Research Highlights

New Insights into Planet Formation from JWST Spectroscopy of Giant Exoplanets

Planets (products of core accretion) are metal-rich in multiple elements

Evidence that the HR 8799 planets accrete both solids and metal-enriched gas from the circumstellar disk

1. Metal Enrichment and Volatile-to-Refractory Abundances in the Four HR 8799 Planets

Collaboration with Jean-Baptiste Ruffio, Yayaati Chachan, Kazumasa Ohno and others; part of GTO 1188 (PI: Hodapp)

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.

2. JWST Spectroscopy of the Young Jupiter Analog AF Lep b

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

I am leading a JWST Cycle 3 GO program to observe AF Lep b, a 3 MJup giant planet orbiting at 9 AU from its star. AF Lep b is the closest Jupiter analog that has ever been imaged, and we have collected 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, AF Lep b will serve as a bridge to link the hot/warm Jupiter population with widely separated imaged planets.

3. 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.

4. JWST Panchromatic Spectroscopy of the First Mature Cold Jupiter to be Imaged

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

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 Paper I, I studied the benchmark brown dwarf companion HD 4747 B anddemonstrated 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). I find that these companions have compositions consistent with solar and their host star compositions, which implies that they most likely form via gravitational instabiliy.

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

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.

Planetary Architectures and Demographics with Gaia Astrometry

Collaborators: William Thompson, Dori Blakely, Brendan Bowler, and others.

1. Direct Measurements of Orbital Mutual Inclinations in Exoplanet Systems

I'm 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.

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

Gaia DR4 (around 2027) and DR5 (after 2030) epoch astrometry will identify thousands of new giant planets and transform studies of planetary demographics and architectures. To prepare for this transition, I co-developed a new absolute astrometry framework that combines Gaia DR2 and DR3, astrometric excess noise, and Hipparcos measurements (Thompson, Blakely, Xuan et al. submitted). The resulting calibrated catalog is integrated into the open-source orbit code Octofitter and supports modeling of Gaia epoch astrometry. With collaborators, we are carrying out intensive ground-based vetting campaigns with Keck and Lick Observatory to rule out false positive scenarios.

Investigating the Origins of Hot Jupiters with Precise Orbits for Their Stellar and Planetary Companions

In collaboration with Marvin Morgan.

I've launched a campaign to refine the orbital eccentricities and inclinations of stellar companions to hot Jupiters. This survey will reveal whether the orbital properties of stellar companions contribute to hot Jupiter formation. My team was awarded two Keck nights in 2026 to initiate the survey.

Characterizing the Keck/NIRC2 Vortex Coronagraph for High-Contrast Imaging

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. This paper is the standard citation for Keck/NIRC2 high-contrast science.