The ______ method of planet-hunting relies on a periodic dimming of the star being orbited.

• A. Doppler
• B. Direct imaging
• C. Microlensing
• D. Transit

Answer: (D)

The transit method detects periodic dimming of a star’s light when a planet passes in front of it from our viewpoint, blocking a small portion of the light. This regular, repeating drop in brightness tells us a planet is orbiting the star and provides two key pieces of information: the planet’s size from how deep the dip is, and its orbital period from how often the dips occur. The method works best when the planet’s orbit is edge-on enough for the planet to cross the star as seen from Earth, and it requires continuous monitoring to catch multiple transits and confirm the signal.

Other planet-hunting methods work differently: the Doppler (or radial velocity) method looks for shifts in the star’s spectral lines caused by the star wobbling due to the planet’s gravity; direct imaging tries to capture actual light from the planet; microlensing relies on gravitational lensing when a foreground star-planet system passes in front of a background star, briefly magnifying its light.