Research Highlights

New Insights into Planet Formation from JWST Spectroscopy of Giant Exoplanets

Key Results

  • Leading 5 JWST programs as PI/Co-PI across Cycles 2-5, totaling more than 110 hours.
  • Building a comprehensive JWST chemical survey of directly imaged companions to delineate planet and star formation pathways.
  • Used H2S to measure S/H and show that directly imaged planets must accrete substantial amounts of solid material.
  • Discovered that distant super-Jupiters (10–100 AU) are as metal-rich as Jupiter and close-in hot and warm Jupiters.
1. Metal Enrichment and Volatile-to-Refractory Abundances in the Four HR 8799 Planets
Directly imaged planets are metal-rich in multiple elements, like Jupiter
Elemental abundances of giant planets and brown dwarfs

I led two atmospheric retrieval studies for the four gas giants around HR 8799 using exquisite 3-5 micron JWST/NIRSpec spectroscopy (R~2700). These iconic planets (5-9 MJup) orbit their star between 16-71 AU, and have posed challenges to planet formation theories. We obtained the first detections of H2S, CO2, CH4, and NH3 in these planets, and made the first direct measurements of solid accretion from S/H. These results mark a new era in precise atmospheric studies of directly imaged planets. Here's the press release story.

Evidence that the HR 8799 planets accrete both solids and metal-enriched gas from the circumstellar disk
Formation schematic for the HR 8799 planets
2. JWST Spectroscopy of the Young Jupiter Analog AF Lep b
Mass-metallicity comparison for AF Lep b and HR 8799 planets

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

I led a JWST Cycle 3 program to observe AF Lep b, one of the most Jupiter-like exoplanets in orbital distance amenable to detailed atmospheric characterization. I found that AF Lep b is as metal-rich as Jupiter and the HR 8799 planets. We also showed that these directly imaged planets have metal enrichments comparable to transiting gas giants. This similarity suggests that the process and efficiency of metal accretion for gas giants may not be strongly dependent on planet mass or orbital distance. You can check out the paper here.

3. JWST Panchromatic Spectroscopy of the First Imaged Mature Cold Jupiter
Panchromatic JWST detections of eps Indi Ab Sanghi et al. 2026

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

I am leading a JWST Cycle 4 GO program to observe eps Indi Ab, the coldest directly imaged planet with Teff~275 K (Matthews et al. 2024). We have obtained a comprehensive SED from 4-25 microns, as well as brown dwarf-quality spectra to measure multiple elemental and isotopic abundance ratios (e.g. D/H, 15N/14N). The first paper from this program has been published by graduate student Aniket Sanghi, who presents the first 25 um image of any exoplanet to date. See: https://doi.org/10.3847/1538-3881/ae74c8

4. JWST 1-14 Micron Spectroscopy of the Iconic T Dwarf Binary Gliese 229 Bab

PI: Xuan

I am leading a JWST Cycle 2 GO program to acquire an extensive spectrum for Gl 229 Bab, now known to be a tight binary brown dwarf. The MIRI spectrum has been published in Xuan et al. 2024d. I found that the two brown dwarfs share the same chemical composition as their host star, as expected for formation via gravitational collapse.

5. A NIRSpec Spectroscopic Search for Astrometric Giant Planet Candidates at Solar System-like Scales

PI: Xuan, Co-PIs: William Thompson & Dori Blakely

I am leading a NIRSpec survey of nine nearby young stars to detect for the first time the most promising astrometric giant planet candidates on solar system-like scales (~5-10 au). Our program is expected to deliver 3-5 new planets (2-10 Jupiter masses) with dynamical masses and measure their atmospheric metallicities using 3-5 µm spectroscopy (R~2700). These planets bridge the gap between close-in Jupiters (~0.1 au) and distant super-Jupiters (~100 au), providing a key sample to study metal enrichment of gas giants in a previously inaccessible phase space.

6. Measuring the Composition and Circumplanetary Environment of a Gap-clearing Protoplanet
VLT/SPHERE image of WISPIT 2 showing protoplanet WISPIT 2b within the disk gap
Image credit: van Capelleveen et al. 2025

PI: Balmer, Co-PIs: Xuan & Choksi

We are using NIRCam and NIRSpec to characterize WISPIT 2b's accreting circumplanetary disk and atmosphere. Among other science, this program will provide a rare opportunity to connect information about a still-accreting protoplanet's location in the disk to its measured abundances.

The Largest High-Resolution Survey of Abundances with Keck/KPIC KPIC logo

Key Results

  • Largest uniform high-resolution abundance survey for 10-30 MJup companions.
  • Established that distant companions with m ≳ 10 MJ (q ≳ 0.01) form through direct fragmentation like stars.
  • Played a major role in KPIC instrumentation, commissioning, and data pipeline development.

I use atmospheric retrievals of KPIC high-resolution spectra to measure the compositions, rotation rates, and radial velocities of directly imaged giant planets and brown dwarf companions. One highlight of this work is Paper III, where I uniformly analyzed eight young, widely separated planetary-mass companions. Their compositions match solar and host-star abundances, indicating that they most likely formed through gravitational fragmentation.

In addition to the science, I am also heavily involved in the instrument commissioning and data pipeline for KPIC. I helped build the prototype of the fiber injection system for KPIC (Mawet, Ruane, Xuan et al. 2017), and worked at Keck Observatory for four months to commission the KPIC Phase II system.

Xuan et al. 2022: High and low-resolution spectroscopy of HD 4747 B Xuan et al. 2024a: Carbon and Oxygen Isotopologue Ratios in HIP 55507 AB Xuan et al. 2024b: Are these planets or brown dwarfs? Speckle Nulling Vortex Fiber Nuller: Instrument Vortex Fiber Nuller: Science Instrument Paper Fiber Injection Unit

Using Tight (<1 AU) Brown Dwarf Binaries to Inform Star Formation

Key Results

  • Resolved Gliese 229 B into a 12-day tight binary, Gliese 229Ba/Bb, and resolved the mass-luminosity discrepancy for this benchmark brown dwarf system.
  • Leading a 25-hour CRIRES+ survey to identify additional tight brown dwarf binaries.

I led a study to spatially and spectrally resolve the first brown dwarf companion, Gliese 229 B, into two nearly equal mass brown dwarfs, Gliese 229Ba and Bb. The two brown dwarfs orbit each other every 12 days, as they orbit a M dwarf every 250 years. The discovery resolved a significant discrepancy between the dynamical mass of Gliese 229 B and its luminosity. Here's the press release story, and the paper published in Nature. I also wrote a Research Briefing with Rebecca Oppenheimer.

I'm leading a 25-hour CRIRES+ program to search for more tight brown dwarf binaries that orbit stellar primaries. The program will observe eight young substellar companions, three times each.

Tracing the Influence of Distant companions on Exoplanetary Systems (TIDES)

In collaboration with Marvin Morgan (UCSB).

Key Results

  • Launched a dedicated survey to measure precise eccentricities and inclinations of stellar companions around ~30 hot Jupiters.
  • Awarded two VLTI/GRAVITY programs (P117 and P117 FTC) and four Keck nights in 2026A and 2026B to initiate the observing campaign.
Highly Eccentric and Misaligned Stellar Companions to Hot Jupiters
Orbital eccentricities and mutual inclinations of stellar binaries hosting hot Jupiters compared with binaries hosting small planets

In Paper I, we combined archival observations with new Keck/NIRC2 imaging to measure the orbits of stellar companions in six hot Jupiter systems. All four systems with well-constrained orbits have highly eccentric stellar companions, and three show significant misalignments between the planet and stellar companion orbits. These results show that stellar companions to HJs have the highly eccentric and misaligned orbits required to drive high-eccentricity migration. Our upcoming papers will examine the full sample and see if these trends hold at the population level, as well as compare the results to a control sample of binaries.

Orbital Architectures of Planetary Demographics with Gaia Astrometry

Collaborators: William Thompson, Dori Blakely and others.

Key Results

  • Made the first mutual inclinations measurements between transiting small planets and outer gas giants.
  • Leveraging the existing Gaia DR2, DR3, and Hipparcos data to find new planets and prepare for DR4.
1. Direct Measurements of Orbital Mutual Inclinations in Exoplanet Systems

I combine different techniques (imaging, radial velocity, astrometry) to measure 3-D 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.

2. Developing Calibrated Astrometric Catalogs and Preparing for DR4 and DR5

I am part of the G23H collaboration, where we have developed a framework to combine Hipparcos, Gaia DR2, and Gaia DR3 absolute astrometry in a single calibrated reference frame (Thompson, Blakely, Xuan et al. 2026). This method can already enable the detection of new giant planets (Do Ó et al. 2026), and the underlying orbit fitting framework, Octofitter (Thompson et al. 2023), is well-suited and prepared to incorporate Gaia DR4 time series astrometry.

High-Contrast Imaging with Keck/NIRC2 Vortex Coronagraphy

Key Results

  • Built and maintain the automated Keck/NIRC2 vortex processing pipeline and product database.
  • Characterized performance across more than 300 stars and built ETC for contrast curve predictions.

I developed and maintain software that automates the data processing workflow for the Keck/NIRC2 vortex coronagraph, and helped set up a database and website to display the data products. With the pipeline, I analyzed imaging data of more than 300 stars, and wrote a paper characterizing the instrument's performance. These tools have been used in many science papers with Keck/NIRC2, including: Wallack et al. (2024), Jones et al. (2025), and Wang et al. (2020).