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As exoplanets lose their atmospheres, mini-Neptunes may become super-Earths.

Mini-Neptunes and super-Earths may share more than just their superlative status.

Four gaseous exoplanets, each slightly smaller than Neptune, appear to be forming super-Earths, rocky worlds up to 1.5 times the width of our own. This is because their stars’ intense radiation appears to be pushing away the planets’ thick atmospheres, according to a paper published on arXiv.org on July 26. The team predicts that if the current rate of atmospheric loss continues, those puffy atmospheres will eventually vanish, leaving behind smaller planets of bare rock.

Scientists can learn more about how other exoplanets lose their atmospheres by studying how these worlds evolve and lose their atmospheres. According to Caltech astronomer Heather Knutson, this can provide information on what types of planets might have habitable environments. “Because you can’t be habitable if you can’t keep an atmosphere,” she explains.

The new study by Knutson and her colleagues confirms an earlier suspicion. The same researchers reported earlier this year that helium appeared to be escaping the atmosphere of one of these mini-Neptunes. However, the team was unsure whether their discovery was a one-time occurrence. “Perhaps we just got very lucky with this one planet, but every other planet is different,” says Caltech exoplanet researcher Michael Zhang.

So the researchers examined three more mini-Neptunes orbiting other stars and compared them to the first planet they discovered. Each of these planets blocks some of the light from its star on occasion (SN: 7/21/21). Zhang, Knutson, and colleagues measured how long each planet blocked the light of its stars and how much of that starlight was absorbed by the helium that envelops the planets. These observations allowed the team to calculate the sizes and shapes of the planets’ atmospheres.

“When a planet loses its atmosphere, it produces a large, sort of cometlike tail of gas,” Knutson explains. If the gas was still bound to the planet, as Neptune is in our solar system, the astronomers would have seen a circle. “We don’t fully understand all of the shapes we see in the outflows,” she says, “but they’re not spherical.”

In other words, each planet is gradually depleting its helium reserves. “I never expected to see such a clear detection on every single planet we looked at,” Knutson says.

The astronomers also calculated how much mass was being lost by the exoplanets (SN: 6/19/17). “This mass loss rate is high enough to strip the atmospheres of at least most of these planets, resulting in some of them becoming super-Earths,” Zhang says.

These rates, however, are only snapshots in time, according to Ian Crossfield, an exoplanet researcher at the University of Kansas in Lawrence who was not involved in this research. “You don’t know exactly how it’s been losing atmosphere throughout its entire history and into the future,” he says of each planet. “We only know what we see today.” Even with such unanswered questions, he adds, the concept of mini-Neptunes transforming into super-Earths “seems plausible.”

Theories and computer simulations of how planets form and lose their atmospheres, according to Crossfield, can help fill in some of the gaps on individual planets.

More mini-Neptune measurements will also be useful. Zhang intends to observe a few more people. “We’ve already looked at one more target, and that target has a pretty strong escaping helium [signal],” he adds. “We’re now five for five.”

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