NASA scientists have been puzzling over a group of planets that seem to be shrinking. The culprit might be radiation.

All kinds of worlds exist beyond our solar system. Faraway alien planets, called exoplanets, can be gas behemoths like Jupiter, rocky globes about the size of our planet, or even "super-puffs" with the density of cotton candy.

But there's a mysterious gap where there should be planets about 1.5 to two times the width of Earth.

A mysterious gap where there should be planets

Among over 5,000 exoplanets that NASA has discovered, there are plenty of super-Earths (which are up to 1.6 times as wide as our planet) and plenty of sub-Neptunes (about two to four times Earth's diameter), but there are hardly any planets in between.

"Exoplanet scientists have enough data now to say that this gap is not a fluke. There's something going on that impedes planets from reaching and/or staying at this size," Jessie Christiansen, a research scientist at Caltech and science lead for the NASA Exoplanet Archive, said in a Wednesday press release.

Scientists think this is because some sub-Neptunes shrink — losing their atmospheres and speeding through the size gap until they are as small as a super-Earth.

Christiansen's latest research suggests those worlds shrink because radiation from the planets' cores pushes their atmospheres away, into space.

The study, published in The Astronomical Journal, on Wednesday, might solve the mystery of the missing exoplanets.

The planets themselves may be pushing their atmospheres away

Shrinking exoplanets may lack the mass (and therefore the gravity) to hold their atmospheres close.

The exact mechanism for the atmosphere loss, however, remains unclear.

The new study supports one hypothesis scientists call "core-powered mass loss," per the release.

Core-powered mass loss is not a trendy new workout plan. It's when a planet's core emits radiation that pushes its atmosphere from underneath, leading to it separating from the planet over time, per the release.

The other hypothesis, called photoevaporation, says that a planet's atmosphere is dissipated by the radiation of its host star.

But photoevaporation is thought to occur by the time a planet is 100 million years old — and core-powered mass loss could happen closer to the planet's one billionth birthday, per the release.

To test the two hypotheses, Christiansen's team looked at data from NASA's retired Kepler Space Telescope.

They examined star clusters that were over 100 million years old. Because planets are thought to be roughly the same age as their host stars, the planets in these clusters would be old enough to have experienced photoevaporation, but not old enough for core-powered mass loss.

The scientists found that most of the planets there retained their atmosphere, making the core-powered mass loss a more likely cause of eventual atmosphere loss.

"However, recent work suggests an ongoing mass-loss sequence where both processes operate," Christiansen wrote on X, the platform formerly known as Twitter, sharing a link to a Harvard assessment posted online in July.

So the mystery isn't solved yet.

According to Christiansen's statement in the release, her work isn't over yet either — especially because our understanding of exoplanets will develop with time.

This article was originally published by Business Insider.

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