What is an exoplanet?
An exoplanet is a planet orbiting a star other than the Sun. Thousands of candidates and confirmed planets reveal the diversity of planetary systems; some differ markedly from objects in our Solar System.
How are they found?
The transit method looks for a small, repeated dip in starlight as a planet passes in front of its star. It gives information about size, but requires a favorable orbital alignment. The radial-velocity method measures periodic shifts in the star’s spectrum caused by the planet’s pull and can yield a lower bound on mass. Direct imaging and gravitational microlensing are other methods. Signals must be checked against alternative explanations such as nearby stars, starspots and measurement error.
Habitability and uncertainty
A star’s habitable zone is a range of distances that could permit surface liquid water under suitable atmospheric conditions; being there does not prove life exists. Atmosphere, magnetic environment, stellar activity and geological history also matter. Scientists study some atmospheric constituents from spectral imprints left as starlight passes through the atmosphere. Detecting one molecule alone does not establish a biological origin; chemical and geological explanations must be considered. Detection methods favor large or close-in planets, so known examples are not an unbiased census.