Some Sub-Neptune Planets Shrink Down to Super-Earth [Study]

Some exoplanets shrink, which could be why few planets are between the sizes of sub-Neptunes and super-Earth. Researchers finally found the gap behind this mystery and concluded it's not a "fluke."

Some Exoplanets Shrink

In a new study, Jessie Christiansen, a scientist at Caltech, and her colleagues examined data gathered by NASA's Kepler 2 mission, which was redesigned to search for exoplanets after the Kepler spacecraft experienced some technical issues.

Since the sun heats our atmosphere, it is estimated that every day, about 90 tons of air from Earth's protective layer shields life from escaping into space. However, scientists estimate that it would take our planet at least 15 trillion years to lose its atmosphere entirely at this incredibly slow rate. Therefore, there is no need for concern on that front.

However, a tiny number of exoplanets-particularly those larger than Earth but smaller than Neptune-are pushing their atmospheres out from within via a mechanism called "core-powered mass loss." This mechanism is thought to be why a puffy, sub-Neptune planet shrinks to a rocky super-Earth.

The team searched two-star clusters for sub-Neptunes orbiting stars -the Hyades cluster, which forms the head of the "Bull" in the Taurus constellation and has around 500 stars, and the Praesepe or Beehive cluster, which contains about 1,000 stars. Their ages are, respectively, 600 million and 800 million years old.

Nearly all stars in this age range had sub-Neptunes with atmospheric conditions circling them, which suggests photoevaporation did not occur.

However, only 25% of the planets orbiting stars older than 800 million years in K2's database showed signs of orbiting sub-Neptunes.

The researchers conclude that a core-powered mass loss most likely causes atmospheric escape on these planets because the older age of those stars is near the one billion-year time frame when the core-powered mass loss is estimated to have occurred.

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Astronomers claim that with this knowledge, they now have enough information to explain why they don't observe many exoplanets between 1.5 and 2 times the size of Earth, the ideal location halfway between a sub-Neptune and a super-Earth.

"Exoplanet scientists have enough data now to say that this gap is not a fluke," Christiansen, the study's lead author, said in a statement. "There's something going on that impedes planets from reaching and/or staying at this size."

What Are Sub-Neptunes and Super-Earth?

Sub-Neptunes are any planet with a radius smaller than Neptune but still massive. These planets include mini-Neptunes and super-Earths.

Super-Earths are a class of planets that are more massive than our planet but lighter than ice giants like Uranus and Neptune. They are twice the size of Earth and up to 10 times its mass. Super-Earth can be made of gas, rocks, or both.