Astronomers explain the strange absence of medium-sized exoplanets | November 21, 2023

Outside our solar system there are distant planets, which are also called exoplanets. These could be gas giants like Jupiter, rocky balls the size of Earth, or even “puffy” objects with the density of cotton candy. NASA has calculated that there are more than five thousand exoplanets, and many of them should theoretically be about one and a half to two times larger than our Earth, but cosmic bodies with exactly these dimensions cannot be found.

As astrophysicists explain in a press release, this is because some sub-Neptunes (planets with a larger volume than Earth and less mass; also called gas dwarfs) are compressed. They lose their atmosphere and accelerate across the size difference until they are as small as super-Earths. And they shrink due to the fact that radiation from the cores of the planets pushes their atmospheres into space.

Collapsing exoplanets may not have enough mass (and therefore gravity) to keep their atmospheres dense. However, the exact mechanism of atmospheric loss remains unclear. The new study supports a hypothesis that scientists call “mass loss due to nuclear energy.” According to it, the planet’s core emits radiation that pushes its atmosphere from below, and this eventually leads to its separation from the planet.

There is another hypothesis. It’s called photoevaporation and states that a planet’s atmosphere is dissipated by radiation from its parent star. But photoevaporation is thought to occur by the time the planet is a hundred million years old, and mass loss due to core energy may occur closer to the planet’s “billionth birthday.”

To test these two hypotheses, scientists studied data from NASA’s Kepler space telescope. By examining star clusters more than a hundred million years old, astronomers discovered that most planets retained their atmospheres. And this suggests that the most likely reason for the possible loss of the atmosphere is the loss of mass due to the energy of the core. At the same time, researchers do not rule out that both processes may be involved simultaneously in the shrinking of planets.

At the same time, scientists have previously proven that exoplanets four to six times larger than Earth can be suitable for life.

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