What Is a Transit? A Planet Crossing a Star and the Dip That Reveals It

When a planet crosses the face of its star from the observer's position, the light received drops by the ratio of the planet's area to the star's, which for a giant planet crossing a sun-like star is around one percent and for an Earth-sized one is around one part in ten thousand. Measuring the depth of that dip therefore gives the size of the planet relative to the star, which is a direct geometric measurement rather than an inference. The time between successive dips gives the orbital period, and the period combined with the star's mass gives the orbital distance. The duration and shape of the dip constrain the orbital geometry. Combining a transit measurement with a velocity measurement of the same system gives both size and mass and therefore density, which indicates whether a planet is rocky, icy or gaseous.

The method has a severe geometric limitation and several practical ones:

• •Only systems aligned so the orbit passes across the star from our position produce transits, which is a small fraction of all systems
• •That fraction falls with orbital distance, so close-in planets are far more likely to be detected than distant ones
• •Detection requires observing several transits to confirm a period, so planets with long years require long campaigns
• •Stellar variability produces dips that mimic planets and must be distinguished
• •Eclipsing binary stars in the background produce false signals that require follow-up to exclude
• •The method gives size and not mass, so a confirmed detection needs a second technique to complete the picture

The most valuable application is one the geometry makes possible and nothing else does. During a transit a small amount of starlight passes through the planet's atmosphere on its way to the observer, and molecules in that atmosphere absorb particular wavelengths, so comparing the spectrum during the transit with the spectrum outside it reveals which wavelengths were removed and therefore what the atmosphere contains. The effect is extremely small and requires careful work, and it has nevertheless detected water, carbon dioxide, methane and several other molecules in the atmospheres of planets many light years away. Observing the planet passing behind its star supplies a further measurement, since the difference between the combined light and the star alone gives the planet's own emission, which indicates temperature. These techniques are the main route to characterising planets rather than merely counting them.

The same geometry operates within our own system and has a long history. Venus crossing the sun happens in pairs separated by eight years, with gaps of over a century between pairs, and observing those events from widely separated locations was the method used in the eighteenth and nineteenth centuries to measure the distance from the Earth to the sun, which was the fundamental unknown scale of the solar system. Expeditions were sent across the world at enormous expense and risk to observe them, and several of those voyages are notable for other reasons. Mercury transits are more frequent and less useful for that purpose. The transits of the moons of Jupiter across their planet were used for a different measurement entirely, since timing them established that light travels at a finite speed, which was the first demonstration of that fact.

The method suits automated searching, since it requires monitoring many stars continuously and looking for periodic dips, which is a task computers do better than people. The Kepler mission stared at a single patch of sky for years and monitored well over a hundred thousand stars, producing thousands of confirmed planets and transforming the subject from a handful of known examples into a statistical science, which allowed the first reasonable estimates of how common planets are. A successor mission surveys the whole sky in shorter segments, targeting brighter and nearer stars whose planets can be studied further. Ground-based surveys contribute and are limited by daylight and weather. The statistical result from those surveys, after correcting for the geometric bias, is that planets are common and that small planets are more common than large ones.

The depth of the dip gives the planet's size relative to the star directly, the interval gives the period and adding a velocity measurement gives density. Only systems aligned edge on from our position transit at all, which favours close-in planets heavily. Starlight passing through the atmosphere during a transit reveals what it contains, which is the main route to characterising planets rather than counting them.