He was affiliated with the Faculty of Earth and Life Sciences at the Vrije Universiteit Amsterdam from 2003 to 2013 as a professor of event stratigraphy, studying rapid changes in the geological record related to mass extinctions.

Smit's main area of research is on the Cretaceous–Paleogene extinction event, which ended the Cretaceous period and killed all non-avian dinosaurs.

He was an early researcher into the now-accepted belief that an asteroid impact was responsible for their extinction; his dissertation, titled "A Catastrophic Event at the Cretaceous–Tertiary Boundary", was related to Luis and Walter Alvarez's recently published theory on the extinction event.

She is one of only a handful of UK-based astronomers in the core survey teams of the James Webb Space Telescope.

[4][5] Smit's son-in-law is Robert Crain, a professor of theoretical astrophysics at the same university.