The discovery of rocky planets like the Earth has been the focus of recent exoplanet explorations. Stars cooler than the sun are the targets of most of the recent missions. There are many red dwarfs in the solar neighborhood.
It's necessary for a planet to have moderate insolation and enough seawater. The occurrence rate of planets satisfying such conditions is small according to previous planet formation models. The formation of a hydrogen-rich atmosphere from the protoplanetary disk and water production is the focus of a new simulation conducted by a PhD student from the University of Tokyo.
They have developed a new planet formation model and predicted the amount of water that planets would have. Several percent of planets with Earth-like radii and insolation around M-type stars have moderate amounts of water. The discovery of planets with warm climates is likely to happen in the next 10 years. The results of the research have been published.
More than 5,000 planets have been found around stars other than the sun. The discovery of so many exoplanets has shown that planetary systems are common in the universe. It has become clear that exoplanets are not the same in terms of size, composition, and distance from the central star.
There are many Earth-sized planets. It's a big deal if any of them have a climate similar to the Earth. Water is an important part of the climate. It is known that a moderate amount of stellar radiation as well as an ocean with a moderate amount of water is required for the maintenance of a warm climate.
The present-day Earth is able to maintain a warm climate because of the functioning of the carbon cycle with plate tectonics and continental weathering.
The current-day Earth's oceans are thought to have been delivered by rocky or icy bodies. The idea of applying this idea to exoplanets around M-type stars led to the prediction that planets with moderate water content are rare, suggesting that habitable planets are not likely to be found.
The production of water in an accumulating atmosphere was proposed as an alternative water acquisition process in the past. As a planet grows in a disk of gas and hydrogen, it creates an atmosphere made of hydrogen.
The rocky surface of the growing planet is believed to be molten due to the heat of the stars. A chemical reaction in the ocean leads to the production of water. It is possible to form a planet richer in water than it would be in a theoretical model.
The amount of hydrous rock acquired by a planet is dependent on the planet formation process. A new planetary population synthesis model has been developed by the authors of this study.
The model can be used to calculate the amount of water acquired in the process. The effect of water production in the primordial atmosphere is included in the model for the first time.
A wide variety of planets of different sizes and atmospheric mass can be found in various locations. The water content for planets is shown in the figure.
Water can be retained by M-type stars when water production in the primordial atmosphere works. The planets formed with the same amount of water as the Earth. Most of the water on these planets is produced in the atmosphere. Several percent of planets with planetary radii between 0.7 and 1.3 times that of the Earth retain enough water to sustain their climates, according to analysis of the data.
More than 100 Earth-sized planets are expected to be found in the habitable zone around M-type stars in the future. According to the results of the study, several of these planets will be water worlds.
The presence of water and other elements in the atmosphere will be revealed by the observations of the atmospheric spectrum of exoplanets. The theoretical predictions of this research are expected to be confirmed by these observations.
More information: Tadahiro Kimura et al, Predicted diversity in water content of terrestrial exoplanets orbiting M dwarfs, Nature Astronomy (2022). DOI: 10.1038/s41550-022-01781-1 Journal information: Nature Astronomy
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