What is an exoplanet, and how are they detected?
A planet orbiting a star other than the Sun. Thousands are now confirmed, and the striking thing is that almost none were detected by seeing them — planets are overwhelmingly outshone by their stars, so nearly all detection is indirect.
The transit method, which has found the most by a wide margin. If a planet's orbit passes between us and its star, the star dims very slightly — typically by a fraction of a percent — once per orbit. Measuring that repeating dip reveals the orbital period and, from the depth, the planet's size.
Its limitation: it only works for the small proportion of systems aligned edge-on to us, so it detects a biased sample.
The radial velocity (Doppler) method. A planet does not orbit a stationary star; both orbit their common centre of mass, so the star wobbles. That motion shifts the star's spectrum slightly blue then red, and measuring it reveals the orbital period and a minimum mass.
Why the two together matter: transit gives size, radial velocity gives mass, and mass with size gives density — which tells you whether it is rocky, icy or gaseous. Neither alone can.
Direct imaging, blocking the star's light to photograph the planet. Only works for large, hot, young planets far from their stars — but it allows spectra of the planet itself.
Gravitational microlensing, where a foreground star's gravity magnifies a background star and a planet adds a brief extra spike. It finds distant planets and the events never repeat, so results cannot be confirmed.
Astrometry, measuring the star's position shift directly.
What has been learned: planets are common, most stars have them; the variety far exceeds expectations, including hot Jupiters orbiting in days and planets around dead stars; and the most common size in the galaxy is between Earth and Neptune — a type absent from our own system.
Atmospheric spectroscopy during transit now detects composition, which is how biosignature searches proceed.