About me

To describe my love for astronomy,
I will resort to a few words by Vera Rubin:
"I became an astronomer because I could not imagine
living on Earth and not trying to understand how the Universe works."

I am starting as a postdoctoral researcher at UC Santa Cruz in fall 2024. Previously, I was a CITA National and MSI fellow at McGill University. I have had the privilege of pursuing my long held childhood interest in astronomy and planetary science since 2016. I grew up in Delhi and finished high school in Singapore on a SIA Youth Scholarship. I then moved to the U.K. to do my undergraduate study at the University of Cambridge, where I finished my B.A. in physics and M.Sci. in astrophysics. I obtained my Ph.D. in planetary science at Caltech.

Research Interests

I have a broad interest in planetary atmospheres, planet formation, protoplanetary discs, and their inter-connectivity. More specifically, I am interested in:

  • Processes that control the observed compositions of planetary atmospheres: planet formation, atmospheric chemistry and dynamics
  • How planets are assembled: the role played by dust dynamics and gas accretion in protoplanetary disks
  • Imprints of protoplanetary disk evolution and formation processes on planet demographics
I use observational, analytical, and numerical methods in my attempts to study these areas. For more details, please take a look at the projects I am involved in.

Current and Past Projects

C.V.

Employment

Postdoctoral scholar, UC Santa Cruz, 2024-
Mentor: Jonathan Fortney
CITA National Fellow & MSI Fellow, McGill University, 2022-2024

Education

Ph.D., California Institute of Technology, 2017-22
Advisor: Heather Knutson
M.Sci., Institute of Astronomy, University of Cambridge, 2016-17
B.A., Natural Sciences (Physics), University of Cambridge, 2013-16

Publications: Interactive list, ADS, Google Scholar

Full C.V.

Current and Past Projects

Spectroscopic Characterization of Exoplanetary Atmospheres

I reduce and analyze data from the Hubble and Spitzer Space Telescopes to characterize planetary atmospheres. Currently, I am studying the transmission spectrum of the ultra-hot Jupiter KELT-20 b. Ultra-hot Jupiters are great targets for characterization and for relating planet formation to composition because all their consituents are in gaseous form and visible to us (Chachan et al. 2023). Previously, I studied the quintessential warm-Neptune HAT-P-11 b in detail and found that its atmosphere is atypically poor in heavy elements (Chachan et al. 2019). I also used Hubble data to study the extremely low-density planet Kepler 79 d and argue that high altitude hazes can explain the spectra and mass loss histories of such planets (Chachan et al. 2020). To interpret atmospheric spectra, I co-developed the open source forward and retrieval modelling code PLATON with Michael Zhang (Github, Zhang et al. 2019, Zhang et al. 2020).

Planet formation, demographics, and orbital architectures

I am particularly intrigued by how dust dynamics in protoplanetary disks affects planet formation and orbital architectures of planets. Recently (Chachan & Lee 2023), I showed that the efficiency of planetary core formation by pebbles (mm-cm sized dust) varies with host star's mass, which can result in a wide range of observed properties of planets (e.g., giant planets are rarer and super-Earths are more common around lower mass stars). In Chachan et al. 2022, I used the Kepler-167 system to put forward an explanation for why cold gas giants (like Jupiter) are often accompanied by inner super-Earths. In Chachan, Lee, & Knutson 2021, I showed that changes in material properties of grains across the water ice line creates strong radial gradients in the dust-to-gas ratio of the disk, which could explain why gas giant planets are most often found at intermediate distances (1-10 au) from their host stars. I also studied the dynamics of dust in binary star systems and its consequences for formation of circumbinary planets in Chachan et al. 2019.

Atmosphere-Interior Interaction

I use semi-analytical and numerical tools to model the atmosphere-interior coupling and evolution of super-Earths. In Chachan & Stevenson 2018, I showed that an enormous amount of hydrogen can be stored in the interiors of super-Earths due to hydrogen's solubility in magma oceans.

Other Projects

I have dabbled in various research projects over the years. These includes studies on Saturn's differential flow and gravitational moments (Chachan & Stevenson 2019), Venus' circumsolar ring (Github), phase transformations of oxides of Iron, and moleculer spintronics.