What Are the Different Exoplanet Detection Methods?

Here is a brief summary of the principal methods used to detect and observe exoplanets. Note that the Extrasolar Planets Encyclopedia lists the method of discovery for each of the confirmed exoplanets in the database.

Radial Velocity (RV) Method

(Doppler Shifts from Host Star's Motion)

This is by far one of the most common methods and is based on detecting the wobble of a planet's parent star as the planet orbits the star. The changes in velocity of the star are detected by means of Doppler shifts in signatures that are imprinted on the light from the star. Read more about the RV method.

Transit Method

This is another of the most common methods used to detect exoplanets, and it is specifically the one used by the Kepler mission. It is based on measuring the properties of the dips in the parent star's light as a planet blocks a small fraction of the star's light when it moves across the star's face. Read more about the transit method.

Direct Imaging

Only a small percentage of exoplanets have been detected by direct imaging (more details in the linked article).

Gravitational Microlensing

This method utilizes the properties of the bending of light as it passes massive objects such as stars. If the star hosts a planet, specific time signatures of the received light are expected. Again, only a small fraction of exoplanets have been observed using this method, but it accesses some parts of the exoplanet parameter space that other methods do not, and may be the most likely method to be able to detect exomoons. Read more about the gravitational microlensing method.

Astrometry

This method, like the RV method, attempts to measure the wobble of the parent star as a planet orbits it, except that the wobble is detected spatially. (In the RV method the wobble was measured purely by means of velocity variations without actually seeing the spatial movement.) The spatial wobble is tiny so the observations required for the astrometry method are extremely challenging. The method is also subtly different to direct imaging because one is not dealing with the image of a planet (it is the image of the star). The host star is a “dot” and the positions of the “dot” are tracked in the time domain. The astrometry method is not currently amenable to “blind” exoplanet searches because it carries a high overhead in terms of observatory time, and groundbased observations are subject to limitations imposed by Earth's atmosphere.

In August 2012, the Extrasolar Planets Encyclopedia listed only one exoplanet as having been discovered by the astrometry method (HD 176051 b). Whilst astrometry by itself cannot currently be used to discover lots of exoplanets, it can be used to provide supplemental information for exoplanets that have already been discovered and observed using a different method. For example, the ambiguity in the planet mass from the velocimetry method can be mitigated by supplemental astrometric observations, although the measurement uncertainties can be larger than those from other methods. The Hubble Space Telescope (HST) has been used in this capacity.

Direct Doppler Shifts from Planetary Material

This method only became feasible in 2012. Whearas the standard RV method examines Doppler shifts from the host star, this method directly measures velocity signatures by means of Doppler shifts from the motion of the planet itself. This is very difficult to do because the signal from the planet is extremely weak and the signal from the parent star is overwhelming. The method was demonstrated with a planet that was already well-studied by other methods. Using infrared observations of the exoplanet $\tau$ boötes b, carbon monoxide signatures from the planet's atmosphere were traced to track the planet's velocity. (Advanced readers see Brogi et al. 2012.) The method provides a means of studying the atmospheres of exoplanets whose orbits are not aligned to show transits across the host start to observers on Earth.

Timing Method

This method is only used for exoplanets that orbit pulsars, which are not regular stars, but spinning neutron stars. Neutron stars are compact objects made of highly compressed neutrons and they result, under certain conditions, from gravitational collapse after regular stars exhaust all the fuel for nuclear burning. A spinning pulsar emits periodic radio signals. Without a planet present, the periodic signature that is measured from the pulsar is extremely regular, stable, and clear. The time period between each pulse is easy to measure with a sufficiently high accuracy that irregularities due to the presence of a planet are predictable and measurable. The first unambiguous exoplanets discovered (in 1992) were in fact exoplanets around pulsars so were discovered using the timing method. (See also When was the first exoplanet discovered? because the situation is not quite as simple as depicted here.)

Learn more about exoplanets with Exoplanets and Alien Solar Systems, which includes comprehensive references to the scientific literature.

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