Between giants and dwarfs: Why don’t astronomers find medium-sized exoplanets?

Scientists have determined where the gap in statistics for Earth-like bodies is coming from. Outside our solar system, there are distant planets, also called exoplanets. These could be gas giant planets like Jupiter, Earth-sized rocks, or even “fluffy” objects with the density of cotton candy. According to NASA calculations, there are more than 5,000 exoplanets, many of which should theoretically be about 1.5 to 2 times the size of Earth, but no objects exactly this size have been discovered.

As astrophysicists explain in a press release, this is because some subneptunes (planets with larger volumes and lower masses than Earth, also known as gas dwarfs) are being compressed. They lose their atmosphere and accelerate beyond their size difference, eventually becoming as small as a super-Earth. And they are shrinking due to the fact that radiation from the planet’s core is pushing the atmosphere out into space. A collapsing exoplanet may not have enough mass (and therefore gravity) to maintain the density of its atmosphere. However, the exact mechanism of atmospheric loss remains unclear. New research supports a hypothesis scientists call “nuclear mass loss.” According to this study, the planet’s core emits radiation that compresses the atmosphere from below, eventually leading to its separation from the planet.

There is also another hypothesis. This is called photoevaporation and means that the planet’s atmosphere is dispersed by radiation from its parent star. However, photoevaporation is thought to begin 100 million years after the planet’s birth, and mass loss due to nuclear energy may occur just before the planet’s “billionth birthday.” . To test these two of his hypotheses, scientists examined data from his NASA Kepler Space Telescope. By studying star clusters that are more than 100 million years old, astronomers have discovered that most planets retain atmospheres. And this suggests that the most likely reason why the atmosphere could be lost is the loss of mass due to nuclear energy. At the same time, the researchers do not rule out the possibility that both processes are simultaneously involved in the planet’s shrinkage.