About

Jonathan J Fortney is a Professor and Chair in the Department of Astronomy & Astrophysics within the Division of Physical & Biological Sciences. His expertise encompasses astrophysics and astronomy, with a focus on planetary atmospheres, planetary interiors, and exoplanets. Fortney's research aims to understand different kinds of planets as classes of astrophysical objects by characterizing their composition and evolution over time. He primarily functions as a modeler and theorist, working closely with observers to study the atmosphere, interior, and thermal evolution of planets both within the solar system and around other stars, at various distances from their parent stars. His work involves studying planets at different ages, from young to old, and across different environments, to better understand their physical properties and developmental processes.

Research topics

• Physics
• Astronomy
• Astrobiology
• Astrophysics
• Meteorology
• Chemistry
• Biology
• Environmental science
• Geology
• Computer Science
• Optics
• Remote sensing
• Botany
• Ecology

Selected publications

• <tt>PICASO 4.0</tt> : Clouds and Photochemistry in Climate Models of Brown Dwarfs and Exoplanets

  The Astrophysical Journal · 2026-03-16 · 1 citations

  articleOpen access

  Abstract We present a major update to the open-source atmospheric modeling package PICASO , designed for simulating the thermal structure and spectra of hydrogen-rich atmospheres of brown dwarfs and exoplanets. This release, PICASO 4.0 , expands upon the existing radiative-convective equilibrium model framework by incorporating several new capabilities. Key additions include the integration of Virga for self-consistent cloud modeling, new flexible treatments for rainout and cold trapping of volatile species, and support for photochemistry. We also introduce a parameterized energy injection scheme to simulate additional external or internal heating processes. These features are motivated by lessons from recent JWST observations that reveal the prevalence of nonequilibrium chemistry and clouds. We benchmark the new functionalities against previously published results in the literature, including the Sonora Diamondback grid, energy injected atmospheres, patchy cloud models, and other photochemical models of WASP-39b. PICASO continues to be actively developed as an open-source package aimed at enabling reproducible, community-driven atmospheric modeling of all substellar objects.

  PublisherDOI

• Cloudy solutions for the clear skies of WASP-80b: 3D cloud feedback on the atmosphere and spectra of a warm Jupiter

  Astronomy and Astrophysics · 2026-02-20

  articleOpen access

  Context . Close-in warm Jupiters orbiting M dwarf stars are expected to exhibit diverse atmospheric chemistry, with clouds playing a key role in shaping their albedo, heat distribution, and spectral properties. Aims . We study WASP-80b, a warm Jupiter orbiting an M dwarf star, using the latest JWST panchromatic emission and transmission spectra to comprehensively characterise its atmosphere, including cloud coverage, chemical composition, and particle sizes, and compare the observations with predictions from the general circulation models (GCMs). Methods . We used a GCM, ADAM (ADvanced Atmospheric MITgcm, formerly known as SPARC/MITgcm), combined with the latest JWST data to study the atmosphere of WASP-80b. A cloud module with radiatively active, tracer-based clouds was integrated with the GCM to study the effects on the atmosphere and the spectrum. Results . We find that the emission and transmission spectra of WASP-80b are only compatible with cloudless atmospheres or with clouds composed of sufficiently large particles, namely Na 2 S (≥10 μm), KCl (≥1 μm), and MgSiO 3 (≥5 μm). For these large-particle cloud cases, efficient gravitational settling confines the clouds to deeper atmospheric layers, resulting in weak spectral signatures. Smaller particles are ruled out due to their strong radiative feedback on the atmospheric structure. Conclusions . Overall, our results suggest that WASP-80b’s atmosphere is either effectively cloud-free or contains clouds composed of large, settled particles whose opacity has little impact on the observable atmosphere. This underscores the importance of particle size and vertical cloud distribution in interpreting exoplanet spectra. Future observations at shorter wavelengths may help distinguish between large-particle cloud scenarios and a truly cloudless atmosphere.

  PublisherDOI

• JWST Spectral Retrieval of Cold Directly Imaged Planet WD 0806 b and the First Measurement of Altitude-dependent <i>K</i> _<i>zz</i> in Exoplanet Atmospheres

  The Astronomical Journal · 2026-03-16

  articleOpen access

  Abstract WD 0806 b is a rare exoplanet companion orbiting a white dwarf, currently with a projected orbital distance of 2500 au. The Spitzer mid-IR photometry suggests that the temperature is as cold as 350 K, making it one of the coldest directly imaged exoplanets. In this paper, we present the Near-infrared Camera (NIRCam) F150W2, F200W, F356W, and F444W broadband photometry and a 3–5 μ m Near-Infrared spectroscopy (NIRSpec) G395M spectrum obtained with the James Webb Space Telescope. We develop a new retrieval framework based on the open-source PICASO software that includes additive and multiplicative systematic parameters. Our retrieval results reveal bounded abundances of H 2 S, CO 2 , CO, NH 3 , H 2 O, and CH 4 . We present a new chemical analysis framework that utilizes retrieved abundances to measure altitude-dependent eddy diffusion coefficients ( K zz ) at multiple quenched pressures. We find that the eddy diffusion coefficients decrease from around 10 4 –10 2 cm 2 s −1 as the atmospheric pressure decreases from from 50 to 20 bar. To our knowledge, this is the first study to report altitude-dependent vertical mixing (or, equivalently, quenched-species-dependent vertical mixing) based on the measured molecular abundances of CO, CH 4 , and CO 2 . With the 1–21 μ m NIRCam, NIRSpec, and the previously published MIRI data, we measure the bolometric luminosity to be log( L / L ⊙ ) = −6.75 ± 0.01 and derive the mass to be 8 ± 1 M J . The retrieval results suggest that WD 0806 b has an elevated C/O ratio of 0.76, or 1.3× solar, subsolar metallicity ([M/H ]= −0.25), and a nearly solar C/S ratio (1.17x solar).

  PublisherDOI

• A carbon-rich atmosphere on a windy pulsar planet

  ArXiv.org · 2025-09-04

  preprintOpen access

  A handful of enigmatic Jupiter-mass objects have been discovered orbiting pulsars. One such object, PSR\,J2322-2650b, uniquely resembles a hot Jupiter exoplanet due to its minimum density of 1.8 g/cm^3 and its ~1900 K equilibrium temperature. We use JWST to observe PSR J2322-2650b's emission spectrum across an entire orbit. In stark contrast to every known exoplanet orbiting a main-sequence star, we find an atmosphere rich in molecular carbon (C3, C2) with strong westward winds. Our observations open up new exoplanetary chemical (ultra-high C/O and C/N ratios of &gt;100 and &gt;10,000, respectively) and dynamical regimes (ultra-fast rotation with external irradiation) to observational study. The extreme carbon enrichment poses a severe challenge to the current understanding of ``black widow'' companions, which were expected to consist of a wider range of elements due to their origins as stripped stellar cores.

  PublisherOA PDFDOI

• The Roasting Marshmallows Program with IGRINS on Gemini South III: Seeing deeper into the metal depleted atmosphere of a gas-giant on the cusp of the hot to ultra-hot Jupiter transition

  ArXiv.org · 2025-07-09

  preprintOpen access

  Ultra-hot Jupiters are a class of gas-giant exoplanets that show a peculiar combination of thermochemical properties in the form of molecular dissociation, atomic ionization, and inverted thermal structures. Atmospheric characterization of gas giants lying in the transitional regime between hot and ultra-hot Jupiters can help in understanding the physical mechanisms that cause the fundamental transition in atmospheres between the two classes of hot gas giants. Using Doppler spectroscopy with IGRINS on Gemini South (1.4 to 2.5 $μ$m), we present the day-side high-resolution spectrum of WASP-122b (T$_{\mathrm{day}}$=2258$ \pm$ 54 K), a gas-giant situated at this transition. We detect the signal from H$_{2}$O, based on which we find that WASP-122b has a significantly metal-depleted atmosphere with metallicity log$_{10}$[Z$_{\mathrm{P}}$/Z$_{\odot}$] = $-$1.48$\pm$0.25 dex (0.033$_{-0.016}^{+0.018}$ $\times$ solar), and solar/sub-solar C/O ratio = 0.36$\pm$0.22 (3$σ$ upper limit 0.82). Drastically low atmospheric metallicity pushes the contribution function to higher pressures, resulting in the planetary spectral lines to originate from a narrow region around 1 bar where the thermal profile is non-inverted. This is inconsistent with solar composition radiative convective equilibrium (RCTE) which predicts an inverted atmosphere with spectral lines in emission. The sub-solar metallicity and solar/sub-solar C/O ratio is inconsistent with expectations from core-accretion. We find the planetary signal to be significantly shifted in K$_{\mathrm{P}}$ and V$_{\mathrm{sys}}$, which is in tension with the predictions from global circulation models and require further investigation. Our results highlight the detailed information content of high-resolution spectroscopy data and their ability to constrain complex atmospheric thermal structures and compositions of exoplanets.

  PublisherOA PDFDOI

• ELemental Abundances of Planets and Brown Dwarfs Imaged around Stars (ELPIS). II. The Jupiter-like Inhomogeneous Atmosphere of the First Directly Imaged Planetary-mass Companion 2MASS 1207 b

  The Astronomical Journal · 2025-07-03 · 13 citations

  articleOpen access

  Abstract 2MASS 1207 b, the first directly imaged planetary-mass companion, has been instrumental in advancing our understanding of exoplanets and brown dwarfs over the past 20 yr. We have performed extensive atmospheric retrieval analyses of 2MASS 1207 b’s JWST/NIRSpec spectrum using petitRADTRANS and a new atmospheric inhomogeneity framework, which characterizes homogeneous atmospheres, patchy clouds, cloud-free hot spots, or the combination of patchy clouds and spots. Among 24 retrieval runs with various assumptions, the most statistically preferred model corresponds to the patchy cloud scheme, with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">eff</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>117</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>4</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> K, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi>g</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo>=</mml:mo> <mml:mn>3.6</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.02</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.03</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> dex, and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>R</mml:mi> <mml:mo>=</mml:mo> <mml:mn>1.39</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>9</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.010</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.008</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> R Jup , along with near-solar atmospheric compositions of [M/H] = −0.05 ± 0.03 dex and C/O = 0.440 ± 0.012. This model suggests ∼9% of 2MASS 1207 b’s atmosphere is covered by thin iron and silicate clouds, producing L-dwarf-like spectra, while the remaining 91% consists of thick iron and silicate clouds, emitting blackbody-like spectra. These thin-cloud patches and thick-cloud regions resemble Jupiter’s belts and zones, respectively, and this scenario is consistently supported by other retrieval runs incorporating inhomogeneous atmospheres. We demonstrate that the weak CO absorption of 2MASS 1207 b can be explained by the veiling effects of patchy thick clouds; the absence of 3.3 μ m CH 4 absorption is attributed to its hot thermal structure, which naturally leads to a CO-dominant, CH 4 -deficient atmosphere. The retrieved atmospheric models also match the observed variability amplitudes of 2MASS 1207 b. Our analysis reveals that the inferred atmospheric properties show significant scatter in less statistically preferred retrieval runs but converge to consistent values among the preferred ones. This underscores the importance of exploring diverse assumptions in retrievals to avoid biased interpretations of atmospheric properties and formation pathways.

  PublisherDOI

• Detecting Land with Reflected-light Spectroscopy to Rule Out Waterworld O₂ Biosignature False Positives

  The Astrophysical Journal · 2025-08-25 · 3 citations

  articleOpen accessSenior author

  Abstract The search for life outside our solar system is at the forefront of modern astronomy, and telescopes such as the Habitable Worlds Observatory (HWO) are being designed to identify biosignatures. Molecular oxygen, O 2 , is considered a promising indication of life, yet substantial abiotic O 2 may accumulate from H 2 O photolysis and hydrogen escape on a lifeless, fully (100%) ocean-covered terrestrial planet when surface O 2 sinks are suppressed. This so-called waterworld false-positive scenario could be ruled out with land detection because exposed land precludes extremely deep oceans (∼50 Earth oceans) given topographic limits set by the crushing strength of rocks. Land detection is possible because plausible geologic surfaces exhibit increasing reflectance with wavelength in the visible, whereas liquid water and ice/snow have flat or decreasing reflectance, respectively. Here, we present reflected-light retrievals to demonstrate that HWO could detect land on an exo-Earth in the disk-averaged spectrum. Given a signal-to-noise ratio (SNR) of 20 spectrum, Earth-like land fractions can be confidently detected with 0.3–1.1 μ m spectral coverage (resolution R ∼ 140 in the visible, R ∼ 7 in the UV, with Earth-like atmosphere and clouds). We emphasize the need for UV spectroscopy down to at least 0.3 μ m to break an O 3 –land degeneracy. We find that the SNR and resolution requirements in the visible/UV imply a large aperture (∼8 m) will be necessary to ensure the observing times required for land detection are feasible for most HWO terrestrial habitable zone targets. These results strongly inform the HWO minimum requirements to corroborate possible oxygen biosignatures.

  PublisherOA PDFDOI

• The Sonora Substellar Atmosphere Models. VI. Red Diamondback: Extending Diamondback with SPHINX for Brown Dwarf Early Evolution

  The Astrophysical Journal · 2025-11-26 · 5 citations

  articleOpen access

  Abstract We extend the Sonora Diamondback brown dwarf evolution models to higher effective temperatures to treat the evolution of younger, higher-mass objects. Due to an upper temperature limit of T eff = 2400 K in the original Sonora Diamondback model grid, high-mass objects ( M ≥ 0.05 M ⊙ = 52.4 M J ) were limited to ages of ≳100 Myr. To include the early evolution of brown dwarfs at T eff &gt; 2400 K, we use existing and new SPHINX cloud-free model atmosphere calculations of temperature structures of M-type atmospheres. These atmospheres range from T eff 2000 to 4000 K, log( g ) 3.0 to 5.5, and metallicity [M/H] −0.5 to +0.5. This combination of Diamondback and SPHINX atmospheres, with a transition across T eff 2000–2400 K, allows us to calculate evolution tracks, and infrared photometry and colors, for ages &gt;1 Myr and masses from above the hydrogen-burning minimum mass down to planetary masses. The Hayashi phase of massive brown dwarf evolution (ages &lt; 10–100 Myr) at low surface gravity leads to nearly constant T eff values, at effective temperatures much lower than would be obtained from simply extrapolating backward from evolution tracks at older ages.

  PublisherDOI

• Understanding the Origins of Super-puff Planets: A New Mass-loss Regime Coupled to Planetary Evolution

  The Astrophysical Journal · 2025-12-02 · 4 citations

  articleOpen access

  Abstract Super-puffs are a class of low-mass, large-radius planets that have challenged planet formation and evolution models. Their high inferred H/He mass fractions, required to explain their physical sizes, would lead to rapid atmospheric escape, raising questions about their long-term retention. Recent modeling work indicates that low-mass planets typically require 50% less H/He mass to match their observed radius, due to the significant roles of the radiative atmosphere and interior heating from the rock/iron core. Here, through a new quantitative analysis of X-ray and EUV (XUV)–driven escape in sub-Neptunes, we find that previous studies overestimated mass loss, as scaling laws in low-gravity regimes deviate greatly from the widely used energy-limited regime. We define a new regime, thermal-energy-mediated photoevaporation, in which thermal-energy conversion critically sets the mass-loss rate. These effects make super-puffs more resilient to mass loss than previously thought. We develop a coupled evolution model integrating this updated thermal evolution framework with a 1D hydrodynamic photoevaporation model. Applying this novel, joint model to observed super-puffs and young low-density planets, we find that their masses, radii, and transit pressures align with predictions assuming either a clear or hazy atmosphere. This indicates that super-puffs have undergone a combination of boil-off and photoevaporative mass loss, with boil-off dominating the process. Our results indicate that low-density planets typically possess both a thick convective envelope and substantial radiative atmosphere, which contribute to their large radii. For this to occur, these planets must have intermediate masses of 5–10 M ⊕ and receive stellar insolation ≲30 F ⊕ , favoring FG-type stars over M dwarfs.

  PublisherDOI

• 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 &amp; 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

Recent grants

• Towards a Comprehensive Understanding of Brown Dwarf Atmospheres

  NSF · $245k · 2015–2018

• Evolutionary Models of Young Jupiters and Hot Jupiters

  NSF · $165k · 2008–2010

• Collaborative Research: Characterizing Cloudy Exoplanet Atmospheres

  NSF · $335k · 2013–2016

• Freeing Giant Planet Interior Models of Assumptions

  NSF · $489k · 2019–2023

• Evolutionary Models of Young Jupiters and Hot Jupiters

  NSF · $197k · 2006–2008

Frequent coauthors

• Mark S. Marley

  University of Arizona

  304 shared

• Caroline Morley

  211 shared

• Jean-Michel Désert

  211 shared

• Kevin B. Stevenson

  Johns Hopkins University Applied Physics Laboratory

  178 shared

• Heather A. Knutson

  California Institute of Technology

  157 shared

• Nikole K. Lewis

  157 shared

• Michael R. Line

  151 shared

• Adam P. Showman

  144 shared

Labs

• Other Worlds LaboratoryPI

Education

• Ph.D., Astronomy

  California Institute of Technology

  2003

• M.S., Astronomy

  California Institute of Technology

  1999

• B.S., Physics

  Harvard University

  1997