Sean Carroll's Mindscape: Science, Society, Philosophy, Culture, Arts, and Ideas · Sean Carroll

Natalie Batalha on What We Know and Will Learn About Exoplanets

August 25, 2025·1 hr 12 min·5 clips

Natalie Batalha was in the room when the first exoplanet around a sun-like star was announced.

Natalie Batalha describes attending a conference as a third-year graduate student in the early-to-mid 1990s where Michel Mayor announced the first exoplanet detection around a normal sun-like star — making her a firsthand witness to the beginning of the field. She joined the Kepler team around 1999-2000 after emailing Bill Borucki, the mission's lead, with a skepticism about the transit photometry method: she studied star spot distributions and knew that star spots modulate stellar brightness on timescales that could mimic planetary transit signals. Borucki told her this was exactly the reason prior Kepler proposals had been rejected and invited her to interview for a postdoctoral position. The Kepler team spent years developing their photometric pipeline on a shoestring ground-based telescope at Lick Observatory in an abandoned, leaky dome before the satellite launched in 2009. Carroll explains the transit photometry method: a planet passing in front of its star blocks a small fraction of the light, producing a periodic dip in the brightness curve that can be detected precisely enough to infer the planet's size and orbital period. The false-positive problem is addressed: eclipsing binary star systems can mimic transit signals, as can large starspots; the team used digital filtering to separate variability timescales and population synthesis to estimate the fraction of stars with confounding spot activity (approximately 30%), ultimately observing 150,000 stars rather than the originally planned 100,000. Kepler operated for four years before losing a second reaction wheel in May 2013 — Batalha's birthday — which ended the primary mission. Engineers devised a workaround called K2 that exploited radiation pressure from the sun to stabilize the telescope's third rotational axis, allowing continued observation of fields along the ecliptic for another four years. The primary Kepler field yielded approximately 5,000 planet candidates, of which about 2,800 have been confirmed. The episode continues into how JWST is now being used to characterize exoplanet atmospheres through transmission spectroscopy, what signatures might eventually indicate biological activity, and why Batalha believes essentially every star in the galaxy likely hosts at least one planet.

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