Transit
In astronomy, a transit refers to the apparent passage of a celestial body across the disk of another, larger body, as seen from the observer’s perspective. The term applies both to planetary transits in the solar system and to the passages of exoplanets in front of their host stars— the latter application having revolutionized the detection of extrasolar worlds.
Transits in the Solar System
From Earth, we can observe the transits of Mercury and Venus across the Sun—the only planets closer to the Sun than we are. These events are rare: the transits of Venus occur in pairs separated by 8 years, with these pairs themselvesmore than a century apart. The most recent ones took place in 2004 and 2012, and the next ones will be in 2117 and 2125. Mercury transits are more frequent: about 13 per century.
The exoplanet transit method
When an exoplanet passes in front of its star as seen from our vantage point, it blocks a tiny fraction of the starlight— a 1% dip for a gas giant, 0.01% for an Earth-like planet. The transit method involves measuring these periodic dips in brightness to detect and characterize the planets. This is the method that has led to the discovery of most of the exoplanets known to date.
What a transit reveals
Precise observation of a transit provides a wealth of information. The depth of the transit indicates the planet’s size relative to the star. Its periodicity indicates the orbital period and thus the distance from the star. The duration and shape of the transit provide information about the planet’s orbit and atmosphere. Transit spectroscopy allows us to analyze theatmosphere of an exoplanet by measuring how starlight is filtered by that atmosphere.
Importance in Astronomy
The Kepler (2009–2018) and TESS (since 2018) missions have revolutionized exoplanetology by continuously monitoring hundreds of thousands of&stars in search of transits. The James Webb Space Telescope takes this technique a step further by analyzing the atmospheres of potentially habitable exoplanets with unprecedented precision.
A concrete example: the TRAPPIST-1 system
The TRAPPIST-1 system, located 40 light-years away, was discovered entirely using the transit method. Seven Earth-like planets orbit a red dwarf star there, and their successive transits have made it possible to determine their masses and radii and even to analyze some of their atmospheres using the James Webb Space Telescope.
Did you know?
For an exoplanet to be detectable via transit, its orbit must be nearly aligned with the Earth-star line of sight. Statistically, only a small percentage of planetary systems can be observed using this method— about 0.5% for an Earth-like planet orbiting a star like the Sun. The actual number of exoplanets is therefore far greater than what we detect.
Frequently Asked Questions
Are all planetary systems observable via transit?
No. Only planets whose orbits are nearly aligned with our line of sight produce visible transits. Other detection methods (radial velocities, direct imaging, gravitational microlensing) complement transits to explore the diversity of planetary systems more broadly.
Can a transit be seen with the naked eye?
For exoplanetary transits, no—the variations in brightness are too slight. For transits of Venus or Mercury, yes, but eye protection is essential, just as it is for a solar eclipse. Venus is easily visible as a black dot on the solar disk; Mercury requires a telescope.
What is the difference between a transit and an occultation?
In a transit, the body passing in front is smaller in apparent size than the one in the background. In an occultation, the opposite is true: one body completely obscures another smaller one (for example, the Moon in front of a star). A total solar eclipse is technically an occultation of the Sun by the Moon.