Two different measurements carry the evidence. A transit is a periodic drop in a star's brightness. A radial-velocity signal is a periodic shift in the wavelengths of a star's spectral lines as it moves slightly toward and away from us.
A planet crossing in front of its star blocks a fraction of its light proportional to the area it covers, so the depth of the dip scales with the planet's size relative to the star.
A planet also pulls on its star. Both bodies orbit a shared centre of mass, so the star traces a small orbit of its own. That motion compresses and stretches its light in a repeating rhythm.
- TRANSIT
- Orbital period, planet radius relative to the star
- RADIAL VELOCITY
- Orbital period, minimum mass, orbital eccentricity
- COMBINED
- Bulk density — the first clue to rocky versus gaseous
Confidence comes from repetition and independence: the same period recovered by different instruments, and ideally by different methods.
A repeating, consistent signal with a stable period, confirmed by more than one instrument or method, supports a planetary companion and constrains its size or mass.
Stars are not quiet. Spots, pulsations and instrument systematics can imitate a periodic signal, which is why some announced detections are later withdrawn. A detection also says little about surface conditions or habitability on its own.
- NASA Science //
- NASA Science //