The discovery of several exoplanets that appear to be shrinking may be the missing link in planetary evolution.

Scientists Four small Neptune-type exoplanets have been discovered around their stars that are losing their atmospheres at a rate consistent with the loss of total mass. This shows that these objects eventually shrink into rocky planets and almost the size of the Earth, and the mother stars are the cause of such evolution.

Although scientists have long thought that these two types of exoplanets are related, but the path by which the mini-Neptunes lose their atmosphere was unknown.

While other mechanisms could be involved, the newly identified smaller worlds suggest that separation The way of stellar radiation is one of the most important things in this evolutionary process. The Milky Way is a large and diverse place, and so far many types of exoplanets have been identified in it, which are very different from what we see in our own solar system.

One of them is mini-Neptune; The most common type of exoplanet detected by the Kepler mission, however, is curiously absent from our own little neighborhood in the galaxy.

These exoplanets are more massive than Earth and less massive than Neptune, but like Neptune is still covered in a thick atmosphere of hydrogen and helium. It seems that these exoplanets do not get smaller than 2 times the radius of the Earth.

The next category is the super-Earths, whose size is between 1 and 1.5 times the radius of the Earth. Thus, between 1.5 and 2 Earth radii, there is a strange gap where exoplanets are very rare. This gap is known as the dwarf planet radius gap.

Scientists believe that this gap exists because exoplanets, above a certain critical limit, have enough mass to maintain a significant primordial atmosphere that makes them their size. It suddenly increases and puts it in the classification of mini-Neptunes. On the other hand, super-Earths that do not have enough mass have either lost their original atmosphere, or had no atmosphere to begin with.

Planetary atmosphere loss process due to radiation from the parent star
Credit: ESA/Hubble, NASA, M. Kornmesser

Question Next is how, if these exoplanets were born with early atmospheres, did they lose their atmospheres?

One potential way is called core-forced mass loss, the internal heat from planet formation. is where the gravitational binding energy is converted to heat and ejects the primordial atmosphere. Another method is called photoevaporation, in which the intense X-ray and ultraviolet radiation of the young central star destroys the exoplanet's atmosphere. An exoplanet is losing its atmosphere and determining its rate. Thus, during this new study that will be published in the Astrophysical Journal, researchers led by Michael Zhang ) astronomers from the California Institute of Technology (Caltech) used spectroscopy to examine the atmospheres of four mini-Neptunes around orange dwarf stars to determine the rate of helium leakage in them.

One of these four mini-Neptunes is called TOI 560b, which With a radius of 2.8 times that of the Earth, it was examined earlier this year. The other three planets are new and include TOI 1430.01 with a radius of 2.1 times that of Earth, TOI 1683.01 with a radius of 2.3 times that of Earth, and TOI 2076b with a radius of 2.52 times that of Earth, respectively.

According to the results obtained All four planets had significant leakage of helium, not due to heat from the energy of the planet's core, but due to evaporation from the light of the parent star. This amount of loss is enough to destroy the atmosphere of these exoplanets within a few hundred million years, which is a very short time scale in cosmic time.

  • Scientists detect secondary atmosphere on an exoplanet

The team's findings also suggest that most of the small Neptunes orbiting Sun-like stars are likely to become super-Earths, and this process through Photoevaporation takes place.

"Many, if not all, of these planets lose their hydrogen-rich mantle and become super-Earths," the team said in a statement. Our results show that most of the small Neptunes orbiting Sun-like stars have primordial atmospheres, and photoevaporation is an efficient mechanism for stripping their atmospheres and becoming super-Earths.

Cover photo: A graphic design of a Shrinking Planet Credit: NASA/ESA/D. Player (STScI)




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