Shrinking planets could explain mystery of universe's missing worlds

(Press-News.org) There's been a breakthrough in the case of the missing planets.

While planet-hunting missions have discovered thousands of worlds orbiting distant stars, there's a severe scarcity of exoplanets that measure between 1.5 and two times Earth's radius. That's the middle ground between rocky super-Earths and larger, gas-shrouded planets called mini-Neptunes. Since discovering this 'radius gap' in 2017, scientists have been sleuthing out why there are so few midsize heavenly bodies.

The new clue arose from a fresh way of looking at the data. A team of researchers led by the Flatiron Institute's Trevor David investigated whether the radius gap changes as planets age. They divvied up exoplanets into two groups -- young and old -- and reassessed the gap. The least common planet radii from the younger set were smaller on average than the least common ones from the older set, they found. While the scarcest size for younger planets was about 1.6 times Earth's radius, it's about 1.8 times Earth's radius at older ages.

The implication, the researchers propose, is that some mini-Neptunes shrink drastically over billions of years as their atmospheres leak away, leaving behind only a solid core. By losing their gas, the mini-Neptunes "jump" the planet radius gap and become super-Earths. As time goes on, the radius gap shifts as larger and larger mini-Neptunes make the jump, transforming into larger and larger super-Earths. The gap, in other words, is the chasm between the largest-size super-Earths and the smallest-size mini-Neptunes that can still retain their atmospheres. The researchers END