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Dimitar Sasselov studies the physics of exoplanets and their atmospheres. With Sara Seager, he modeled how a transiting gas giant's atmosphere would imprint absorption features on the host star's radiation spectrum.

During an exoplanet transit, a small fraction of the stellar light passes through gas above the planetary limb. Strong atomic absorption can therefore make the apparent blocking area depend on wavelength. Their calculation identified promising sodium, potassium and helium features and emphasized the role of cloud height in interpreting the signal.^[ref]

Gabriela Mallén-Ornelas coauthored, with Sara Seager, an analytic study of what an exoplanet transit reveals about a planet and its host star. A light curve supplies more than a depth: durations and the orbital period also constrain the geometry.

Their result shows that the mean stellar mass density can be inferred from the light curve under stated assumptions, including a circular orbit and negligible limb darkening in the observing band. These conditions matter: a claim of a unique solution under a specified model does not make the same inference model-independent.^[ref]

By Sara Seager and Gabriela Mallén-Ornelas. Section3.1,equations1–4 specify depth, contact durations and the circular-orbit scale. The2002 manuscript was published in The Astrophysical Journal585,1038–1055(2003).

Sara Seager develops physical methods for interpreting exoplanets. With Dimitar Sasselov, she calculated atmospheric transmission during an exoplanet transit: absorption in the planetary limb makes the measured transit depth depend on wavelength. Their work identified strong atomic features and cloud height as observationally useful diagnostics.^[ref]

Her models of solid planets also connect measured mass and radius to bulk composition through an equation of state. They showed how such measurements can distinguish broad iron-rich, silicate-rich and water-rich families while leaving finer compositional ambiguities. A measured mean mass density therefore constrains a family of interior models rather than uniquely revealing an interior.^[ref]

By Sara Seager and Dimitar Sasselov. The2000 study calculates transmission absorption through an atmospheric annulus and discusses how weak contrast and clouds affect sensitivity. The Astrophysical Journal537,916–921.