Astronomers using NASA’s Hubble Space Telescope have discovered an immense cloud of hydrogen dubbed “The Behemoth” bleeding from a planet orbiting a nearby star.
The planet is about the size of Neptune and the cloud of gas 50 times as large.
enormous, comet-like feature is about 50 times the size of the parent star. The hydrogen is evaporating from a warm, Neptune-sized planet, due to extreme radiation from the star.
This phenomenon where hydrogen evaporates away due to the heat of a star has never been seen around a planet so small.
Astronomers say it may offer clues to how other planets with hydrogen-enveloped atmospheres could have their outer layers evaporated by their parent star, leaving behind solid, rocky cores.
“This cloud is very spectacular, though the evaporation rate does not threaten the planet right now,” explains the study’s leader, David Ehrenreich of the Observatory of the University of Geneva in Switzerland.
“But we know that in the past, the star, which is a faint red dwarf, was more active. This means that the planet evaporated faster during its first billion years of existence because of the strong radiation from the young star. Overall, we estimate that it may have lost up to 10 percent of its atmosphere over the past several billion years.”
The planet, named GJ 436b, is considered to be a “Warm Neptune”, because of its size and because it is much closer to its star than Neptune is to our sun. Although it is in no danger of having its atmosphere completely evaporated and stripped down to a rocky core, this planet could explain the existence of so-called Hot Super-Earths that are very close to their stars.
These hot, rocky worlds were discovered by the Convection Rotation and Planetary Transits (CoRoT) and NASA’s Kepler space telescope. Hot Super-Earths could be the remnants of more massive planets that completely lost their thick, gaseous atmospheres to the same type of evaporation.
Because the Earth’s atmosphere blocks most ultraviolet light, astronomers needed a space telescope with Hubble’s ultraviolet capability and exquisite precision to find “The Behemoth.”
Bill Condie is a science journalist based in Adelaide, Australia.
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