page_title: Helium planet
section_title: Hydrogen evaporation from giant planets
hierarchical_section_title: Helium planet / Formation / Hydrogen evaporation from giant planets
caption_attribution_description: English: PIA19345: How to Make a Helium Atmosphere – fossil state 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. Both water and methane consist in part of hydrogen. Eventually, billions of years later (a "Gyr" equals one billion years), the abundances of the water and methane would be greatly reduced. Since hydrogen would not be abundant, the carbon would be forced to pair with oxygen, forming carbon monoxide. Helium Atmospheres on Warm Neptunes Spitzer Space Telescope For more information about Spitzer, visit http://spitzer.caltech.edu and http://www.nasa.gov/spitzer.
context_page_description: A helium planet is a planet with a helium-dominated atmosphere. This contrasts with ordinary gas giants such as Jupiter and Saturn, whose atmospheres consist primarily of hydrogen, with helium as a secondary component only. Helium planets might form in a variety of ways. Gliese 436 b is a possible helium planet.
context_section_description: A helium planet might form via hydrogen evaporation from a gaseous planet orbiting close to a star. The star will drive off lighter gases more effectively through evaporation than heavier gasses, and over time deplete the hydrogen, leaving a greater proportion of helium behind.
  A scenario for forming helium planets from regular giant planets involves an ice giant, in an orbit so close to its host star that the hydrogen effectively boils out of the atmosphere, evaporating from and escaping the gravitational hold of the planet. The planet's atmosphere will experience a large energy input and because light gases are more readily evaporated than heavier gases, the proportion of helium will steadily increase in the remaining atmosphere. Such a process will take some time to stabilize and completely drive out all the hydrogen, perhaps on the order of 10 billion years, depending on the precise physical conditions and the nature of the planet and the star. Hot Neptunes are candidates for such a scenario.
  The loss of hydrogen also leads to a depletion of methane in the atmosphere. On ice giants, methane naturally forms a cycle of melting, evaporation, breakdown and subsequent recombination and condensation. But as hydrogen gets depleted, a fraction of the carbon atoms will not be able to recombine with free hydrogen in the atmosphere and over time this will lead to an overall loss of methane. With time, the methane in the atmospheres of hot ice giants will also get depleted.
caption: Formation of a helium planet from a hot giant planet, possibly like Gliese 436 b.
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