space
did you know a tiny dip in starlight can reveal a planet?
a distant world's silhouette can give away its size.
10% radius → 1% dip
a silhouette turns size into brightness
a simplified full transit across a uniformly bright star.
- measure starlight
- spot repeated dips
- estimate relative size
the idea
when a planet passes in front of its star from our viewpoint, it blocks a small fraction of the light. repeated dips can reveal the planet's orbital period and help estimate its size relative to the star.
how it works
a larger silhouette blocks more of the stellar disc. regular repetition suggests an orbiting object and gives a candidate period. the duration and detailed shape provide further clues.
go deeper
how do you spot something you cannot see directly?
when a planet passes in front of its star from our viewpoint, it blocks a small fraction of the light. repeated dips can reveal the planet's orbital period and help estimate its size relative to the star.
measure the combined brightness very carefully over time. the planet's silhouette leaves a temporary dip in the light curve, even when the telescope cannot resolve the planet as a separate point.
use the depth and the rhythm
a larger silhouette blocks more of the stellar disc. regular repetition suggests an orbiting object and gives a candidate period. the duration and detailed shape provide further clues.
not every dip is a planet. stellar variability, eclipsing stars, and instrumental effects can imitate parts of the signal, so astronomers check other evidence before confirming the interpretation.
how does the dip reveal size?
for an opaque planet fully in front of a uniformly bright star, the blocked fraction is the ratio of their disc areas. a planet with one-tenth the star's radius blocks about one-hundredth of its light.
brightness dip ≈ (planet radius / star radius)²; (0.1)² = 1%
why are there planets we miss?
- the orbit must align so that the planet crosses the star from our viewpoint. most orientations do not produce a transit.
- real stars are not uniformly bright across their discs. modelling effects such as limb darkening improves the size estimate.
sources & further reading
concepts: transits · area ratios · exoplanets
2 minute read