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Most of the gas in the universe is a mixture of hydrogen and helium. It is thought that the primordial atmospheres of many planets also begin in this way. However, over billions of years, as the planets evolve, the atmosphere may change. Hydrogen can react with other chemicals, and both and helium can escape into the atmosphere. Venus, Earth, and Mars are thought to have secondary atmospheres, with their primary hydrogen/helium envelopes lost and/or replaced.
Energy loss is difficult. Lighter elements are easily lost, but hydrogen can be preserved by combining it into molecules such as methane and ammonia. Gravity helps keep molecules in place, and the magnetic field can slow down the energy of radiation emitted from space. Proximity to the star will also be important, both because of the light it emits and because it can heat the atmosphere and expand it to the point where the gravity is very low.
Given all these complications, it can be difficult to know what to expect to find on exoplanets. But a study published Wednesday in the journal Nature describes how helium is being lost from the atmosphere of an exoplanet orbiting the star LHS 1140, about 50 light-years away. Depending on how much helium is being released, we can tell about the rest of the atmosphere.
