Research

Exoplanets are planets orbiting other stars than our sun - planets exterior to our solar system. Like our solar system planets, exoplanets can also host atmospheres. There are multiple methods to study their atmospheres. For example, when the star light incident on the planet's atmosphere passes through it, it can carry information about its physical state and and chemical contents to us. Studying this light spectroscopically is called transit spectroscopy and has been very instrumental to gain key insights into the contents of exoplanetary atmospheres. Planets also emit their own thermal emission due to their temperature. This thermal emission can be detected to study their atmospheres using techniques like direct imaging and eclipse spectroscopy. Star light can also get reflected from the planet atmosphere and reach the observer. Studying this reflected light is called reflection spectroscopy. I work on developing theoretical models of exoplanet atmospheres that can be used to simulate the transmission spectrum, thermal emission spectrum, reflected light spectrum and polarization signature of exoplanets and brown dwarfs. These theoretical models are available and open-sourced in the PICASO repository. I use these theoretical models to find ways to measure atmospheric contents and properties of these far-away worlds. I work on varied type of exoplanets and substellar objects like gas giant exoplanets, sub-Neptunes, and brown dwarfs. I use these models to explain, understand, and make inferences from observations of these worlds from space-based and ground-based telescopes like JWST, HST, Keck, Gemini, etc. Please get in touch with me if you, like me, also wonder about the physics and chemistry of these worlds and their atmospheres !

GJ 436b is a Neptune-like exoplanet in terms of its mass and radius. However, its temperature is a lot warmer (~700 K) than Neptune. Past studies with telescopes like Spitzer had found that it's atmosphere is potentially poor in a gas like methane and more enriched in a gas like carbon monoxide than theoretical expectations. This is a tell-tale sign of disequilibrium chemistry in a planet's atmosphere. It was hypothesized that the interior of this Neptune-like planet is tidally heated due to its eccentric orbit. This heating of its atmosphere is reflected in this unexpected chemical composition due atmospheric dynamics. To test this hypothesis, I led a study of its atmosphere with JWST with the MANATEE collaboration. We observed the planet's eclipse with JWST to measure its panchromatic thermal emission spectrum for the first time between 2.4-12 microns. Surprisingly, we found that the JWST measurements of the planet's emission flux were totally inconsistent with the past Spitzer measurements at 3.6 and 4.5 microns (top left figure). We detect a weak evidence for the presence of carbon dioxide in GJ 436b's atmosphere with the JWST spectrum (bottom left figure). We also put stringet upper limits on the planet's albedo. We show that the JWST spectrum doesn't necessarily demand a planet with a tidally heated interior and can be explained with a nominal interior heat if cloudless conditions are assumed. Our atmospheric modeling shows that the atmosphere is likely very enriched in "metals" and it is also quite likely that the dayside of the planet is enshrouded with thick aerosol cover. More JWST observations (e.g., transmission spectrum) are required to put more stringent constraints on its composition and cloud cover. The published paper is Mukherjee et al. (2025, ApJL).

The chemical composition of transiting exoplanets can be significantly altered due to atmospheric processes like atmospheric dynamics, photochemistry, and interior heat flux from the planet. In this work, we have done a theoretical survey of a large parameter space to investigate how each of these processes alter the observable chemistry of diverse exoplanetary atmospheres - from gas giants to sub-Neptunes and from hot to warm planets. We have found fascinating trends in chemical behavior for gases like methane, carbon monoxide, carbon dioxide, etc. We have found that a gas that can be good tracer of the vigor of vertical mixing for one type of exoplanets can become completely insensitive to it in another. Similar trends were also found for other important planet properties like internal heat flux of planets. Sulfur molecules have been at the forefront of exoplanetary atmosphere studies since 2023 as gases like sulfur dioxide are formed in the upper atmosphere of giant planets due to photochemistry- opening up the possibility of probing sulfur in exoplanets and also probing photochemical processes. The left plot shows a fascinating behavior of sulfur bearing gases we have uncovered. We have found that gases like sulfur dioxide are prominent sulfur bearers for warm to hot exoplanets. But for colder planets, gases like carbon disulfide and carbon sulfide become the dominant sulfur bearing gases produced by photochemical processes. Moreover, atomic sulfur also are always the most dominant sulfur bearing gas for the photosphere of planets across all temperatures. To see a lot more fascinating theoretical chemical trends of exoplanets, see the published paper Mukherjee et al. (2025, ApJ). Our efforts to test these predictions with JWST observations of diverse exoplanet atmospheres is also well underway. We have reported some initial observed trends in this paper Fu et al. (2025, ApJ).

The chemical composition of directly imaged exoplanets and brown dwarfs can also be significantly altered due to highly uncertain atmospheric processes like atmospheric dynamics. In this work, we have developed a grid of radiative-convective models with self-consistent treatment of vertical mixing induced disequilibrium chemistry across a large range of object temperature, gravity, metallicity, and elemental abundance ratios. We call this the Sonora Elf Owl grid of models. We have shown that processes like vertical mixing and parameters like object metallicity have degenerate effects on various chunks of the observable spectrum of these planets and brown dwarfs (top left panels). Atmospheric dynamics is one of the least understood processes in the atmosphere of exoplanets and brown dwarfs. The parameter describing it is uncertain by 7-8 orders of magnitude. We use our theoretical model grid to constrain this highly uncertain parameter for a sample of field brown dwarfs with different temperatures. We use archival observational spectrum from Spitzer and AKARI telescopes to derive constraints on the strength of vertical mixing in their atmosphere. The bottom left figure shows our constraints. Surprisingly, we find that the strength of dynamics in their atmospheres are much weaker than expected from convection. With more self-consistent forward model grids, we show that this is likely because we are probing the strength of vertical mixing in sandwiched radiative regions in the deep atmosphere of these objects instead of convective regions. This is consistent with the theoretical predictions from my previous work in Mukherjee et al. (2023, ApJ) and the findings of Miles et al. (2020, ApJ). See the full published paper Mukherjee et al. (2024, ApJ). The models can be found in L-type, Y-type, and T-type .

Black holes feed on the matter in their surroundings. Infalling matter on the black hole loose their gravitational energy through radiation forming disk like structures called accretion disks. These accretion disks have been spectroscopically inferred in many solar mass black hole and AGN (SMBH) systems. Blazars are black hole systems where the jet outflows from the BH points at a very small angle to our line of sight. Blazars emit across a huge range of wavelengths from radio to very high energy gamma rays. Blazars also show time variability in various timescales - from hours to years. Modeling Spectral energy distribution (SED) of blazars show that the emission in blazars are from ultra-relativistic particles which are traveling at speeds very close to the speed of light. These particles emit radiation through processes like Synchroton and Inverse- Compton radiation. Accretion disks also show variability in a large number of timescales. The origin of these highly relativistic jet outflows are not well understood. Since they originate very close to the central engine and the only source of matter in that region is the matter in the accretion disk, it is thought that the matter around the accretion disk gets accelerated and collimated as a jet through complex processes. If this is so, one might expect to observe certain temporal connection between the variability of accretion disks and jets. Chatterjee et al. (2009) and several other studies have observed such connections in Seyferts. Disk variability also is well known to show a characteristic timescale related with black hole mass observed by McHardy et al (2005) . Recently, Chatterjee et al. (2018) observed a similar characteristic timescale in jet variability as well. This raises a question and the opportunity to investigate the connections between these two timescales. We have performed numerical modeling of emission variability in the jet as well as the disk. We are investigating various scenarios of a disk-jet connection. We have also predicted the nature of Power Spectral Densities of future multi-wavelength jet variabilty observations. Here is our recent publication on the the disk-jet connection in blazar.