When the planet passes in front of the star, it is called Lingxing. When the planet moves behind the star, it is called secondary eclipse. Observing secondary eclipse can let us know a lot of information about the planet, especially its atmospheric composition.
To study their atmospheric composition, we need to use a spectrometer with a spectrometer on a large telescope, which can observe the spectrum emitted by distant celestial bodies. Light waves are generated by electrons moving inside atoms. The motion of electrons inside atoms of various substances is different, so they emit different light waves. Studying the luminescence and absorption of light by different substances has important theoretical and practical significance, and has become a special discipline - spectroscopy.
The planet does not emit light, but it can reflect the light of the star. The specific light reflected depends on the planet's atmosphere. Specific molecules in the atmosphere will absorb the reflected light at a specific wavelength and leave the information of some chemical elements. When it goes behind the star, the reflected light suddenly disappears. At this specific time, those unique reflected lights are gone. Through observation in different wave bands and comparison with the light of the star, We can know what chemical elements the planet's atmosphere contains.
For those star systems whose orbits are perpendicular to our line of sight, planets will not transit stars. To study their atmosphere, we need powerful telescopes. At the end of 2014, with the help of European very large telescopes, we studied the constellation of shepherd τ B the atmosphere of this planet that will not transit stars first sees the light of this planet. Of course, this is the reflected stellar light. When these lights come to the earth, as long as we distinguish that its characteristics are not caused by the earth's atmosphere, we can judge that these characteristics are planetary, so we can get the signal of the planet's atmosphere and further judge the atmospheric characteristics of this planet.
Now, sodium has been found on exoplanets, and water and hydrogen have been found on gj1214b. What elements exist on the planet is directly related to whether life can be found and whether it is suitable for us to live there.
Generally speaking, on planets with a temperature of one or two thousand degrees, most of the carbon in the atmosphere will combine with oxygen to form carbon monoxide. What is really worth studying are those planets with a temperature of less than one thousand degrees, where the carbon will combine with oxygen to form methane, which is a signal of life. In addition, ozone composed of three oxygen atoms is also an important signal of the existence of life.