Most of the gas in the Universe is a mixture of hydrogen and helium. It's thought that the initial atmospheres of most planets also start out that way. However, over billions of years, as planets evolve, the composition of their atmospheres may shift. Hydrogen can react with other chemicals, and both it and helium can be lost to space. Venus, Earth, and Mars are thought to have second atmospheres, with their original hydrogen/helium envelopes having been lost and/or transformed.

The dynamics of loss are complicated. Lighter elements are lost more easily, but hydrogen can be protected by being incorporated into molecules like methane and ammonia. The gravity of the body can help retain some molecules, and a magnetic field can limit radiation's ability to blast material out of the atmosphere. Proximity to a star will matter too, both because of the radiation it produces and because it can heat the atmosphere and expand it to where gravity's influence is less substantial.

Given all these complications, it can be difficult to know what to expect to find on exoplanets. But a study in Wednesday's issue of Nature describes observations of helium being lost from the atmosphere of an exoplanet orbiting the star LHS 1140, about 50 light-years away. Based on the rate at which the helium is being lost, we can infer something about the remaining atmosphere.

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WD 1856 b is the only confirmed case of a planet that survived the death of a Sun-like star. It’s a Jupiter-size world orbiting a white dwarf—the burned-out remnant of a Sun-like star. Now, a team of astronomers has used the James Webb Space Telescope to take a closer look at this planet for the first time, and what they found makes an already strange system even stranger.

A feeding frenzy

WD 1856 b was an accidental discovery. Astronomers pointed the TESS observatory at a sample of roughly 2,000 white dwarfs in 2020. These stars are the remains of a Sun-like star that have already gone through a red-giant phase, leaving behind an Earth-size body that’s primarily composed of elements like carbon and oxygen. The TESS team was searching for small objects like comets or asteroids that might transit across the face of these dead stars.

What they found in the WD 1856 system was a gas giant. “As soon as they looked at it, they said, okay, that’s weird,” said Christopher O’Connor, a theoretical astrophysicist at Cornell University and co-author of the recent Nature study on WD 1856 b.

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