Mini-Neptunes and super-Earths may have a lot more in common than just being superlatives.
Four gaseous exoplanets, each slightly smaller than Neptune, appear to be evolving into super-Earths, rocky worlds up to 1.5 times the width of our home planet. That’s because the intense radiation from its stars appears be moving away the thick atmospheres of the planets, the researchers report in a paper posted July 26 on arXiv.org. If the current rate of atmospheric loss holds, the team predicts, those puffy atmospheres will eventually disappear, leaving behind smaller, bare-rock planets.
Studying how these worlds evolve and lose their atmospheres can help scientists understand how other exoplanets lose their atmospheres. And that, says Caltech astronomer Heather Knutson, may provide insights into what kinds of planets might have habitable environments. “Because if you can’t maintain an atmosphere,” she says, “you can’t be livable.”
The new study by Knutson and colleagues reinforces a previous suspicion. Earlier this year, the same researchers reported that the helium seemed to be escaping the atmosphere of one of these mini-Neptunes. But the team wasn’t sure if their discovery was unique. “Maybe we got really lucky with this planet, but all other planets are different,” says exoplanet researcher Michael Zhang, also of Caltech.
The team then observed three more mini-Neptunes orbiting other stars and compared those worlds to the first planet they had observed. Each of these planets occasionally blocks some of its star’s light (Serial number: 7/21/21). Zhang, Knutson and their colleagues tracked how long each planet blocked out the light from its stars and how much of that starlight was absorbed by the helium that enveloped the planets. Together, these observations allow the team to measure the sizes and shapes of the planets’ atmospheres.
“When a planet is losing its atmosphere, you get a big comet-like tail of gas coming off the planet,” says Knutson. If the gas were instead still attached to the planet, as is the case with Neptune in our solar system, astronomers would have seen a circle. “We don’t fully understand all the shapes we see in the outflows,” he says, “but we see that they are not spherical.”
In other words, each planet is constantly losing its helium. “I never would have imagined that on every planet we looked at, we would see such a clear detection,” says Knutson.
The astronomers also calculated how much mass those exoplanets we are losing (Serial number: 06/19/17). “This rate of mass loss is high enough to strip the atmosphere of at least most of these planets, so that some of them, at least, will become super-Earths,” says Zhang.
However, these rates are just snapshots in time, says Ian Crossfield, an exoplanet researcher at the University of Kansas in Lawrence who was not involved in this work. For each planet, “exactly how it has been losing atmosphere throughout its history and into the future is not known,” he says. “All we know is what we see today.” Even with such open-ended questions, he adds, the idea of mini-Neptunes becoming super-Earths “seems plausible.”
Theories and computer simulations of how planets form and lose their atmospheres can help fill in some of the blanks on individual planets, Crossfield says.
Measurements of more mini-Neptunes will also help. Zhang plans to watch another handful. Also, “we’ve already looked at one more target, and that target also has a pretty strong helium leak.” [signal],” he says. “Now we have five times five.”