About

Professor Andrew J Skemer is a faculty member in the Astronomy & Astrophysics department at UC Santa Cruz. His research group focuses on a diverse range of topics including exoplanet imaging, brown dwarfs, exomoons, instrumentation, and astrobiology. He is actively involved with the James Webb Space Telescope (JWST) and is completing a new instrument called SCALES, which will be deployed at the Keck Observatory. SCALES utilizes adaptive optics, coronagraphs, and integral field spectroscopy to obtain spectra of directly imaged exoplanets. Professor Skemer advises graduate students on projects related to these areas and collaborates with colleagues specializing in exoplanet observations and instrumentation.

Research topics

• Physics
• Astronomy
• Astrophysics
• Optics
• Astrobiology
• Computer Science
• Geology
• Remote sensing

Selected publications

• The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems. VII. Molecular Mapping Performance with JWST/MIRI MRS: VHS 1256 b as a Case Study

  The Astronomical Journal · 2026-04-16

  articleOpen access

  Abstract VHS 1256 b was the first planetary-mass companion to be observed with the James Webb Space Telescope’s Mid-Infrared Instrument (JWST/MIRI) using the Medium-Resolution Spectrometer (MRS). The MRS provides high-quality integral-field spectral data in the mid-infrared (IR) wavelengths from 4.9–18 μ m. This data set serves as a testbed for applying cross-correlation techniques to characterize exoplanet atmospheres. We implement the so-called molecular mapping approach, which consists of performing a spectral cross-correlation between each spectral pixel and atmospheric model templates. We compare these results with those obtained from cross-correlation of the extracted spectrum. Using a self-consistent Exo-REM atmospheric model grid, we constrain the temperature, surface gravity, C/O ratio, and metallicity, finding values consistent with those obtained from other analysis methods. We detect CO (S/N ∼ 25) and H 2 O (S/N ∼ 76), with tentative detections of NH 3 and CH 4 (S/N ∼ 3). We test cross-correlation to measure trace-species abundances and isotopic ratios. We measure a volume mixing ratio (VMR) of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo stretchy="false">[</mml:mo> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">NH</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> </mml:msub> <mml:mo stretchy="false">]</mml:mo> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:mn>5.7</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.14</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.15</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> and an isotopic ratio <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow/> <mml:mrow> <mml:mn>12</mml:mn> </mml:mrow> </mml:msup> <mml:mi mathvariant="normal">C</mml:mi> <mml:msup> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mrow> <mml:mn>13</mml:mn> </mml:mrow> </mml:msup> <mml:mi mathvariant="normal">C</mml:mi> <mml:mo>=</mml:mo> <mml:mn>77</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>13</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , both consistent with free-chemistry retrievals. The derived NH 3 VMR, combined with the measured temperature and radius, is consistent with VHS 1256 b having a mass above the deuterium-burning limit. These results demonstrate the diagnostic power of mid-IR spectroscopy and highlight cross-correlation as a robust method for characterizing directly imaged exoplanets, even in future higher-contrast regimes where spectral extraction becomes challenging. Future MIRI MRS observations across a wider range of temperatures and masses will further expand our understanding of planetary atmospheric chemistry.

  PublisherDOI

• Dynamical Architectures of S-type Transiting Planets in Binaries. II. A Dichotomy in Orbital Alignment of Small Planets in Close Binary Systems

  The Astronomical Journal · 2026-01-09

  articleOpen access

  Abstract Stellar multiplicity plays a crucial role in shaping planet formation and dynamical evolution. We present a survey of 54 TESS objects of interest (TOIs) within 300 pc that exhibit significant Hipparcos−Gaia astrometric accelerations. We identified 35 TOIs with stellar companions at projected separations between 0 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mtext>″</mml:mtext> </mml:mrow> </mml:mover> </mml:math> 1 and 2″ (or 10–200 au). We also identified 12 TOIs that could host planetary-mass or brown dwarf companions, including six that are newly discovered. Furthermore, we perform 3D orbital characterization for 12 binaries hosting confirmed planets or planet candidates, allowing us to constrain the line-of-sight mutual inclination, Δ I los , between the planetary and binary orbits. Combining our sample with previous measurements, we apply Bayesian hierarchical analysis to a total of 26 binary systems with S-type transiting planets ( r p &lt; 5 R ⊕ ). Specifically, we fit the Δ I los distribution with both single (Rayleigh) and mixture models (two-component Rayleigh and Rayleigh-isotropic mixture). We find the mixture models are strongly favored ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mi>Z</mml:mi> <mml:mo>≳</mml:mo> <mml:mn>13.9</mml:mn> </mml:math> , or ≈5 σ ), indicating the observed planet-binary Δ I los values likely originate from two underlying populations: one nearly aligned ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>σ</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>2</mml:mn> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mtext>°</mml:mtext> </mml:mrow> </mml:mover> <mml:msubsup> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.9</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.7</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> ) and one with more scattered mutual inclinations ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>σ</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>23</mml:mn> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mtext>°</mml:mtext> </mml:mrow> </mml:mover> <mml:msubsup> <mml:mrow> <mml:mn>6</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>7.1</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>8.8</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> ). Alternatively, the misaligned systems can be equally well described by an isotropic distribution of inclinations. This observed dichotomy likely reflects different dynamical histories. Notably, the misaligned population only emerges in systems with stellar periastron distances &gt;40 au, while systems with close-in or eccentric stellar companions (periastron distances &lt;40 au) preserve planet−binary alignment.

  PublisherDOI

• Follow-up Exploration of the TWA 7 Planet–Disk System with JWST NIRCam

  The Astrophysical Journal Letters · 2025-07-10 · 10 citations

  articleOpen access

  Abstract The young M star TWA 7 hosts a bright and near face-on debris disk, which has been imaged from the optical to the submillimeter. The disk displays multiple complex substructures such as three disk components, a large dust clump, and spiral arms, suggesting the presence of planets to actively sculpt these features. The evidence for planets in this disk was further strengthened with the recent detection of a point source compatible with a Saturn-mass planet companion using JWST/MIRI at 11 μ m, at the location a planet was predicted to reside based on the disk morphology. In this Letter, we present new observations of the TWA 7 system with JWST/NIRCam in the F200W and F444W filters. The disk is detected at both wavelengths and presents many of the same substructures as previously imaged, although we do not robustly detect the southern spiral arm. Furthermore, we detect two faint potential companions in the F444W filter at the 2 σ –3 σ level. While one of these companions needs further follow-up to determine its nature, the other one coincides with the location of the planet candidate imaged with MIRI, providing further evidence that this source is a sub-Jupiter-mass planet companion rather than a background galaxy. Such discoveries make TWA 7 only the second system, after β Pictoris, in which a planet predicted by the debris disk morphology has been detected.

  PublisherDOI

• Fringing analysis and forward modeling of Keck Planet Imager and Characterizer (KPIC) spectra

  Journal of Astronomical Telescopes Instruments and Systems · 2025-08-13 · 1 citations

  article
  PublisherDOI

• The Watery Atmosphere of HD 209458 b Revealed by Joint <i>K</i> - and <i>L</i> -band High-resolution Spectroscopy

  The Astronomical Journal · 2025-09-15 · 2 citations

  articleOpen access

  Abstract We present a joint analysis of high-resolution K - and L -band observations of the benchmark hot Jupiter HD 209458 b from the Keck Planet Imager and Characterizer. One half-night of observations was obtained in each bandpass, covering similar preeclipse phases. The two epochs were then jointly analyzed using our atmospheric retrieval pipeline based on petitRADTRANS to constrain the atmospheric pressure–temperature profile and chemical composition. Consistent with recent results from JWST observations at lower spectral resolution, we obtain an oxygen-rich composition for HD 209458 b (C/O &lt; 10 −3 at 95% confidence) and a lower limit on the volatile metallicity similar to the solar value ([(C + O)/H] &gt; −0.2 at 95% confidence). Leveraging the large spectral grasp of the multiband observations, we constrain the atmospheric H 2 O mixing ratio to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mi mathvariant="normal">H</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mml:msub> <mml:mi mathvariant="normal">O</mml:mi> <mml:mi>V MR</mml:mi> </mml:msub> <mml:mo>&gt;</mml:mo> <mml:mo>−</mml:mo> <mml:mn>3.1</mml:mn> </mml:math> at 95% confidence, and obtain 95% upper limits on the atmospheric mixing ratios of CO (&lt;10 −4.8 ), CH 4 (&lt;10 −4.5 ), NH 3 (&lt;10 −5.8 ), H 2 S (&lt;10 −3.3 ), and HCN (&lt;10 −5.6 ). The limits on CH 4 , NH 3 , and HCN are consistent with recent results from JWST transmission spectroscopy, demonstrating the value of multiband, ground-based high-resolution spectroscopy for precisely constraining trace-species abundances in exoplanet atmospheres. The retrieved low-C/O, moderate-metallicity composition for HD 209458 b is consistent with formation scenarios involving late accretion of substantial quantities of oxygen-rich refractory solids and/or ices.

  PublisherDOI

• Constraining giant exoplanet atmospheres with SCALES medium-spectral-resolution angular/spectral differential imaging

  2025-09-18

  article
  PublisherDOI

• High-contrast L-band Integral Field Spectroscopy of HD 33632 Ab

  ArXiv.org · 2025-12-05

  preprintOpen access

  We present LBTI/ALES 3.07-4.08 micron spectroscopic observations of HD~33632~Ab, a ~53 M_Jup directly imaged companion to an F8 star. Spectroscopic measurements of HD 33632 Ab now span 1-4 micron, and we perform the first spectroscopic analysis covering this full range. The data are compared to isolated brown dwarf template spectra, indicating that HD 33632 Ab is similar to L8/9 field brown dwarfs. Synthetic atmosphere model spectra from multiple model families are fit, with cloudy models providing the best fits, consistent with expectations for an L-dwarf. Evolutionary model predictions for the bulk properties of HD 33632 Ab are highly constrained by the precise dynamical mass found for the object. In particular, predictions for surface gravity are narrowly peaked, log(g)=5.21+/-0.05, and not dependent on the effects of clouds or cloud dispersion. We find significant tension between the surface gravities and object radii inferred from atmosphere model fits and those predicted by evolutionary models. We conclude with a comparison to the spectra of the HR 8799 c, d, and e, and emphasize the case that HD 33632 Ab, and other L/T transition directly imaged companions with constrained masses, will serve an essential role in understanding the complex physical processes governing the appearance of clouds in planetary atmospheres.

  PublisherOA PDFDOI

• Water Dissociation and Rotational Broadening in the Atmosphere of KELT-20 b from High-resolution Spectroscopy

  The Astronomical Journal · 2025-05-23 · 9 citations

  articleOpen accessCorresponding

  Abstract We present atmospheric retrievals from Keck/KPIC Phase II observations of the ultrahot Jupiter (UHJ) KELT-20/MASCARA-2 b. Previous free retrievals of molecular abundances for UHJs have been impacted by significant model biases due to variations in vertical abundance profiles, which we address by including molecular dissociation into our retrieval framework as an additional free parameter. We measure the abundance of CO ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">CO</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">MMR</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:mn>2</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>5</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.5</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.6</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> ) and obtain a lower limit on the abundance of H 2 O ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mi mathvariant="normal">H</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mml:msub> <mml:mi mathvariant="normal">O</mml:mi> <mml:mi>MMR</mml:mi> </mml:msub> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mn>5</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1.0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.8</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , &gt;−3.0 at 95% confidence) in the atmosphere of KELT-20 b. These abundances yield an atmospheric <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="normal">C</mml:mi> <mml:mo>/</mml:mo> <mml:mi mathvariant="normal">O</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.1</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.4</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> (C/O &lt; 0.9 at 95% confidence) and suggest a metallicity approximately solar to 10 × solar. H 2 O is dissociated at pressures below <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="normal">log</mml:mi> </mml:mrow> <mml:msub> <mml:mrow> <mml:mi>P</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> <mml:mrow> <mml:mi mathvariant="normal">O</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:mn>1.</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.7</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.5</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> bar, roughly consistent with predictions from chemical equilibrium models, and suggesting that the retrieved composition is not a result of assumptions about the vertical mixing profiles. We also constrain the rotational velocity of KELT-20 b to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>v</mml:mi> <mml:mi>sin</mml:mi> <mml:mi>i</mml:mi> <mml:mo>=</mml:mo> <mml:mn>7.5</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.7</mml:mn> </mml:math> km s −1 , suggesting the presence of a jet comparable to the sound speed in the direction of the planet’s rotation, assuming the actual rotation of the planet is tidally locked.

  PublisherDOI

• Reproduction package for the paper "Chasing the storm: Investigating the application of high-contrast imaging techniques in producing precise exoplanet light curves"

  Zenodo (CERN European Organization for Nuclear Research) · 2025-11-04

  datasetOpen access

  This is a basic reproduction package for the paper "Chasing the storm: Investigating the application of high-contrast imaging techniques in producing precise exoplanet light curves" by Sutlieff et al. (2025). It aims to provide the most important data products to check and reproduce the main results of the paper.

  PublisherDOI

• Large-amplitude variability driven by giant dust storms on a planetary-mass companion

  Science Advances · 2025-11-26

  articleOpen access

  Large-amplitude variations are commonly observed in the atmospheres of directly imaged exoplanets and brown dwarfs. VHS 1256B, the most variable known planet-mass object, exhibits a near-infrared flux change of nearly 40%, with red color and silicate features revealed in recent JWST spectra, challenging current theories. Using a general circulation model, we demonstrate that VHS 1256B's atmosphere is dominated by planetary-scale dust storms persisting for tens of days, with large patchy clouds propagating with equatorial waves. This weather pattern, distinct from the banded structures seen on solar system giants, simultaneously explains the observed spectra and critical features in the rotational light curves, including the large amplitude, irregular evolution, and wavelength dependence, as well as the variability trends observed in near-infrared color-magnitude diagrams of dusty substellar atmospheres.

  PublisherDOI

Recent grants

• Collaborative Research: Extending Spectroscopy of Directly Imaged Planets into the Thermal Infrared

  NSF · $332k · 2016–2020

• Collaborative Research: A New Instrument for the Large Binocular Telescope to Expand Study of the Composition of Exoplanetary Atmospheres

  NSF · $676k · 2016–2022

Frequent coauthors

• Philip M. Hinz

  University of California, Santa Cruz

  179 shared

• Dimitri Mawet

  133 shared

• Jarron Leisenring

  105 shared

• Vanessa P. Bailey

  Jet Propulsion Laboratory

  98 shared

• Denis Defrère

  KU Leuven

  95 shared

• Jason Wang

  94 shared

• Jordan Stone

  92 shared

• Michael P. Fitzgerald

  University of California, Los Angeles

  88 shared

Education

• Ph.D., Astronomy

  California Institute of Technology

  2009

• B.S., Physics

  Harvard University

  2004