His discovery of dark absorption bands in the spectra of Jupiter, Saturn, and Neptune led to the identification of some of the chemical constituents of their atmospheres.
The radioactivity of the surface materials of both Mars and Venus have been studied with a gamma-ray detector, the isotopic composition of their atmospheres analyzed with a mass spectrometer, and their magnetic fields measured.
In a few cases planets can even be seen directly, using clever tricks to blot out the light of the stars they orbit, and researchers are starting to analyse the light from planets to determine the characteristics of their atmospheres.
Here the situation is complicated by the vast extent of their atmospheres, so that radiation originating at very different depths in the star is observed at the same time.
Some of these planets, researchers hope, will become candidates for follow-up studies of their atmospheres by the James Webb Space Telescope, scheduled for launch in 2018.
Astronomers have discovered about 2000 planets around other stars, but they have few hard facts about what they are like, such as the contents of their atmospheres.
All planets lose a small portion of their atmospheres to space, if individual gas molecules get hot enough and reach escape velocity.
To determine just how conducive to life these exoplanets may be, scientists want to learn more about the exact chemical make-up of their atmospheres, the surface pressure and range of temperatures found on the planet.
Because these are around stars closer to Earth, astronomers will be able to follow them in more detail with other telescopes, measuring their masses and probing the composition of their atmospheres for signs of life. .
Some stars vary widely in the chemical composition of their atmospheres, even though the stars are about the same age and were all born within the same cloud of dust and gas.
