How can scientists tell what is in the atmosphere
of a planet that is thousands of light years away from us? The answer is,
they can't tell definitively, but they can make inferences. You will get
some practice making inferences in an upcoming activity, but to give you
some more information about planetary atmosphere, think about the planets
in our solar system.
Jovian planets, like Jupiter and Saturn, have large masses and are very gaseous. These planets have so much mass that they can keep even the very very small, very very light particles of most gases from escaping their gravity. Planets with smaller masses, like that of Mercury, or even Earth, don't have enough gravity to hold onto the lighter gases, like Hydrogen and Helium. These gases can escape the gravitational pull of the planet and drift off into space. The amount of speed a particle needs to escape the gravity of a particular planet is that planet's escape velocity.
So, if you are examining a planet that has the mass of a Jovian planet, you can infer that the planet has a thick atmosphere with a high percentage of the lighter gases, such as Hydrogen, Helium, and Lithium. It may also have heavier gases, but they will be present in much smaller amounts. Smaller planets that can't hold onto light gases may or may not have an atmosphere. Mercury doesn't, but Venus and Earth do. These atmospheres are likely to have a very low percentage of Hydrogen and Helium, because they don't have enough mass and gravity to hold onto them.