mutual inclinations between giant planets and their debris discs in HD 113337 and HD 38529. MNRAS, 499, 5059

  7. Xuan, J.W., Bryan, M.L., Knutson., et al (2020) A Rotation Rate for the Planetary-mass Companion DH Tau b. AJ, 159, 979

  8. Xuan, J.W., Mawet, D., Ngo, H., Ruane., et al. (2018) Characterizing the Performance of the NIRC2 Vortex Coronagraph at W. M. Keck Observatory. AJ, 156, 156

  9. Xuan, J.W., Perrin, M., and Mawet, D., et al. in review at ApJL.Atmospheric abundances and bulk properties of the binary brown dwarf Gliese 229 Bab from JWST/MIRI spectroscopy.

  10. Zhang, Y., Xuan, J.W., Mawet, D., et al. (2024) Atmospheric characterization of the super-Jupiter HIP 99770 b with KPIC.AJ, 168, 131

  11. Finnerty, L., Xuan, J.W., Xin, Y., et al. (2023) Atmospheric metallicity and C/O of HD 189733 b from high-resolution spectroscopy. AJ, 167, 43

  12. Sanghi, A., Xuan, J.W., Wang, J., et al. (2024) Efficiently Searching for Close-in Companions around Young M Dwarfs using a Multi-year PSF Library.AJ, 168, 215

  13. Costes, J., Xuan, J.W., Vigan, A., et al. (2024) Fresh view of the hot brown dwarf HD 984 B through high-resolution spectroscopy. A&A, 686, A294

  14. Hejazi, N., Xuan, J.W., Cristofari, P., in review at ApJL. Chemical Links between a Young M-type T Tauri star and its Substellar Companion: Spectral Analysis and C/O Measurement of DH Tau A

  15. Echeverri, D., Xuan, J.W., Jovanovic, N., et al. (2023) Vortex Fiber Nulling for Exoplanet Observations: Implementation and First Light. JATIS, 9, 035002

  16. Echeverri, D., Xuan, J.W., Monnier, J., et al. (2024) Vortex Fiber Nulling for Exoplanet Observations: First Direct Detection of M Dwarf Companions around HIP 21543, HIP 94666, and HIP 50319. ApJL 965, L15

  17. Xin Y., Xuan, J.W., Mawet, D., et al. (2023) On-sky speckle nulling through a single-mode fiber with the Keck Planet Imager and Characterizer. JATIS 9, 035001

  18. Hsu C.-C., Wang, J.J, Xuan, J.W. et al. (2024) Rotation and Abundances of the Benchmark Brown Dwarf HD 33632 Ab from Keck/KPIC High-resolution Spectroscopy. ApJ 971, 9.

  19. Inglis, J., Wallack, N., Xuan, J.W., et al. (2024) . Atmospheric Retrievals of the Young Giant Planet ROXs 42B b from Low- and High-resolution Spectroscopy

  20. Mawet, D., Ruane, G., Xuan, W., et al. (2017) Observing Exoplanets with High-dispersion Coronagraphy. II. Demonstration of an Active Single-mode Fiber Injection Unit. ApJ, 838, 92

Click below for a full list of my publications (ADS).

Outreach and Mentoring

In the summer of 2019, I worked for 6 weeks as Teaching and Residential Assistant for the Summer Science Program, where I am alumnus (2014). In the Astrophysics camp, students participate in an intensive research project on asteroid orbital determination, organizing their own observations and reducing real telescope data. As TA, I helped with late-night observing runs, homework sessions, and answering questions. I also lived together with the students, organized fun activites including sports, board games, boba and hiking trips. It was an incredibly rewarding experience to see the participants grow both as scientists and human beings.

During college and at Caltech, I worked as TA for several physics and astronomy classes, including Observational Astrophysics (twice), Intro to Astronomy, Advanced Intro Physics, Bayesian Statistics, and High Energy Astrophysics.

At Caltech, I advised two undergraduate students, Aniket Sanghi (now grad at Caltech), and Gavin Wang (junior at JHU). Aniket's paper on advancing reference star differential imaging for the Keck/NIRC2 vortex has been published in AJ, and Gavin is currently working on an atmospheric retrieval paper using Keck/KPIC data.

I am currently helping an undergraduate student from UIUC to apply for graduate school as part of the Caltech Accountability Partners Program, Future Ignited.