Developed by Patrick Sirejean

2 Techniques for Detecting Extrasolar Planets

High precision techniques are required to detect planets in orbit around stars other than our Sun.

Stars are between 1 million and 10 billion times brighter than the planets. The planets are therefore too close and too dim to be directly detected by traditional instruments. This is the equivalent to detecting a candle a few millimeters from a lighthouse from a distance of 10 kilometers .

Indirect techniques are the most used today. They consist in detecting :

  • Either the star’s very small motion that periodically approaches and recedes from the observer due to the presence of a planet. This is the radial velocity method.
  • Either the tiny periodic variations in the intensity of the star’s luminosity due to the planet’s repeated passage in front of the star’s disk. This is the transit method.
  • Two Complementary Indirect Techniques

    The observation of a planet by either of these two techniques allows for the measurement of mass (radial velocity) and its diameter (transit), without trying to collect the light it emits. One can derive the planet’s density, and therefore it’s nature: gaseous (like Jupiter, Saturn, Uranus and Neptune) or terrestrial (like the Earth, Venus, Mars and Mercury). The average density of a terrestrial planet is 3 times greater than the density of gaseous planets.

    The star and the planet rotate around their center of gravity: the star appears to move in the sky, it recedes from and approaches the observer, in phase with the planet’s rotation.

    This movement is detected in the star’s spectra, which periodically shifts towards the red and then towards the blue.

    The amplitude of this radial velocity variation is greater with increasing planet mass and its distance to the parent star.

    When the astronomer observes an extra solar system edge-on, he/she can discern the passage – or transit- of the planet in front of the star disk, by the slight dimming of luminosity produced. The importance of the planet’s transit is greater with the planet’s increasing diameter.

    It is more likely to observe this phenomenon for planets with a short orbital period or at smaller distances from their star.

    Planet Configuration
    Type   (e.g. Jupiter) Density = 1.326 g/cm3
      (e.g. Earth) Density = 5.515 g/cm3
    D 0 0




    Planet Linear Velocity : 0

    Radial Velocity Method

    of the Star

    Radial Velocity




    Transit Method

    Relative Brightness