Astrometry relates to the extremely accurate measuring of the position of an astronomical object in the night sky. Observers use a system of Right Ascension and declination (usually abbreviated to RA and dec) to record the object's co-ordinates. These co-ordinates are similar to the latitude and longitude system that we use on Earth.

As we know with our own Solar System, objects that orbit it cause our Sun to wobble slightly, with this wobble more pronounced for more massive objects (strictly those with the highest planet:star mass ratio) and those that are closest to their parent star. Many objects can be seen to be moving against a fixed background of stars and galaxies; it is only when these very subtle movements are seen to repeat that we can start to think that we are seeing the effects of an exoplanet on its host star (see Figure 1).

Figure 1: The characteristic movement of a star caused by its exoplanet over 3 years.
Credit: ESA (European Space Agency)

To date (late-2018), only a handful of systems are thought to have been detected using this method, although its effectiveness has been proven to confirm previously discovered exoplanets.

An excellent example of this effect was presented in 2018 when a comparison of data from Beta Pictoris from the Gaia and Hipparcos missions yielded a mass for beta Pictoris b, of 11±2 Jupiter masses. This estimate compares favourably with previous estimates of ~ 13 Jupiter masses.

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