<P> Light given off by a star is un-polarized, i.e. the direction of oscillation of the light wave is random . However, when the light is reflected off the atmosphere of a planet, the light waves interact with the molecules in the atmosphere and become polarized . </P> <P> By analyzing the polarization in the combined light of the planet and star (about one part in a million), these measurements can in principle be made with very high sensitivity, as polarimetry is not limited by the stability of the Earth's atmosphere . Another main advantage is that polarimetry allows for determination of the composition of the planet's atmosphere . The main disadvantage is that it will not be able to detect planets without atmospheres . Larger planets and planets with higher albedo are easier to detect through polarimetry, as they reflect more light . </P> <P> Astronomical devices used for polarimetry, called polarimeters, are capable of detecting polarized light and rejecting unpolarized beams . Groups such as ZIMPOL / CHEOPS and PlanetPol are currently using polarimeters to search for extrasolar planets . The first successful detection of an extrasolar planet using this method came in 2008, when HD 189733 b, a planet discovered three years earlier, was detected using polarimetry . However, no new planets have yet been discovered using this method . </P> <P> This method consists of precisely measuring a star's position in the sky, and observing how that position changes over time . Originally, this was done visually, with hand - written records . By the end of the 19th century, this method used photographic plates, greatly improving the accuracy of the measurements as well as creating a data archive . If a star has a planet, then the gravitational influence of the planet will cause the star itself to move in a tiny circular or elliptical orbit . Effectively, star and planet each orbit around their mutual centre of mass (barycenter), as explained by solutions to the two - body problem . Since the star is much more massive, its orbit will be much smaller . Frequently, the mutual centre of mass will lie within the radius of the larger body . Consequently, it is easier to find planets around low - mass stars, especially brown dwarfs . </P>

Match the type of measurement to what we could learn about a star (or its planets)