Dr Masahiro Ikoma released his research results.
Recent exoplanet exploration has focused on the discovery of temperate rocky planets like the Earth, which are often called habitable planets. Most of the recent missions are targeting stars cooler than the Sun. Such stars are known as red dwarfs or M-type stars, which are numerous in the solar neighbourhood. It is known that not only moderate insolation but also an adequate amount of seawater is necessary for a planet to maintain a temperate climate. Previous planet formation models, however, predict that the occurrence rate of planets satisfying such conditions around M-type stars is very small. New simulations conducted by Tadahiro Kimura, a doctoral student from the University of Tokyo and Prof. Masahiro Ikoma from the Division of Science, NAOJ have focused on the formation of an hydrogen-rich atmosphere from the protoplanetary disk and the water production via the reaction between the atmosphere and the magma ocean. They have developed a new planet formation model and, thereby, have predicted the amounts of seawater that exoplanets orbiting M-type stars would have. As a result, their estimate shows that several percent of planets with Earth-like radii and insolation orbiting M-type stars have moderate amounts of seawater. This suggests that the discovery of planets with temperate climates in the next decade is likely. The research results have been published in Nature Astronomy on 2022/09/29, GMT.
Since the first detection in 1995, more than 5000 planets orbiting stars other than the Sun (exoplanets) have already been detected. The detection of such a large number of exoplanets has shown that planetary systems exist commonly in the Universe. On the other hand, it has also become clear that exoplanets are diverse in terms of size, composition, distance from the central star and insolation. Among the planets detected so far, there are many Earth-sized planets. Whether any of them have a temperate climate like the Earth, which are often called habitable planets, is a matter of great interest. Water is necessary for life on Earth, but water also plays an important role in climate. It is known that the maintenance of temperate climates needs a moderate amount of stellar radiation as well as an ocean with a moderate amount of water. The present-day Earth is able to maintain a warm climate due to the functioning of the carbon cycle with plate tectonics and continental weathering; if the amount of oceanic water were several dozen times greater than on Earth, the carbon cycle would be restricted, resulting in an extremely hot or cold climate.
A widespread idea is that the present-day Earth’s oceans were delivered by water-bearing rocky or icy bodies. Previous studies applying this idea to exoplanets around M-type stars gave the prediction that planets with moderate water content are very rare, suggesting that although M-type stars are the main target of future habitable planet searches, habitable planets are highly unlikely to be found.
On the other hand, the production of water in an accumulating atmosphere was proposed as an alternative water acquisition process in previous research by Prof. Ikoma and his colleague. Generally, as a planet grows in a protoplanetary disk, it gravitationally acquires gas from the disk and forms an atmosphere composed mainly of hydrogen. In addition, the rocky surface of the growing planet is thought to be molten due to the heat of celestial impacts (Fig. 1); namely, the planet is covered with a magma ocean. At this time, a chemical reaction between atmospheric hydrogen and oxides in the magma ocean leads to producing water. Taking into account the effects of such a water-producing reaction, it is possible to form a planet richer in water than in conventional theoretical models.
The amount of hydrous rock acquired by a planet and the amount of water obtained from water-producing reactions are highly dependent on the planet formation process. In this study, Tadahiro Kimura and Masahiro Ikoma have developed a new planetary population synthesis model to re-estimate the frequency of aqua planets in extrasolar systems around M-type stars. The model follows the mass growth and orbital evolution of planets based on the latest planet formation theories, and can calculate the amount of water acquired in the process. In addition to the previously considered acquisition of hydrous rocks, the model also newly incorporates the effect of water production in the primordial atmosphere.
Numerical simulations using this model show that a wide variety of planets of different sizes and atmospheric masses are produced in various locations (see Fig. 2). The calculated water content for planets in the habitable zone is shown in Fig. 3. As shown in the figure, exoplanets orbiting M-type stars can retain very diverse amounts of water when water production in the primordial atmosphere works. Some of these planets have formed with similar amounts of seawater to that of the Earth. Most of the seawater on these planets is brought through the atmospheric water production. Analysis of the computational data has led to the prediction that several per cent of planets with planetary radii between 0.7 and 1.3 times that of the Earth retain adequate amounts of water to sustain temperate climates (around 0.1-100 times the Earth’s seawater content).
It is expected that nearly 100 Earth-sized planets will be detected in the habitable zone around M-type stars in ongoing and future exoplanet exploration programmes such as TESS and PLATO. The results of this study predict that several of these planets will be aqua planets with Earth-like warm climates. Observations of the atmospheric spectra of exoplanets by the infrared space telescopes JWST and Ariel will also reveal the presence of water molecules and other elements in the atmosphere. These observations are expected to validate the theoretical predictions of this research and lead to a better understanding of the formation process of aqua planets such as the Earth.
This research is supported by the Grant-in-Aid for Scientific Research on Innovative Areas “A Paradigm Shift by a New Integrated Theory of Star Formation: Exploring the Expanding Frontier of Habitable Planetary Systems in Our Galaxy” Group A3 “Understanding the Formation, Evolution, and Diversity of Planetary Atmospheres” (18H05439) and the Grant-in-Aid for Young Scientists “Modeling the Evolution of Primordial atmospheres with Water Production and Estimating the Amounts of Water Acquisition for Terrestrial Exoplanets” (22J11725).
Information of Publication
Title：Predicted diversity in water content of terrestrial exoplanets orbiting M dwarfs
Authors：Tadahiro Kimura, Masahiro Ikoma
 Protoplanetary disk
When a new star is formed, the material nebular gas forms a disk around the star. This is called the protoplanetary disk and consists of hydrogen-dominated gas and solids (dust). Planets are generally thought to form in these disks.
 Magma ocean
The rocky planet’s surface is molten due to the frequent impact of celestial bodies on the planet during the formation phase. Furthermore, hydrogen gas, the main component of the atmosphere during this period, has a strong heat-keeping effect, so that the molten surface does not cool down and the entire planet is thought to be covered with magma. This is called a magma ocean.
 Habitable zone