About

Nikole Lewis is an Associate Professor of Astronomy at Cornell University, based in the Space Sciences Building. Her research focuses on probing exoplanet atmospheres through a combination of observational and theoretical techniques. She works with both space- and ground-based campaigns to measure spectra and photometry of exoplanet atmospheres, and develops one-, two-, and three-dimensional models to guide and interpret these observations. Her interests also include laboratory work that provides constraints on key atmospheric processes such as cloud and haze formation, as well as the development of instrumentation for facilities aimed at characterizing exoplanets and their environments.

Research topics

• Physics
• Astrobiology
• Astronomy
• Computer Science
• Meteorology
• Astrophysics
• Artificial Intelligence
• Chemistry
• Botany
• Organic chemistry
• Biology
• Thermodynamics
• Environmental science
• History
• Materials science
• Algorithm

Selected publications

• NASA’s Pandora SmallSat Mission: Simulating the Impact of Stellar Photospheric Heterogeneity and Its Correction

  The Astronomical Journal · 2026-05-08

  articleOpen access

  Abstract Stellar photospheric heterogeneity is a dominant astrophysical systematic impacting exoplanet transmission spectroscopy. NASA’s Pandora SmallSat Mission is designed to address this challenge through contemporaneous visible-band photometry and near-infrared spectroscopy of exoplanet host stars. Here, we present an end-to-end simulation study quantifying Pandora’s ability to infer stellar photospheric properties and correct stellar contamination using out-of-transit observations. We construct eight representative stellar activity scenarios and generate 160 simulated Pandora datasets, incorporating time-dependent stellar spectra, instrument response, and noise. Given accurate models, Bayesian retrievals of joint visible photometry (0.4–0.7 μ m) and near-infrared spectroscopy (0.9–1.6 μ m, R ≈ 120) recover photospheric temperatures with typical uncertainties of ≈30 K, with no significant bias. Models with two spectral components (i.e., a quiescent photosphere and spots) are strongly favored in 95% of cases; one-component models are preferred when true spot filling factors fall below a detection threshold of ≈0.3%. We propagate the true and inferred stellar parameters to compute true, inferred, and residual contamination signals under physically motivated spot geometries. For simple spot distributions, contamination signals of 10 2 –10 3 ppm are reduced to ≲10 ppm—well below Pandora’s expected transmission spectroscopy precision (30–100 ppm). For more complex spot distributions, geometric degeneracies limit deterministic corrections, leaving residual contamination at the 10 3 ppm level that must be mitigated using additional constraints, such as spot-crossing events and joint stellar–planetary retrievals of transmission spectra. These results define regimes in which stellar contamination can be corrected from stellar observations alone and show how Pandora stellar observations can identify cases where additional information is required.

  PublisherDOI

• JWST-TST DREAMS: The Nightside Emission and Chemistry of WASP-17b

  ArXiv.org · 2025-10-07

  preprintOpen access

  Theoretical studies have suggested using planetary infrared excess (PIE) to detect and characterize the thermal emission of transiting and non-transiting exoplanets, however the PIE technique requires empirical validation. Here we apply the PIE technique to a combination of JWST NIRSpec G395H transit and eclipse measurements of WASP-17b, a hot Jupiter orbiting an F-type star, obtained consecutively (0.5 phase or 1.8 days apart) as part of the JWST-TST program to perform Deep Reconnaissance of Exoplanet Atmospheres through Multi-instrument Spectroscopy (DREAMS). Using the in-eclipse measured stellar spectrum to circumvent the need for ultra-precise stellar models, we extract the first JWST nightside emission spectrum of WASP-17b using only transit and eclipse data thereby performing a controlled test of the PIE technique. From the WASP-17b nightside spectrum, we measure a nightside equilibrium temperature of $1005 \pm 256$ K and find tentative evidence for nightside SO2 absorption ($\ln B = 1.45$, $2.3σ$). In context with the dayside, the temperature of the nightside is consistent with (1) previous eclipse mapping findings that suggest relatively inefficient day-night heat transport, and (2) a non-zero bond albedo of $0.42^{+0.06}_{-0.10}$. SO2 on the nightside, if confirmed, would represent the first direct evidence for transport-induced chemistry, matching previous model predictions, and opening a new door into the 3D nature of giant exoplanets. Our results suggest that PIE is feasible with JWST/NIRSpec for two epochs separated in time by significantly less than the rotation period of the host star.

  PublisherOA PDFDOI

• Condensation Clouds in Substellar Atmospheres with Virga

  ArXiv.org · 2025-08-20

  preprintOpen access

  Here we present an open-source cloud model for substellar atmospheres, called Virga. The Virga-v0 series has already been widely adopted in the literature. It is written in Python and has heritage from the Ackerman & Marley (2001) model (often referred to as eddysed), used to study clouds on both exoplanets and brown dwarfs. In the development of the official Virga-v1 we have retained all the original functionality of eddysed and updated/expanded several components including the back-end optical constants data, calculations of the Mie properties, available condensate species, saturation vapor pressure curves and formalism for fall speeds calculations. Here we benchmark Virga by reproducing key results in the literature, including the SiO2 cloud detection in WASP-17 b and the brown dwarf Diamondback-Sonora model series. Development of Virga is ongoing, with future versions already planned and ready for release. We encourage community feedback and collaborations within the GitHub code repository.

  PublisherOA PDFDOI

• The HUSTLE Program: The UV to Near-IR Transmission Spectrum of the Hot Jupiter KELT-7b

  The Astronomical Journal · 2025-08-21

  articleOpen access

  Abstract The ultraviolet and optical wavelength ranges have proven to be a key addition to infrared observations of exoplanet atmospheres because they offer unique insights into the properties of clouds and hazes and are sensitive to signatures of disequilibrium chemistry. Here, we present the 0.2–0.8 μ m transmission spectrum of the T eq = 2000 K Jupiter KELT-7b, acquired with Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3)/UVIS G280 as part of the HUSTLE Treasury program. We combined this new spectrum with the previously published HST WFC3/IR G141 (1.1–1.7 μ m) spectrum and Spitzer photometric points at 3.6 and 4.5 μ m, to reveal a generally featureless transmission spectrum between 0.2 and 1.7 μ m, with a slight downward slope toward bluer wavelengths, and an asymmetric water feature in the 1.1–1.7 μ m band. Retrieval models conclude that the 0.2–1.7 μ m spectrum is primarily explained by a high H − abundance (∼10 −5 ), significantly above the equilibrium chemistry prediction (∼10 −12 ), suggesting disequilibrium in KELT-7b’s upper atmosphere. Our retrievals also suggest the presence of bright inhomogeneities in the stellar surface, and tentative evidence of CO 2 at the Spitzer wavelengths. We demonstrate that with the UV–optical coverage provided by WFC3 UVIS/G280, we are able to confirm the presence and constrain the abundance of H − , and obtain evidence for bright stellar inhomogeneities that would have been overlooked using infrared data alone. Observations redward of 1 μ m with JWST should be able to further constrain the abundance of H − , as well as confirm the presence of CO 2 inferred by the two Spitzer data points.

  PublisherDOI

• Constraining Ongoing Volcanic Outgassing Rates and Interior Compositions of Extrasolar Planets with Mass Measurements of Plasma Tori

  ArXiv.org · 2025-06-09

  preprintOpen accessSenior author

  We present a novel method of constraining volcanic activity on extrasolar terrestrial worlds via characterization of circumstellar plasma tori. Our work generalizes the physics of the Io plasma torus to propose a hypothetical circumstellar plasma torus generated by exoplanetary volcanism. The quasi-steady torus mass is determined by a balance between material injection and ejection rates from volcanic activity and corotating magnetospheric convection, respectively. By estimating the Alfvén surfaces of planet-hosting stars, we calculate the torus mass-removal timescale for a number of exoplanets with properties amenable to plasma torus construction. Assuming a uniform toroidal geometry comparable to Io's "warm" torus, we calculate quasi-steady torus masses inferable from the optical depth of atomic spectral features in torus-contaminated stellar spectra. The calculated quasi-steady masses can be used to constrain the volcanic outgassing rates of each species detected in the torus, providing quantitative estimates of bulk volcanic activity and interior composition with minimal assumptions. Such insight into the interior state of an exoplanet is otherwise accessible only after destruction via tidal forces. We demonstrate the feasibility of our method by showcasing known exoplanets which are susceptible to tidal heating and could generate readily-detectable tori with realistic outgassing rates of order 1 ton s$^{-1}$, comparable to the Io plasma torus mass injection rate. This methodology may be applied to stellar spectra measured with ultraviolet instruments with sufficient resolution to detect atomic lines and sensitivity to recover the ultraviolet continuum of GKM dwarf stars. This further motivates the need for ultraviolet instrumentation above Earth's atmosphere.

  PublisherOA PDFDOI

• The Photochemical Plausibility of Warm Exo-Titans Orbiting M Dwarf Stars

  The Astrophysical Journal Letters · 2025-11-03

  articleOpen accessCorresponding

  Abstract The James Webb Space Telescope (JWST) has begun to spectrally characterize small exoplanets orbiting M dwarf stars, but interpretation of these spectra is ambiguous, with stellar, instrumental, or atmospheric origins possible for apparent spectral features. Consequently, the interpretation of JWST small exoplanet spectra follows a Bayesian approach, with less theoretically plausible interpretations facing a higher burden of proof. Here, we use photochemical modeling to evaluate the plausibility of warm exo-Titans, exoplanets with N 2 –CH 4 atmospheres analogous to Titan but orbiting closer to their host stars. Consideration of warm exo-Titans is motivated by arguments from planet formation, as well as tentative evidence from observations. Using TRAPPIST-1e as a case study, we show that the higher instellation experienced by warm exo-Titans reduces their CH 4 lifetime <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>τ</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mspace width="0.1em"/> <mml:mtext>CH</mml:mtext> <mml:mspace width="0.1em"/> </mml:mrow> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> </mml:math> relative to true Titan by orders of magnitude, reducing the probability of observing them. We constrain the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>τ</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mspace width="0.1em"/> <mml:mtext>CH</mml:mtext> <mml:mspace width="0.1em"/> </mml:mrow> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> </mml:math> on a warm exo-Titan to be ≤0.1× (and most likely ≤0.02×) true Titan, implying the absolute probability of detecting a warm exo-Titan is &lt;0.1 and likely &lt;0.01. This finding is consistent with recent JWST nondetections of CH 4 -dominated atmospheres on warm terrestrial exoplanets. The low prior probability means that the standard of proof required to claim a warm exo-Titan detection is high, and we offer specific suggestions toward such a standard of proof. Observation of oxidized carbon species would corroborate a putative warm exo-Titan detection. Confirmed detection of warm exo-Titans would signal the need to fundamentally rethink our understanding of the structure, dynamics, and photochemistry of Titan-like worlds.

  PublisherDOI

• Effects of Ultraviolet Radiation on Sub-Neptune Exoplanet Hazes Through Laboratory Experiments

  ArXiv.org · 2025-05-19

  preprintOpen access

  Temperate sub-Neptune exoplanets could contain large inventories of water in various phases, such as water-worlds with water-rich atmospheres or even oceans. Both space-based and ground-based observations have shown that many exoplanets likely also contain photochemically-generated hazes. Haze particles are a key source of organic matter and may impact the evolution or origin of life. In addition, haze layers could provide a mechanism for lower-atmospheric shielding and ultimately atmospheric retention. Often orbiting close to M-dwarf stars, these planets receive large amounts of radiation, especially during flaring events, which may strip away their atmospheres. M-dwarf stars are known to have higher stellar activity than other types of stars, and stellar flares have the potential to accelerate atmospheric escape. In this work, we present results on laboratory investigations of UV radiation effects simulating two different stellar flare energies on laboratory-produced exoplanet hazes made under conditions analogous to water-world atmospheres. We find that both simulated flares altered the overall transmittance and reflectance of the hazes, and higher energy "flares" make those alterations more pronounced. On a larger scale, these laboratory-made hazes show potential signs of degradation over the simulated flaring period. Our results provide insight into the effects that stellar flaring events have on potential exoplanet haze composition and the ability for water-world-like exoplanets to retain their atmospheres.

  PublisherOA PDFDOI

• JWST-TST DREAMS: Sulfur dioxide in the atmosphere of the Neptune-mass planet HAT-P-26 b from NIRSpec G395H transmission spectroscopy

  St Andrews Research Repository (St Andrews Research Repository) · 2025-09-19 · 1 citations

  preprintOpen access

  We present the James Webb Space Telescope (JWST) transmission spectrum of the exoplanet HAT-P-26 b (18.6 Earth masses, 6.33 Earth radii), based on a single transit observed with the JWST NIRSpec G395H grating. We detect water vapor (ln B = 4.1), carbon dioxide (ln B = 85.6), and sulfur dioxide (ln B = 13.5) with high confidence, along with marginal indications for hydrogen sulfide and carbon monoxide (ln B &lt; 0.5). The detection of SO2 in a warm super-Neptune sized exoplanet (radius about 6 Earth radii) bridges the gap between previous detections in hot Jupiters and sub-Neptunes, highlighting the role of disequilibrium photochemistry across a broad range of exoplanet atmospheres, including those cooler than 1000 K. Our precise measurements of carbon, oxygen, and sulfur indicate an atmospheric metallicity of about 10 times solar and a sub-solar C/O ratio. Retrieved molecular abundances are consistent within 2 sigma with predictions from self-consistent models including photochemistry. The elevated CO2 abundance and possible H2S signal may also reflect sensitivities to the thermal structure, cloud properties, or additional disequilibrium processes such as vertical mixing. We compare the SO2 abundance in HAT-P-26 b with that of ten other JWST-observed giant exoplanets, and find a correlation with atmospheric metallicity. The trend is consistent with the prediction from Crossfield (2023), showing a steep rise in SO2 abundance at low metallicities, and a more gradual increase beyond 30 times solar. This work is part of a series of studies by our JWST Telescope Scientist Team (JWST-TST), in which we use Guaranteed Time Observations to perform Deep Reconnaissance of Exoplanet Atmospheres through Multi-instrument Spectroscopy (DREAMS).

  PublisherDOI

• Constraining Ongoing Volcanic Outgassing Rates and Interior Compositions of Extrasolar Planets with Mass Measurements of Plasma Tori

  The Astrophysical Journal Letters · 2025-06-23 · 1 citations

  articleOpen accessSenior authorCorresponding

  Abstract We present a novel method of constraining volcanic activity on extrasolar terrestrial worlds via characterization of circumstellar plasma tori. Our work generalizes the physics of the Io plasma torus to propose a hypothetical circumstellar plasma torus generated by exoplanetary volcanism. The quasi-steady torus mass is determined by a balance between material injection and ejection rates from volcanic activity and corotating magnetospheric convection, respectively. By estimating the Alfvén surfaces of planet-hosting stars, we calculate the torus mass-removal timescale for a number of exoplanets with properties amenable to plasma torus construction. Assuming a uniform toroidal geometry comparable to Io’s “warm” torus, we calculate quasi-steady torus masses inferable from the optical depth of atomic spectral features in torus-contaminated stellar spectra. The calculated quasi-steady masses can be used to constrain the volcanic outgassing rates of each species detected in the torus, providing quantitative estimates of bulk volcanic activity and interior composition with minimal assumptions. Such insight into the interior state of an exoplanet is otherwise accessible only after destruction via tidal forces. We demonstrate the feasibility of our method by showcasing known exoplanets that are susceptible to tidal heating and could generate readily detectable tori with realistic outgassing rates of order 1 t s −1 , comparable to the Io plasma torus mass injection rate. This methodology may be applied to stellar spectra measured with ultraviolet instruments with sufficient resolution to detect atomic lines and sensitivity to recover the ultraviolet continuum of GKM dwarf stars. This further motivates the need for ultraviolet instrumentation above Earth’s atmosphere.

  PublisherDOI

• JWST-TST High Contrast: Medium-resolution spectroscopy reveals a carbon-rich circumplanetary disk around the young accreting exoplanet Delorme 1 AB b

  Astronomy and Astrophysics · 2025-10-28 · 2 citations

  articleOpen access

  Context. Young accreting planetary-mass objects are thought to draw material from a circumplanetary disk composed of gas and dust. While the gas within the disk is expected to disperse within the first million years, strong accretion has nonetheless been detected in older systems, including the 30-45 Myr-old planetary-mass companion Delorme 1 AB b. Aims. We conducted spectroscopic observations with the James Webb Space Telescope’s Mid-Infrared Instrument ( JWST /MIRI) to investigate the presence of circumplanetary material around this young accreting planet and to characterize the planet’s atmospheric properties and composition. Methods. We performed forward modeling using atmospheric models to characterize the planet’s atmosphere, combining our MIRI observations with archival ground-based near-infrared data. We used slab models to analyze the circumplanetary gas and investigated H 2 emission. Results. We derived the atmospheric parameters of Delorme 1 AB b, finding an effective temperature of T eff = 1725 ± 134 K. To achieve a satisfactory fit to the observed spectrum, a secondary component is required, consistent with dust emission from a circumplanetary disk (CPD), characterized by a blackbody temperature of T bb = 295 ± 27 K and an effective radius of R bb = 18.8 ± 2.7 R Jup . Beyond 10 μm, the spectral energy distribution (SED) becomes dominated by this circumplanetary disk rather than the planet itself. We detected strong emission from HCN and C 2 H 2 , along with tentative evidence of the isotopologue 13 CCH 2 , while no O-bearing species such as CO, CO 2 , or H 2 O are observed in the CPD spectrum. This suggests that the gas in the CPD has an elevated C/O. We also identified spatially extended H 2 emission around the planet, tracing warm gas, with indications that it may be at a higher temperature than the non-extended component. Conclusions. The mid-infrared spectrum of the planetary-mass companion Delorme 1 AB b reveals the first detection of bright C-bearing species in a CPD together with an outflow traced by H 2 extended emission, which could be interpreted as a disk wind. The hot dust continuum emission suggests an inner cavity in the CPD. The presence of warm gas in the CPD provides constraints on the disk’s chemical composition and physical conditions, opening up new avenues for disk studies. The study of these long-lived “Peter Pan” disks will enhance our understanding of how accretion persists in evolved low-mass systems and shed light on their formation, longevity, and evolutionary pathways in planetary systems.

  PublisherDOI

Frequent coauthors

• Jonathan J. Fortney

  University of California, Santa Cruz

  157 shared

• Mark S. Marley

  University of Arizona

  131 shared

• Hannah R. Wakeford

  100 shared

• Kevin B. Stevenson

  Johns Hopkins University Applied Physics Laboratory

  85 shared

• Adam P. Showman

  81 shared

• Tiffany Kataria

  Jet Propulsion Laboratory

  77 shared

• Drake Deming

  77 shared

• Heather A. Knutson

  California Institute of Technology

  74 shared

Education

• Ph.D., Planetary Sciences

  University of Arizona

  2012

• Master of Arts, Astronomy

  Boston University

  2004

• Bachelors, Physics and Mechanical Engineering

  Worcester Polytechnic Institute

  2002

Awards & honors

• Zonta Amelia Earhart Fellowship