English: PIA19345: How to Make a Helium Atmosphere

http://photojournal.jpl.nasa.gov/catalog/PIA19345

NASA's Spitzer Space Telescope observed a proposed helium planet, GJ 436b. This diagram illustrates how hypothetical helium atmospheres might form. These would be on planets about the mass of Neptune, or smaller, which orbit tightly to their stars, whipping around in just days. They are thought to have cores of water or rock, surrounded by thick atmospheres of gas. Radiation from their nearby stars would boil off hydrogen and helium, but because hydrogen is lighter, more hydrogen would escape. It's also possible that planetary bodies, such as asteroids, could impact the planet, sending hydrogen out into space. Over time, the atmospheres would become enriched in helium.

With less hydrogen in the planets' atmospheres, the concentration of methane and water would go down.

Helium Atmospheres on Warm Neptunes Spitzer Space Telescope

For more information about Spitzer, visit http://spitzer.caltech.edu and http://www.nasa.gov/spitzer.