Established in 2017, the 51 Pegasi b Fellowship is named for the first exoplanet discovered orbiting a Sun-like star and includes now over 60 Fellows who conduct theoretical, observational, and experimental research in planetary astronomy. The fellowship award provides up to $450,000 to support independent research over three years, including salary and discretionary fund.
The fellowship also provides flexible funding at a critical early-career juncture to enable scientific growth and leadership in the field, mentorship by an established faculty member at the host institution, and an option to apply for a fourth year of funding to support the fellow in a future astronomy faculty, or permanent staff position at a U.S. university or nonprofit institution.
Mukherjee models planetary atmosphere-interior interactions to probe persistent mysteries about the galaxy’s most abundant planet type: sub-Neptunes. As the name suggests, this type of planet is between the size of Neptune and Earth. Yet they are missing from our own solar system, and Mukherjee wants to know why.
“Our solar system has just eight planets, each providing a single example of how planets form," Mukherjee said. "With more than 5,000 confirmed extrasolar planets, we can test planet formation in a statistically robust way.”
Holding space for exoplanet research
Until the James Webb Space Telescope (JWST) began releasing precise data across a wide array of spectral wavelengths, only large planets similar to Jupiter or Saturn—which are much rarer than sub-Neptunes—could be characterized in detail with confidence. When the first JWST observations became available in 2022, Mukherjee was part of a community of exoplanet scientists from around the world who convened at UC Santa Cruz to analyze this information for a large Saturn-like exoplanet.Mukherjee said he was astonished as findings from this effort reached his father in India through a local Bengali newspaper. Buoyed by the power and reach of scientific discovery, Mukherjee then fixed his curiosity on sub-Neptunes, which pose a host of intriguing questions—including why and how they form, what they are composed of, and whether they can sustain life.
“Because sub-Neptunes have an intermediate size, their interiors can influence their atmospheres by a huge amount. For example, magma oceans on the surface can spew out gases and change what we’re observing," Mukherjee explained. "The theoretical models that we are using to interpret these observations need to account for various interconnected effects, like clouds and the influence of sub-Neptune interiors on their atmospheres, to fully capture the complexity of these worlds.”
A believer in open source and diversity
As JWST provides increasingly sensitive observations of sub-Neptunes, it reveals discrepancies with most models currently describing these planets. As a planetary theorist and devoted open-source modeler, Mukherjee has demonstrated his eagerness to provide a more inclusive framework on which others can build. His development of open-source software, as well as mentorship of students from underrepresented backgrounds, reflects his commitment to making scientific community more accessible.During his fellowship at Arizona State University (ASU), Mukherjee will develop and test a multi-part model that, unlike its predecessors, accounts for the chemical composition and cloud cover of sub-Neptunes, as well as interactions between their atmospheres and interiors, where gases spewed from possible magma oceans on these planets can dramatically alter their observable chemistry.
The Heising-Simons Foundation states that Mukherjee’s contributions are timely, as hundreds of hours of JWST data focus on sub-Neptunes and a new generation of Extremely Large Telescopes come online. With his model, Mukherjee expects to reduce the uncertainties that cloud today’s understanding of sub-Neptunes, and reveal the compositions and origin stories of these abundant, compelling planets.
Mukherjee will receive his Ph.D. in astronomy and astrophysics from UC Santa Cruz this spring, and is expected to begin his fellowship at ASU in September 2025.
