How do we know what the properties of extrasolar planets are? What type of devices are we using to know this information?

The way to detect the extrasolar planets is to look at the light coming from the star, since there is no way of looking at the planet directly. The observations are based on the fact that there is some gravitational interaction between the star and the planet as the planet orbits. The planet orbits the star because of the action of the star's gravity. But the effect is somewhat symmetric, and the star also moves because of the presence of the planet. However, since the star is much more massive than the planet, it is the star that remains almost at rest, and the planet that moves more around. But the star is moving around a little bit, and we can detect this small displacement of the star.

As the planets moves around the star, the star also moves around a litte bit, sometimes going toward the Earth, sometimes going away from it. We can measure these movements by looking at the light coming from the star and looking for the Doppler shift (the Doppler shift causes the wavelength of the light we observe to change because of the motion of the source). To get the Doppler shift, astronomers must use spectrographs to split the light from the star into the individual colours it contains, and then look for tiny displacement of these 'spectral lines'. By knowing what gravitational effects the planets have on the stars, we can figure out their rough mass and distance to the star. We know from the mass of these stars that most of them are giant gas planets, somewhat like Jupiter. ( this webpage explains what the Doppler shift is and how it use for extrasolar planets, if you want more details about this.)

Another way to find out about the properties of these planets is to look at the difference in the light we get from the star when the planets passes in front of the star and when it is hidden behind it. Using this technique sodium has been detected in the atmosphere of planets. Spectrographs are used to see what the light we receive from the stars and planets is made of, which allows the astronomers to tell what is the composition of the planets. Thats what we call transit spectroscopy.