transit method

History

The idea of using planet transits for detection originated in the mid-20th century, but the real revolution was made by the Kepler telescope, launched in 2009. Its principal investigator, William Borucki, had fought for decades to get this project approved — and ultimately discovered thousands of worlds.

How it works

Imagine a light curve of a star — a graph like a cardiogram. When a planet eclipses the star, a 'dip' appears on the graph. The depth of the dip shows how big the planet is, and the time between dips shows how fast it orbits the star. Sensitive photometers record brightness changes of hundredths of a percent.

History

The idea of using planet transits for detection originated in the mid-20th century, but the real revolution was made by the Kepler telescope, launched in 2009. Its principal investigator, William Borucki, had fought for decades to get this project approved — and ultimately discovered thousands of worlds.

How it works

Imagine a light curve of a star — a graph like a cardiogram. When a planet eclipses the star, a 'dip' appears on the graph. The depth of the dip shows how big the planet is, and the time between dips shows how fast it orbits the star. Sensitive photometers record brightness changes of hundredths of a percent.

History

The first successful detection of an exoplanet transit occurred in 1999 for the star HD 209458. The theoretical possibility of the method had been discussed since the mid-20th century, but technical difficulty required space-based surveys. The breakthrough was achieved by William Borucki, who since the 1980s had been developing the concept of the Kepler mission. Launched in 2009, Kepler observed about 150,000 stars simultaneously and discovered thousands of candidates, proving that planets are common in the Galaxy. In 2018, TESS followed, continuing transit surveys of nearby stars.

How it works

When a planet crosses a star's disk, the total radiation flux decreases by an amount proportional to the planet's disk area: ΔF/F = (R_p/R_s)². The transit light curve has a characteristic shape with gradual ingress/egress and a flat bottom (if the star is uniform). The period between transits gives the orbital period, and the transit duration gives the impact parameter and inclination. The secondary transit (when the planet passes behind the star) allows measuring the planet's own emission. Transit spectroscopy reveals absorption lines in the atmosphere — for example, water vapor, methane, carbon dioxide.

Open problems & fun facts

High rate of false positives due to binary stars or starspots; Difficulty in detecting Earth-like planets with long periods because of shallow and infrequent transits; Limitations of ground-based telescopes due to the atmosphere, requiring space instruments

💡 The most famous transit in history was the passage of Venus across the Sun's disk in 1769, which allowed calculation of the Earth-Sun distance. The modern transit method has found a planet resembling Tatooine from Star Wars — Kepler-16b, orbiting a binary star!