What does a dip in starlight reveal about a transiting planet?

A star that looks like a single point can briefly dim when a planet crosses in front of it. We may never see the planet as a separate dot, yet its passage can leave a measurable signature in the combined light. This event is a transit, and a graph of the received brightness over time is a light curve.

The alignment must be right. A planet can orbit its star without ever crossing the stellar disk from our viewpoint. When a transit does occur, the planet first overlaps the edge, then may move fully in front of the star, and finally leaves the other edge. Brightness falls gradually during entry and returns during exit because the overlapping area changes continuously.

For a uniformly bright stellar disk and an opaque circular planet fully inside its outline, the deepest blocked fraction is the ratio of their projected areas. Each area is pi times radius squared, so dividing gives depth = (planet radius / star radius)². A planet with one tenth of the star's radius blocks one hundredth of its area: a 1% dip. If the unobscured brightness is called 1.00, that example reaches 0.99 during the full transit.

This is a relative measurement. The depth gives a radius ratio, not an absolute planetary radius, unless the star's radius is also estimated. It does not directly give planetary mass. Repeated similar dips can establish an orbital period when they come from the same orbiting body. Transit duration and shape carry further information about the crossing, although interpreting them requires a model of the star and orbit.

Real stars complicate the clean area comparison. Their disks are often dimmer near the edge, an effect called limb darkening. A grazing planet may never sit fully inside the stellar outline. Starspots can change the received light, and an unresolved neighboring star can add extra light that makes a transit look shallower. A smaller observed dip can therefore reflect a larger total light supply rather than a smaller planet.

A repeating dip also needs checking before it becomes a planet discovery. Two stars passing in front of each other can produce an eclipse signal resembling a planetary transit. Astronomers compare the pattern across repeated events and use additional observations, including the star's motion, to test such alternatives. Different measurements constrain different parts of the explanation.

The power of a transit is that geometry turns a small loss of light into a clue about an unseen world. The limitation is equally useful: every inference depends on what else contributes to that light. A faint dip can reveal a planet's relative size and orbital rhythm, but careful comparison is what turns a promising shadow into evidence for a planet.