(ORDO NEWS) — Among the already known exoplanets, most of all are sub-Neptuns – as a rule, they are two and a half times larger than Earth, are located very close to parent stars and are subject to their hard radiation. Nevertheless, they retained the remains of the primary atmosphere – including from water vapor. Recently, scientists have suggested that oceans are forming on the surface of such planets, which means that the appearance of life cannot be ruled out.
The paradox with the atmosphere
About four thousand exotic planets are now discovered in distant systems that are not at all similar to the solar one. We have rocky Mercury, Venus, Earth and Mars, gas Jupiter and Saturn, the ice giants Uranus and Neptune, and the asteroid belt. Most exoplanets are transitional types between rocky and giants, which are not in our system.
“Observational data show: exoplanets, whose radius is equal to one to four terrestrial ones, are very numerous, and in the solar system there is a gap between these extreme values,” says Valery Simatovich, head of the solar system research department at the Institute of Astronomy of the Russian Academy of Sciences.
Another oddity is that exoplanets are very close to their stars: their periods of revolution do not exceed one hundred days, and more often are limited to tens. For comparison: Mercury has 88 days.
Many stars have compact planetary systems where objects are located in similar orbits. All of them are strongly influenced by stellar winds and would be literally sterile if not for the atmosphere. How they survived there – this riddle is trying to solve numerous research groups.
“According to the theory of the formation of systems, everything starts with a protoplanetary cloud of light gases – molecular hydrogen and helium, a dust fraction and a large amount of water ice. Close to a star, a large silicate core of the planet simply does not have time to form – the ice quickly evaporates. But behind the so-called snow line where the radiation of a young star is not so strong, more massive cores are formed. They attract the surrounding gas, accumulate the primary atmosphere and become gas giants, “explains the scientist.
What does the correct neptune look like?
Usually in the system there are several giant planets, which compete with each other for gas, dynamically affect other bodies and migrate to the parent star. She, young and active, meets them with harsh ultraviolet radiation, flashes, plasma emissions – and strips off the primary gas shells. But very massive objects hold these shells – in part or in whole. And then, as the orbit shrinks, they turn into hot Jupiters.
Smaller planets lose their atmosphere more actively, sometimes completely. Among them, the two most numerous populations stand out. The first – with radii equal to or slightly larger than the earth, these are the so-called super-earths. The second – with radii of two to four terrestrial: members of this group are known as sub-Neptuns. That is, they are smaller than our Neptune, which, in turn, is four times larger than Earth.
Between them is the so-called photoevaporation gorge, or the Fulton fission, named after the author of the article, who was the first to present data on two peaks in the distribution of exoplanets by radii and masses. Today, scientists believe that photoevaporation by the hard radiation of a young parent star, as well as the stripping of the planet’s gas atmosphere by the stellar wind during migration, are the most likely mechanisms for the formation of secondary atmospheres of super-earths and sub-Neptuns.
Sub-Neptune, or gas dwarf, consists of a gaseous atmosphere and a rocky core that, if melted, forms a magma ocean on the surface (left). If the planet very quickly migrates to the parent star, an ocean of water can appear on its surface.
It is known that at the stage of formation, the rocky core of sub-Neptunians melts due to gravitational compression – and a magma ocean is generated on its surface.
“It plays an important role: it is an additional mechanism for the loss of the primary atmosphere, which can be light – hydrogen-helium and heavy – from water vapor. During rapid migration due to the snow line, the light atmosphere can be completely lost. Heavy – partially, and then the ocean appears. It is believed that water is present in it in all states – ice, liquid and vapor. The density of such a planet is greater than that of sub-Neptuns with a partially preserved light atmosphere of hydrogen and helium,” says Valery Simatovich.
Last year, the TESS space telescope discovered one super-Earth (TOI-270 b) and two sub-Neptuns (TOI-270 c and TOI-270 d) near TOI-270. These are the smallest and most closely spaced exoplanets detected by the transit method – when observing the passage of objects against the background of the parent star.
Both sub-Neptuns appear to be half water ice. True, TOI-270 c is very hot, and TOI-270 d is moderate, but too massive to form an ocean. In addition, a lot of water is lost there due to the greenhouse effect.
However, the authors of the work note, the parent star, the red dwarf TOI-270, is inactive: it is unlikely that it will sterilize its planets. At the same time, it is bright enough, so that in the vicinity you should look for a habitability zone – orbits that allow rocky planets to have liquid water on the surface.
The solar system is still the only known one where there is life. However, in red dwarfs, the habitable zone can be located much closer – these stars are not as massive and hot as our Sun.
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