NASA puzzles over why some exoplanets are shrinking

As astronomers have used large telescopes to scan the skies for exoplanets, they've noted something odd: Worlds of a certain size are conspicuously missing in space.

The number of confirmed exoplanets — planets orbiting stars other than the sun — has risen to 5,539, according to NASA, with over 10,000 more candidates under review. But exoplanet hunters believe the universe is likely teeming with many trillions of planets.

These exoplanets come in different sizes. Of the midsize worlds, they are mostly divided into two groups, known as super-Earths and mini Neptunes (or sometimes sub-Neptunes). Although both kinds are larger than Earth and smaller than Neptune, super-Earths can be as much as 1.75 times the size of our home planet, and mini Neptunes are double to quadruple the size of Earth.

But if worlds were lined up in order of size, there would be a noticeable gap smack-dab in between super-Earths and mini Neptunes — planets that are about 1.5 to 2 times the size of Earth.

"Exoplanet scientists have enough data now to say that this gap is not a fluke," said Jessie Christiansen, an Infrared Processing and Analysis Center research scientist at Caltech, in a statement. "There’s something going on that impedes planets from reaching and/or staying at this size."

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The absent midsize planets are the focus of a new study published in The Astronomical Journal. Christiansen led the research team, which looked at the Milky Way star clusters Praesepe, sometimes known as the beehive, and Hyades, located on the face of the Taurus constellation, using NASA’s Kepler Space Telescope data.

The size gap could be caused by certain mini Neptunes actually shrinking over time. If such a world doesn’t have enough mass, and thus gravitational force, it could lose its atmosphere by its core emitting radiation that pushes the atmosphere away.

Over time, these less-massive mini Neptunes would shrink down to the size of a super-Earth. Scientists believe this could occur about 1 billion years into a planet's life.

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"Exoplanet scientists have enough data now to say that this gap is not a fluke."

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But there's another possibility that could answer for this behavior. So-called photoevaporation, thought to occur in a planet’s first 100 million years, happens when a planet's atmosphere is blown away by the radiation of its host star, similar to what would happen if you pointed a hair dryer at an ice cube, Christiansen said.

The coauthors are leaning toward the first explanation — with the source of the radiation coming from within the planet's core — because the star systems they were observing are thought to be about 600 to 800 million years old. If photoevaporation were the cause, it likely would have happened hundreds of millions of years earlier, they figure, and the planets would have barely any atmospheres left to speak of.

But to put the question to bed, scientists will need to conduct several follow-up studies in the future to prove or disprove the researchers' findings.