Anjali Piette

I'm a Carnegie Postdoctoral Fellow at the
Earth & Planets Laboratory, Carnegie Institution for Science.

I study the atmospheres of a wide range of exoplanets, from temperate low-mass planets to highly-irradiated gas giants and isolated brown dwarfs. I'm especially interested in what the emission spectra of these atmospheres can tell us about their temperature profiles, energy transport, potential surface/interior properties and atmospheric chemistry/clouds. In my work, I use self-consistent atmospheric models to understand how various physical processes affect the temperature profile and emission spectrum of the atmosphere. I also use atmospheric retrievals to understand what we can learn about these planets with observations from facilities such as the upcoming James Webb Space Telescope.


I am interested in the atmospheres of a wide range of exoplanets, 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. Please see below for selected recent results. For more publications, please see Publications.


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. Piette, A. A. A. & Madhusudhan, N. (2020), ApJ, 904, 154


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


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


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