Sheila A. Sagear

Welcome to my website! I am a Ph.D. Candidate in Astronomy at the University of Florida. With my advisor Prof. Sarah Ballard, I study exoplanets around the smallest and coolest stars.
I recieved my Bachelor's degree in Astronomy & Physics from Boston University in 2020. In the past, I have been a Flatiron Pre-Doctoral Fellow in the Center for Computational Astrophysics at the Flatiron Institute, an intern at the Kepler/K2 Guest Observer office at NASA Ames Research Center and the FastML group at CERN.

Click here to download my CV.

Research

The Orbital Eccentricities of Planets in the Kinematic Thin and Thick Galactic Disks

With Gaia DR3 astrometry along with stellar chemistry from the Kepler-APOGEE sample, we calibrate a classification model to categorize Kepler stars with the thin or thick Galatcic disks using only Galactic kinematic criteria. We find evidence that planets associated with the kinematically thick Galactic disk are, on average, more eccentric than planets in the kinematically thin disk. This work has been submitted to AAS journals.

The Orbital Eccentricity-Radius Relation for Planets Orbiting M Dwarfs

The orbital eccentricity-radius relation for small planets gives us clues as to how planets form and evolve, especially in terms of late-stage dynamical evolution. We investigate the eccentricity—radius relation in a sample of planets orbiting M dwarf stars. With data from the TESS and Kepler missions, we constrain orbital eccentricities for a sample of ~200 planets. We present evidence for a positive eccentricity-radius relationship with elevated eccentricities for planets larger than 3.5 R_earth, similar to the trend for planets orbiting Sun-like stars. This work has been submitted to AJ and is in review as of July 2025.

zoomies: A Tool to Infer Stellar Age from Vertical Action in Gaia Data.

Stellar ages are fundamental to understanding a range of astronomical processes, including Galactic dynamics, stellar evolution, and planetary system formation. However, extracting age information for main-sequence stars is complex, especially for the lowest-mass stars. We leverage Gaia DR3 astrometry to construct a stellar age prediction model based only on stellar dynamical properties, namely the vertical action. We calibrate the hierarchical stellar age–vertical action relation on asteroseismic ages for red-giant-branch stars and isochrone ages for main-sequence turn-off stars. We describe a framework called "zoomies" based on this calibration, by which we can infer ages for any star given its vertical action. The tool is open-source and intended for community use.

The Orbital Eccentricities of Planetary Systems Orbiting M dwarfs

Leveraging the photoeccentric effect , we combine Kepler transit light curves with stellar density information from spectroscopy and parallaxes from Gaia to constrain the orbital eccentricities for over 150 planets around nearby M dwarfs. Within a Bayesian hierarchical framework, we draw out the underlyling eccentricity distribution for planets around M dwarfs and find two distinct populations: single-transit, higher eccentricity planets and multi-transit, low-eccentricity planets. We use planet occurrence information to constrain the volume-limited occurrence rate of eccentric M dwarf planets. Read the paper in PNAS

Upper Limits on Planet Occurrence around Ultracool Dwarfs

As an undergraduate at Boston University, I worked with my advisors Prof. Philip Muirhead and Dr. Julie Skinner to constrain the upper limit of planet occurrence around late-M and early-L dwarfs (or "ultracool dwarfs") in our local neighborhood. We conducted a planet search around 827 ultracool dwarfs observed by NASA's K2 mission, and found none. Using this null result, we constrained the upper limit of planet occurrence around ultracool dwarfs in the local neighborhood, accounting for our transit detection efficiency and transit probability.

Papers

Selected Presentations

Software

I published an open-source Python package called zoomies, which is an open-source tool to constrain stellar ages using only kinematic information (vertical action from Gaia), a galactic potential model, and an external stellar calibration sample. The package includes tools to calibrate a stellar age--vertical action relation and apply it to your desired stellar sample. Since the age relation comes from purely kinematic information (galactic orbits), the relation is nearly completely mass-independent, meaning you can use this tool to constrain ages for any stars from supergiants to M dwarfs!

I also published an open-source Python package called photoeccentric, which includes a suite of tools to process Kepler transit lightcurves, calculate stellar densities, and perform transit fitting to constrain eccentricities using only photometric data + stellar density priors.

I contribute to lightkurve, a user-friendly, open-source Python package for working with Kepler, K2, and TESS data.

Contact

Trulli
My lovely group at the OWL Exoplanet Workshop in Santa Cruz, CA.
L to R: Natalia Guerrero, Chris Lam, and Quadry Chance.