nontransit

Nontransit refers to the absence of a transit event in observational astronomy. A transit occurs when a celestial body, most commonly a planet, passes directly between an observer and a background light source, typically causing a temporary dip in the observed brightness of a star. Nontransit describes the situation in which such an alignment does not occur during a given monitoring window. This can result from the orbital plane being inclined away from the observer’s line of sight, from gaps in timing, or from the long orbital period of a distant planet.

In exoplanet surveys, transits are used to infer the presence of planets by detecting periodic dips in starlight. However, most planets do not transit from Earth, making nontransit the common outcome of random orientation. A nontransit light curve can still provide information when combined with other methods, such as radial velocity measurements, astrometry, or analyses of transit timing variations from potential nearby transiting planets.

The likelihood that a planet will transit depends on the system’s geometry; transit probability is higher for planets close to their star and decreases with increasing orbital distance. The concept emphasizes that a lack of observed transit does not rule out the existence of planets around a star.

Terminology notes: nontransit is primarily used in the context of transit astronomy and is distinct from non-transiting planets, which are planets inferred without observed transits.