A mystery surrounding hundreds of planets that have gone missing from their star systems may finally have been solved by scientists.

Hot Neptunes, alien worlds the size of Neptune that sit perilously close to their stars, quickly lose their atmosphere under certain conditions, causing them to evolve into a different type of planet.

The distant worlds, which are mysteriously absent in our nearest star systems, turn into super-Earths – large, rocky planets up to ten times heavier than Earth.

“This is the first time that a planet has been observed to lose its atmosphere so quickly that it can impact its evolution,” said study author Dr. Vincent Bourrier.

The University of Geneva team says the find may finally explain why there are far fewer hot Neptunes in the universe than there should be. The research was published in the journal Astronomy & Astrophysics.

Scientists had puzzled for years as to why there were no medium-sized hot planets in other star systems.

They had found many large hot planets – around the size of Jupiter – as well as numerous smaller, Earth-sized ones, but none close to the size of Neptune.

This mysterious “desert” of hot Neptunes suggests two explanations: Either such alien worlds are rare, or they were once plentiful and have mysteriously disappeared.

Scientists studying images from NASA’s Hubble Space Telescope have found two warm Neptunes on the edge of the desert that are rapidly losing hydrogen from their atmospheres.

One of the planets, named GJ 3470b, is losing its hydrogen 100 times faster than the second, known as GJ 436b.

The two planets sit about 2.3 million miles from their star – about one-tenth the distance between Mercury and the sun.

The fact that warm, Neptune-sized planets lose their atmosphere at this pace suggests that their hotter twins are incredibly short-lived.

Hot Neptunes would have shrunk to become mini-Neptunes or would have eroded completely to leave only their rocky core.

“This could explain the abundance of hot super-Earths that have been discovered,” said study author Dr. David Ehrenreich.