Research

I'm interested in the atmospheres of a wide range of planets, from temperate low-mass planets to highly-irradiated gas giants and isolated brown dwarfs. In particular, I work on the thermal profiles and emission spectra of these atmospheres, and what they can tell us about their complex physical processes.

 

Recently, I have worked on projects spanning self-consistent models of mini-Neptune atmospheres, thermal inversions in hot Jupiters, atmospheric retrievals of isolated brown dwarfs and self-consistent modelling of a highly-irradiated brown dwarf. See below for more details and links to the papers. For more publications, please see this page.

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Self-consistent mini-Neptune atmospheric P-T profiles across a range of internal temperatures, irradiation temperatures and infrared opacities for clear (top panels) and hazy (bottom panels) atmospheres. 

Piette, A. A. A. & Madhusudhan, N. , ApJ, in press

Temperature profiles and emission spectra of mini-Neptune atmospheres

 

With their relatively large scale heights and large planet-star contrasts, mini-Neptunes are currently ideal targets towards the goal of characterising temperate low-mass exoplanets. Indeed, atmospheric observations of mini-Neptunes orbiting M-dwarfs are beginning to provide constraints on their chemical and thermal properties, while also providing clues about their interiors and potential surfaces. 

 

In this work, we explore various aspects of mini-Neptunes, including radiative/convective energy transport, boundary conditions for the interior, and their potential habitability. We use self-consistent atmospheric models to investigate the effects of irradiation, internal flux, metallicity, clouds and hazes on the temperature profiles and thermal emission spectra of temperate mini-Neptune atmospheres. We find a range of physically-motivated atmospheric conditions that allow for liquid water under the H2-rich atmospheres of planets such as K2-18 b, and find that observations of thermal emission with JWST/MIRI spectrophotometry can place useful constraints on the habitability of such planets. Our results underpin the potential of temperate mini-Neptunes such as K2-18 b as promising candidates in the search for habitable exoplanets.

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Top: The parametric temperature profile introduced in this work. The P-T points are defined by the changes in temperature between them, as well as the absolute temperature at 3.2 bar.

Bottom: Observed and retrieved spectra for the isolated T-dwarf 2MASS J2339+1352. Data error bars include the retrieved model error. (Data from Buenzli et al. 2014, ApJ, 782, 77)

Piette, A. A. A., Madhusudhan, N. (2020) , MNRAS, 497, 5136

Considerations for atmospheric retrieval of high-precision brown-dwarf spectra

Isolated brown dwarfs provide remarkable laboratories for understanding atmospheric physics in the low-irradiation regime, and can be observed more precisely than exoplanets. As such, they can provide a glimpse into the future of high-SNR observations of exoplanets.

 

In this work, we investigate several new considerations that are important for atmospheric retrievals of high-quality thermal emission spectra of sub-stellar objects. We propose a new parametric pressure-temperature profile for brown dwarfs, which is able to capture the steep temperature gradients in brown dwarf atmospheres while avoiding commonly-encountered numerical artefacts. We also demonstrate an approach to include model uncertainties in the retrieval, focusing on uncertainties introduced by finite spectral and vertical resolution. We validate our retrieval framework by applying it to a simulated data set and then apply it to the HST/WFC3 spectrum of the T-dwarf 2MASS J2339+1352, obtaining sub-solar abundances of H2O and CH4 in the object at ~0.1 dex precision. 

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Assessing spectra and thermal inversions due to TiO in hot Jupiter atmospheres

Thermal inversions have been detected in several hot Jupiter atmospheres, motivating new avenues to understand the many factors affecting their temperature structures. TiO has long been proposed to cause such thermal inversions, and has been detected in the optical and near-infrared. As such detections rely on the accuracy of the TiO cross-sections used, the recently reported TOTO TiO line list provides a new opportunity to investigate these dependences.

In this work, we find that the improvement in the TOTO line list compared to a previous line list results in observable differences in the spectra of hot Jupiters, particularly in the optical at high resolution. We also explore the interplay between temperature structure, irradiation, and composition with TiO as the primary source of optical opacity, using 1D self-consistent atmospheric models. Among other trends, we find that the propensity for thermal inversions due to TiO peaks at C/O 0.9, consistent with recent studies. Using these models, we further assess metrics to quantify thermal inversions due to TiO, compared to frequently used Spitzer photometry, over a range in C/O, irradiation, metallicity, gravity, and stellar type.

Inversion maps showing the strength of the inversion/non-inversion of equilibrium P-T profiles as a function of equilibrium temperature and C/O ratio. Top panel: true temperature contrast. Middle and bottom panels: temperature contrast measured by different photometric metrics. In this parameter space, the J-band metric is better able to indicate the presence of a thermal inversion.

​Piette, A. A. A., Madhusudhan, N., McKemmish, L. K., Gandhi, S., Masseron, T., Welbanks, L. (2020), MNRAS, 496, 3870

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Thermodynamic conditions at the boundary between the H2O layer and the H/He atmosphere (HHB) for the range of internal model solutions found. In some cases, this boundary occurs in the liquid water phase, meaning that liquid water could be present beneath the atmosphere. 

Madhusudhan, N., Nixon, M. C., Welbanks, L., Piette, A. A. A., Booth, R. A. (2020), ApJL, 891, L7

The Interior and Atmosphere of the Habitable-zone Exoplanet K2-18b

Mini-Neptunes orbiting M-dwarfs provide an excellent opportunity to study low-mass exoplanets in the habitable zone. For example, H2O was recently detected in the atmosphere of the habitable-zone planet K2-18b. The bulk density of K2-18b is between that of Earth and Neptune and may suggest the presence of a H/He atmosphere, though the extent of such an atmosphere, and the conditions which lie beneath, are currently unknown.

In this work, we use both the bulk properties of K2-18b and its transmission spectrum to place constraints on its atmospheric and interior properties. We find the atmosphere to be H/He-rich with a water mixing fraction of 0.02%-14.8%, consistent with previous studies. Using the atmospheric properties retrieved from the observed transmission spectrum, we self-consistently model the atmosphere of K2-18b considering a range of possible conditions, including internal temperature and composition. We couple these atmospheric models to a self-consistent internal structure model and find a range of solutions consistent with the measured mass and radius of the planet. We find that the water layer beneath the H/He atmosphere can be in the supercritical or liquid phase, and some solutions allow for liquid surface water at habitable temperatures and pressures. These results demonstrate that the potential for habitable conditions may not necessarily be limited to Earth-like rocky planets.

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Model night side spectrum (left) and temperature profile (right) for KELT-1 b. Dashed black lines correspond to blackbody spectra at 750, 1000 and 1500 K, respectively. Data are shown by black points and error bars, and are consistent with an internal temperature of ~1100 K.

von Essen, C., Mallonn, M., Cowan, N. B., Piette, A., Madhusudhan, N., Agol, E., Antoci, V., Poppenhaeger, K., Stassun, K. G., Khalafinejad, S., Tautvaišienė, G., manuscript in review

TESS unveils the optical phase curve of KELT-1b. Thermal emission and ellipsoidal variation from the brown dwarf companion, and activity from the star

KELT-1b is an inflated, highly-irradiated brown dwarf with a mass of ~27 Jupiter masses. Observations of this extreme object provide excellent opportunities to study the physical processes of highly-irradiated sub-stellar objects.

 

In this work, the detection and analysis of the phase curve of KELT-1b, observed by the Transiting Exoplanet Survey Satellite (TESS), are presented. The phase curve is analysed using a six-component model which includes ellipsoidal variations caused by the large mass of KELT-1b. The resulting brightness temperatures obtained for the day and night sides are 3201 +/- 147 K and 1484 +/- 110 K, respectively. Using the new TESS data and existing Spitzer measurements, we model both the day and night sides of KELT-1b using 1D self-consistent atmospheric models, and find models for the day and night sides which are consistent with each other. The night side brightness temperatures are consistent with an internal temperature of ~1100 K, which is higher than expected from brown dwarf evolution models and may be linked to the inflated radius of KELT-1b. This object may therefore provide exciting opportunities to understand the mechanisms behind inflated radii of highly-irradiated giant planets and sub-stellar objects.