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Evolution of Neptune's atmosphere, page 5
The Size of the Planet matters




Mercury
(small)

Venus
(medium sized)

Jupiter
(very large)

Hydrogen (H) less than 1 hour days over the age of the Universe
Helium (He) roughly 1 day 100 times the age of the solar system over the age of the Universe
Oxygen (O) 800 million years over the age of the Universe over the age of the Universe

This table shows the time it takes, on average, for a few common atmospheric gases to escape the gravity of a planet. The table shows that as the planets get larger (more massive), it is harder for the gases to escape. The table shows that the average time for any gas to escape from Jupiter's gravity is much too long. The same is true for all the Giant planets. Therefore it is likely that all the gases in Neptune's atmosphere are probably the original gases swept up by the proto-Neptune.

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