How did Sub-Neptune form and evolve?

Recently, an international joint research team used the Guo Shoujing Telescope (LAMOST) in China and combined with the data of the International Gaia and Kepler space telescopes to obtain the evolution of planetary radius distribution with the age and metal abundance of the host star. The relevant research results were recently published in the Journal of Astronomy.

It is reported that since the 1990s, more than 5,000 exoplanets have been discovered by humans. But the richest types of planets discovered so far are neither gas giants nor rocky planets, but planets not found in the solar system and somewhere between Earth and Neptune.

Among them, those that are slightly larger than the Earth (about 1 to 2 times the radius of the Earth) are called “super-Earth”, and those that are slightly smaller than Neptune (about 2 to 4 times the radius of the Earth) are called “Sub-Neptune”. What is the structure of these planets? How did they form and evolve?

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The internal structure of different types of planets (Image: ESA)

With the discovery of exoplanets in large numbers, the statistics of its large samples show that there is a trough in the planet’s appearance near the two Earth’s radius of planets, called the “planetary radius valley”. The discovery provides key new clues and directions for revealing the planet’s internal structure.

Previous studies suggest that the super-Earth on the left side of the Radius Valley is an enlarged version of the Earth, with a thin atmosphere wrapped around a larger stony core, but the structure of sub-Neptune on the right is unclear.

Regarding the formation mechanism of this planetary radius valley, the theoretical models proposed in the current study can generally be divided into two categories: evolutionary models and primitive formation models.

Evolutionary models suggest that Sub-Neptune consists of a stony core and a thick cladding of gas. Under the influence of radiation from external stars (called photoevaporation) or heat stored in the planet’s core (called planetary nuclear heat), part of the sub-Neptune’s atmospheric envelope is stripped away, leaving only the stony core, known as the super-Earth.

Primitive evolutionary models suggest that radius valleys are natural consequences of planet formation, such as some studies suggest that their sides correspond to two types of planets of different compositions: dense stony super-Earths and water-rich/ice-rich sub-Neptunes (such as “ocean planets” whose surfaces are covered by hundreds to thousands of kilometers of oceans).

The international joint research team used the kinematic method of estimating age, that is, with the help of OBSERVATIONS of LAMOST and Gaia to accurately characterize the motion velocity of the host star of the Kepler planetary system, and to estimate the age, and studied the evolution of the exoplanet “radius valley” with the age and metal abundance of the host star.

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Planet “Radius Valley” (Image: Fulton et al. 2017) and schematic diagram of its formation model

They found that as they age, the average radius of sub-Neptune gradually decreases, while the average radius of super-Earths is almost constant. This most likely means that Sub-Neptune contains a sufficiently thick envelope of gas when it is young, and as it ages, the planet gradually cools, shrinks, and the radius decreases. This result is more supportive of evolutionary models, suggesting that (at least in part) Sub-Neptune is a gas dwarf giant.

They also found that the planet Radius Valley had initially formed early in star formation and became more pronounced as we grew older. At the same time, the ratio of super-Earths and sub-Neptunes is also increasing, which is also consistent with the expectations of evolutionary theory: some sub-Neptunes (gas dwarf giants) are stripped of their atmospheres and evolve into super-Earths.

Quantitatively, planetary radii were initially formed by photo-evaporation in the early stages and further strengthened by planetary nuclear thermothermia in the later period. In addition, the study found that sub-Neptune is more likely to appear around iron-rich and magnesium-rich/silicon/calcium stars, suggesting that metallic elements (iron, magnesium, silicon, calcium, etc.) play an important role in the formation of sub-Neptune.

The researchers revealed that this article is the third article in the series “Spatial Distribution and Age Evolution of Exoplanets” (SHORT PAST, Chinese referred to as “Crossing”), and more follow-up work on the “Crossing” series is underway and in preparation. (Source: China Science Daily Shen Chunlei)

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