Transit Method
Detecting a planet when it passes in front of its star.
When a planet crosses its star, it blocks a tiny fraction of the star's light. A larger planet blocks more, and the dip in the light curve deepens.
A star's measured brightness decreases slightly when a planet crosses between the star and the observer. The dip is typically a fraction of a percent, and it repeats on a fixed schedule.
- 01A body is orbiting the star, in a plane close to our line of sight.
- 02The interval between repeats gives the orbital period.
- 03The depth of the dip gives the planet's radius relative to the star's.
- Observation
Repeated, regularly spaced dips in a star's light curve.
- Measurement
Depth of the dip, its duration, and the time between repeats.
- Physical model
An opaque disc crossing a bright disc blocks light in proportion to the ratio of their areas.
- Inference
Planet radius relative to the star, plus orbital period and orbital geometry.
- +Planet radius, relative to the host star's radius
- +Orbital period
- +Orbital geometry along the line of sight
- +Atmospheric clues, when combined with spectroscopy during transit
What a method cannot settle matters as much as what it can. Each note below is tagged by how firm the statement is.
- OBSERVATION
A transit only occurs if the orbit happens to be aligned with our line of sight. Most planetary systems are not, so non-detection is not evidence of absence.
- INFERENCE
Transit depth alone gives no mass, and therefore no density. Without mass, a measured radius cannot distinguish a rock from a low-density gas envelope.
- HYPOTHESIS
Starspots, eclipsing background binaries and instrument artefacts can mimic a dip. Candidates are treated as unconfirmed until an independent method agrees.
TRAPPIST-1 system
Repeated transits of several small planets across a nearby cool dwarf star allowed their radii and orbital periods to be measured before any of them was directly imaged.
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