Transits

The transits method has given rise to the largest number of exoplanet detections, notably thanks to the Kepler satellite ? .

When a planet passes in front of its star, it hides a small part of its surface, which results in a very slight dip in its brightness. The advantage of this method is the reproducibility of the observable transits (every orbit in the same conditions).

Passage of a planet in front of the disc of a star
Hans Deeg (Instituto de Astrofisica de Canarias)

To make this observation, the star’s flux must be precisely recorded. Ground observations are limited by alternating day and night and by atmospheric fluctuations. This is the method that benefits most from space-borne instruments.

If we know the mass and radius of the star, observing transits enables us to find out the size of the planet and its distance from the star:

  • The dip in the relative brightness of the star gives a measurement of the relative size of the planet with respect to the star:

$$\frac{dL_{st}}{L_{st}}=\left( \frac{R_{pl}}{R_{st}} \right)^2$$

  • Observation of two successive transits gives the orbital period ?  of the planet. If we know the mass of the star, Kepler’s 3rd law will give us the semi-major axis ?  of the planet’s orbit.
  • Detection of the secondary transit provides information about the planet’s atmosphere, temperature and composition.

The first transits detected were those of the gas giant exoplanet HD 209458 b. The orbital period ? of 3.5 days shows that the planet is 8 times closer to its star than Mercury is to the Sun. Its hydrogen atmosphere is so hot, 1,000°C, that it evaporates like a comet ? .

Geometry of the transit and occultation ? (secondary transit) of an exoplanet

This technique has so far 07/10/2026 led to

objects.

Advantages:

  • Can be used to determine the orbital inclination ? of the planet.
  • Several stars are observed simultaneously.
  • Can be used to study terrestrial planets ? close to their star.
  • Can be used to perform transmission spectroscopy ?  of the atmosphere.
  • Secondary transit (occultation ? ) gives access to the emission spectrum of the atmosphere.

Drawbacks:

  • Requires a well-oriented orbit so that the planet passes through the star-observer axis.
  • The transit does not constrain the mass of the object and false positives are possible (spots on the surface of the star, binary stars). Confirmation by another method is necessary but can be difficult.