A planetary transit is an astronomical event in which a planet passes directly between an observer and the star it orbits, causing the planet to block a small fraction of the stars light. From Earth, the best known transits involve Mercury or Venus crossing the visible disc of the Sun. Transits of planets around other stars can also be observed when their orbital orientation causes the planet to pass across the stellar disc from our line of sight.
The event is essentially a miniature eclipse, but the terminology is normally applied when the foreground body appears much smaller than the background star. During a transit, the planets silhouette moves across the star and produces a small, measurable reduction in brightness. This reduction is called a transit signal or transit depth and depends strongly on the relative sizes of the planet and star.
A planetary transit requires precise orbital alignment. The planet must be located between the observer and its star, and its orbital plane must be oriented so that the planet crosses the apparent stellar disc. Most planetary systems do not produce observable transits from Earth because their orbital planes are tilted in directions that prevent the planet from crossing the star from our viewpoint.
The probability of transit depends on the sizes of the star and planetary orbit and is generally greater for planets orbiting close to their stars. This geometric bias is important because it means transit surveys do not detect all planets equally. They are particularly sensitive to planets with short orbital periods and systems whose orbital planes happen to be oriented close to edge on from Earth.
During a transit, the observed brightness of the star falls by a small amount because part of its emitting surface is covered by the planet. To first approximation, the fractional loss of light is related to the square of the ratio between the planetary radius and stellar radius. A planet with a radius one tenth that of its star therefore blocks approximately one hundredth of the stellar disc area, ignoring complications such as stellar limb darkening.
In real observations, the brightness curve is more complex because stars are not uniformly bright across their discs. The centre of a star generally appears brighter than its edge, producing limb darkening that affects the precise shape of the transit light curve. Astronomers model these effects to determine planetary and stellar parameters accurately.
The transit light curve contains more information than simply confirming that a planet exists. Its depth can provide an estimate of the ratio between planetary and stellar radii. The duration and shape of the transit provide information about orbital speed, orbital geometry, the relative sizes of the bodies and the position of the transit path across the stellar disc. Repeated transits reveal the orbital period, because the interval between similar brightness dips corresponds to the planets orbital period. Combining transit measurements with other observations can allow astronomers to estimate planetary density, atmospheric properties and orbital characteristics. Transit timing variations can also reveal gravitational interactions between planets that may otherwise be difficult to observe directly.
Planetary transits have become one of the most important methods for discovering exoplanets. Space telescopes and ground based surveys monitor large numbers of stars and search for repeated, periodic reductions in brightness. The Kepler and K2 missions demonstrated the enormous power of this approach, while the Transiting Exoplanet Survey Satellite has surveyed much of the sky for transiting planets around relatively nearby bright stars.
The method can detect planets much smaller than stars because modern photometric instruments can measure extremely small changes in stellar brightness. Repeated signals, statistical validation and independent observations are used to distinguish genuine planets from false positives such as eclipsing binary stars, instrumental effects and stellar variability.
Transits can also provide a method for studying planetary atmospheres. When a planet passes in front of its star, a small portion of the starlight can pass through the planets atmosphere before reaching the observer. Atmospheric gases absorb particular wavelengths of light, so measurements at different wavelengths can reveal spectral signatures associated with atmospheric constituents. This technique is called transmission spectroscopy. The signal is extremely small because the atmosphere is a thin layer compared with the planetary radius, and stellar activity can complicate interpretation. Nevertheless, transit observations have allowed astronomers to investigate atmospheres and clouds on a range of exoplanets and have become an important component of modern planetary science.
The word transit has a broader astronomical meaning because it refers generally to one celestial object passing across the apparent disc of another from an observers perspective. A planetary transit is therefore defined by apparent alignment rather than by physical proximity in space. Mercury transits and Venus transits across the Sun are examples within our Solar System, although Venus transits are much rarer.
A transit should not be confused with an occultation, in which one object passes in front of another and blocks or hides it from view, although the concepts overlap geometrically. It should also be distinguished from a solar eclipse, because the apparent size of the foreground body is central to the usual terminology and because a planetary transit normally produces a much smaller change in brightness.
Planetary transits are valuable because they transform an otherwise invisible planet into a measurable change in starlight. They combine orbital geometry, photometry, stellar physics and planetary science in a single observational method. Their principal limitation is geometric selection: only planets whose orbital planes are suitably aligned can transit from the viewpoint of a particular observer. Transit observations also depend on the star being sufficiently quiet and the signal being distinguishable from astrophysical and instrumental noise. For these reasons, transit surveys are often combined with radial velocity measurements, direct imaging, gravitational microlensing and other methods. The resulting combination of techniques provides a more complete understanding of planetary systems than any single detection method can provide.
