File:Heliumplanet.jpg
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English: 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.
There are several hypotheses for how a helium planet might form. 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. A helium-rich planetary object may also form from a low-mass white dwarf, which gets depleted of hydrogen via mass transfer in a close binary system with a second, massive object like a neutron star. One scenario involves an AM CVn type of symbiotic binary star composed of two helium-core white dwarfs surrounded by a circumbinary helium accretion disk formed during mass transfer from the less massive to the more massive white dwarf. After it loses most of its mass, the less massive white dwarf may approach planetary mass. Helium planets are expected to be distinguishable from regular hydrogen-dominated planets by strong evidence of carbon monoxide and carbon dioxide in the atmosphere. Due to hydrogen depletion, the expected methane in the atmosphere cannot form because there is no hydrogen for the carbon to combine with, hence carbon combines with oxygen instead, forming CO and CO2. Due to the atmospheric composition, helium planets are expected to be white or grey in appearance. Such a signature can be found in Gliese 436 b, which has a predominance of carbon monoxide, and is hypothesized to be a helium planet. |
| Date | |
| Source | Own work |
| Author | Pablo Carlos Budassi |
Planet concept illustrated especially for Wikimedia Commons by Pablo Carlos Budassi. Source: https://www.pablocarlosbudassi.com/2021/02/planet-types.html Background image by ESO/Serge Brunier: https://commons.wikimedia.org/wiki/File:ESO_-_Milky_Way.jpg Suggestions for improving this image are welcome: pablocarlosbudassi@gmail.com
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| current | 15:18, 15 August 2023 | | 1,080 × 1,080 (357 KB) | Celestialobjects (talk | contribs) | Uploaded own work with UploadWizard |
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