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It is possible that mini-Neptunes and super-Earths share a great deal more in common than their status as superlatives. Four gaseous exoplanets, each somewhat smaller than Neptune, appear to be transforming into super-Earths, rocky worlds up to 1.5 times the breadth of Earth. In a recent publication, researchers claim that the powerful radiation of their stars appears to be eroding the planets’ thick atmospheres. The research estimates that if the current rate of atmospheric loss continues, these puffy atmospheres would eventually disappear, leaving behind smaller planets of bare rock.

Scientists can learn how other exoplanets lose their atmospheres by examining how these worlds evolve and lose their atmospheres. And this, according to Caltech astronomer Heather Knutson, can provide information on what types of planets may have livable environments. “Because if you cannot maintain an atmosphere, you cannot be habitable,” she explains.

The latest study by Knutson and her colleagues confirms a previous suspicion. These same researchers announced earlier this year that helium appeared to be fleeing the atmosphere of one of these mini-Neptunes. The crew was uncertain as to whether or not their discovery was unique. “Perhaps we just got lucky with this one planet, but every other planet is different,” says Michael Zhang, an exoplanet researcher at Caltech.

The researchers then examined three other mini-Neptunes orbiting other stars and compared them to the first planet discovered. Each of these planets occasionally obstructs a portion of its star’s light (SN: 7/21/2). Zhang, Knutson, and his colleagues monitored how long each planet obstructed its star’s light and how much of that sunlight was absorbed by the planets’ helium envelopes. These observations allow the scientists to determine the sizes and forms of the atmospheres of planets.

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“When a planet loses its atmosphere, a large, comet-like tail of gas emerges from the planet,” explains Knutson. Astronomers would have observed a circle if the gas were still connected to the planet, as is the situation with Neptune in our solar system. She explains, “We do not entirely comprehend all the shapes we observe in the outflows, but we can see that they are not spherical.”

In other words, each planet is losing helium slowly. “I never would have anticipated that we would find such a clear detection on every planet we examined,” Knutson says.

Astronomers also determined the amount of mass these exoplanets were shedding (SN: 6/19/17). “This mass loss rate is sufficient to strip the atmospheres of the majority of these planets, transforming at least some of them into super-Earths,” says Zhang.

Ian Crossfield, an exoplanet researcher from the University of Kansas in Lawrence who was not engaged in this research, explains that these rates are merely snapshots in time. It is unknown exactly how each planet has lost atmosphere during its whole history and into the future, he said. “The only information we have is what we observe now.” Even with such unanswered concerns, he continues, the hypothesis that mini-Neptunes transform into super-Earths “appears reasonable.”

Crossfield states that theories and computer simulations of how planets create and lose their atmospheres can assist fill in some of the gaps about specific planets. Measuring more mini-Neptunes will also be beneficial. Zhang intends to observe an additional dozen. In addition, “we’ve already examined another target, and that target has a quite robust escaping helium signal,” he says. The current score is five for five.

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