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Siddharth Gandhi

I am a research fellow in the Astronomy and Astrophysics group at the University of Warwick working with Dr Matteo Brogi. My work focuses on atmospheric modelling of exoplanets and comparisons with space and ground based observations to constrain their properties, with the eventual goal of studying rocky Earth-like planets in the habitable zone which have the potential for biosignatures.

I joined Dr Brogi's group at the University of Warwick in July 2019 after completing my PhD at the University of Cambridge, advised by Dr Nikku Madhusudhan.

You can find information about me here, and on my personal website. Feel free to get in touch if you would like to know more about my research.

My Research

My research is focused on answering some of the fundamental questions in exoplanetary science. These include:

  • What is the composition and diversity of exoplanet atmospheres?

  • What are the physical and dynamical processes that can occur in the atmospheres of exoplanets?

  • How did these planets form and how did they migrate to their current distances from their host star?

  • What do these exoplanets tell us about the uniqueness of our own solar system?

Some of the recent contributions I have made:

  • Development of a self-consistent equilibrium model custom built for exoplanetary atmospheres
  • A retrieval architecture which allows constraints on disequilibrium processes
  • First combined retrieval using observations with the Hubble Space Telescope, Spitzer telescope and ground based high resolution observations from the VLT in a fully Bayesian framework
  • First detections of numerous species in the atmospheres of hot Jupiters from ground based high resolution and space based observations

Below I discuss some of the research areas which I am actively involved in.

Modelling Cloudy planets at High Spectral Resolution

Recently, we have begun obtaining observations of warm Neptunes and super Earths, many of which show thick cloud cover in their atmosphere. These clouds obscure the atmospheric signal and make detecting species more difficult. However, high resolution (R > 20,000) ground based observations have the ability to probe a much wider range of altitudes, and thus potentially observe above the effects of clouds. My work has been to explore the detectability of species such as H2O, CH4, NH3 and CO by modelling two cloudy warm Neptunes, GJ 436 b and GJ 3470 b. The figure below shows the effect of clouds on observations. These features are present above the clouds and our calculations show H2O and the other species are potentially detectable even with quite thick clouds with just a few nights of observing time on a number of telescopes around the world. We also find that the Earth's own atmosphere can potentially obscure detections of some species also present on the Earth, such as H2O. This work has recently been published in the Monthly Notices of the Royal Astronomical Society and can be found here.

Effect of cloud cover on spectral extent

Molecular Cross Sections

One of the most important considerations when observing exoplanet atmospheres is the opacity of the atmosphere. Each molecule present in the atmosphere has its own opacity which varies according to its abundance and with wavelength. To accurately detect and constrain molecules such as H2O in exoplanet atmospheres, we require detailed knowledge of all of the quantum mechanical transitions that can occur. My recent work has been to compile the latest generation of opacities for high resolution spectroscopy, where accuracy is critical for detection. We use these new cross sections to explore previous detections of H2O, CO, HCN and CH4 in hot Jupiters and model the spectra of a super Earth, warm Neptune and hot Jupiter. This work has recently been published in the Monthly Notices of the Royal Astronomical Society and can be accessed here with the opacities publicly available on the Open Science Framework.

Molecular cross sections for the various species which are prominent in hydrogen/helium Ruch atmospheres

Retrievals of Low and High Resolution Observations

To find out the properties of the exoplanet atmospheres, such as the H2O abundance or the temperature profile, we use retrievals to compare our models against the observations. Retrievals often explore many millions of spectral models to constrain the atmospheric properties in a Bayesian framework. These have been used extensively to analyse observations from the Hubble Space Telescope as well as other low resolution observations. I have developed a new retrieval framework, HyDRA, from the ground up, to efficiently retrieve such low resolution observations. Furthermore, I have also combined the low resolution observations with ground based high resolution data to perform hybrid retrievals. Such retrievals incorporating high resolution data have only very recently been proposed owing to their complexity and computational expense, requiring state of the art computational and statistical tools for efficiency. One of the most promising avenues I have been exploring has been the complementarity of low and high resolution observations, which allows for more stringent constraints on the abundances of species such as H2O and CO with the hybrid approach (see below). My recent paper on these hybrid retrievals has been published in The Astronomical Journal and can be found here.

Hybrid constraints for water and CO

Radiative-Convective and Chemical Equilibrium Models

Equilibrium Models provide a key tool to test our understanding of exoplanetary atmospheres. These model the atmosphere assuming all of the input physics, such as the chemical reactions and the radiative processes. Equilibrium models provide the benchmarks to test our understanding of atmospheric processes and can be compared against observations to constrain out of equilibrium processes in the atmosphere. I have developed a new equilibrium code, GENESIS, custom built for exoplanetary atmospheres. GENESIS uses the latest mathematical and numerical tools to model a wide range of exoplanets, encompassing irradiated as well as non-irradiated atmospheres. This has been used to model a number of hot Jupiters (see below), as well as warm Neptunes and super Earths throughout my research. My paper discussing GENESIS and its applications to a number of phenomena in hot Jupiter atmospheres can be found here.

Simulation of 5 hot Jupiters with GENESIS

About Me

I completed my undergraduate in 2015 at St Catharine's College at the University of Cambridge studying Natural Sciences. I was always fascinated by theoretical physics, as well as mathematics, and how it applies to the world (or worlds!) around us. I was always fascinated by all of my subjects, from quantum mechanics to astrophysical fluid dynamics and relativity. It was especially nice to see similar principles and ideas in very different areas of physics. I then started my PhD in 2015 in exoplanetary atmospheres supervised by Dr Nikku Madhusudhan, which I completed in 2019. Studying exoplanets was ideal for me given its fundamental interdisciplinary nature. I have also been a fan of science fiction, especially those which explore the cosmos and other worlds. Perhaps it is not surprising I ended up studying exoplanets!

As well as research, I am also a big fan of playing and watching cricket. Many a summer afternoon has been spent batting for my local team, though I'm not really good enough to be out there for too long! I also enjoy rowing, and coxed for most of my time during my time in Cambridge.

Selected Publications

Seeing above the Clouds with High Resolution Spectroscopy

Molecular Cross Sections for High Resolution Spectroscopy of Hot Jupiters, Warm Neptunes and Super Earths

HyDRA-H: Simultaneous Hybrid Retrieval of Exoplanetary Emission Spectra

Retrieval of Exoplanet Emission Spectra with HyDRA

GENESIS: New Self-Consistent Models of Exoplanetary Spectra

My full list of publications can be found here. A full CV is also available on request.

Write to:

Siddharth Gandhi,
Department of Physics,
University of Warwick,
Coventry CV4 7AL

Contact details: