When it comes to detecting exoplanets, there are several techniques available at the disposal of the exoplanet hunter. Each technique, quite often sensitive to different types of planets, is briefly summarised below and shown in Fig. 1. Amongst these techniques the direct imaging and observations of transiting exoplanets have had the greatest impact on atmosphere characterisation and as such these methods will be discussed in more detail in the subsections below.

M-D_Diagram

Fig.1 Planetary mass as a function of semi-major axis. The shapes represent the various detection techniques; radial velocity (red circles), transits (blue diamonds), timing (black downward triangle), microlensing (orange upward triangle) and direct imaging (green stars).

The main techniques used to detect exoplanets are:

Direct imaging:

The exoplanet is imaged directly using large telescopes fitted with adaptive optics and coronagraphs. The technique is most sensitive to the warmer, bright (young) and massive exoplanets on wide and/or eccentric orbits (large sky projected separations). The separation from the host star allows for spectra to be obtained directly and allows for the direct measurement of the luminosity.

Radial velocity:

The exoplanet is detected by measuring the Doppler shift in the host star light, a consequence of the gravitational affects between the two bodies. The technique is most sensitive to exoplanets with a large mass orbiting close to their host star perpendicular to the plane of the sky. The radial velocity technique allows for a minimum mass (dependant on orbital inclination) to be calculated.

Transits:

The exoplanet is detected by measuring a periodic decrease in the flux received from the host star, as a consequence of the exoplanet transiting in front of the host star. The transiting technique is most sensitive to large exoplanets orbiting close to their host star stars and provides an accurate determination of the planetary radius relative to the host star.

Microlensing:

The exoplanet is detected by measuring characteristic light curve changes caused by changes in the lensing effect observed when a star with a planet passes in front of a distant star. The technique is limited to distant one time events and by the lack of accurate determinations of the planet and orbit parameters. It is however a very valuable technique due to the lack of strong radii or mass biasses making it ideal for statistical population studies.

Transit timing variations:

The exoplanet is detected by observing a change in periodic phenomena due to the presence of an exoplanet. Examples include a change in transit time (known as TTV) of one planet, due to the presence of others in multiple planet systems and pulsar timing, where anomalous movement (measured at radio wavelengths) can be used to infer the presence of a planet.

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