Planets with temperate climate likely to be discovered in the next decade

Sep. 30 () –

Japanese scientists have developed a new model of planet formation and, with it, have predicted the amounts of seawater that worlds 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 (red dwarfs, cooler than the Sun and most common star in our stellar neighborhood) have moderate amounts of seawater.

This suggests that the discovery of planets with temperate climates is likely in the next decade, reports the University of Tokyo. The results of the research were published in Nature Astronomy.

Moderate insolation and an adequate amount of seawater are necessary for a planet to maintain a temperate climate. Previous models of planet formation, however, predict that the rate of occurrence of planets satisfying such conditions around M-type stars is small.

New simulations by Tadahiro Kimura, a doctoral student at the University of Tokyo, and Professor Masahiro Ikoma of the NAOJ Science Division, have focused on the formation of a hydrogen-rich atmosphere from the protoplanetary disk. and the production of water through the reaction between the atmosphere and the magma ocean.

Since the first detection in 1995, more than 5,000 planets orbiting stars other than the sun (exoplanets) have been detected. The detection of such a large number of exoplanets has shown that planetary systems commonly exist 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 Earth’s is a topic of great interest. Water is necessary for life on Earth, but water also plays an important role in climate.. Maintaining temperate climates is known to require a moderate amount of stellar radiation as well as an ocean with a moderate amount of water.

Today’s Earth is capable of maintaining a warm climate due to the functioning of the carbon cycle with plate tectonics and continental weathering; if the amount of ocean water were several dozen times greater than on Earth, the carbon cycle would be restricted, which would result in extremely hot or cold weather.

A generalized idea is that the oceans of the present Earth were generated by rocky or icy bodies that contained water. Previous studies applying this idea to exoplanets around M-type stars led to the prediction that planets with moderate water content are rare, suggesting that although M-type stars are the primary target of future planet searches livable, it is very unlikely that habitable planets will be found.

On the other hand, the production of water in a cumulative atmosphere was proposed as an alternative water acquisition process in previous research by Professor 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.

Additionally, the rocky surface of the growing planet is thought to be molten due to heat from celestial impacts; that is, the planet is covered by an ocean of magma. At this time, a chemical reaction between atmospheric hydrogen and oxides in the magma ocean leads to the production of 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 hydrated rock acquired by a planet and the amount of water obtained from the reactions that produce water are highly dependent on the planet’s formation process.

In this study, Tadahiro Kimura and Masahiro Ikoma have developed a new planetary population synthesis model to re-estimate the frequency of aquatic planets in extrasolar systems around M-type stars.

The model follows the mass growth and orbital evolution of planets based on the latest theories of planet formation and can calculate the amount of water gained in the process. In addition to the previously considered acquisition of hydrated rocks, the model also recently incorporates the effect of water production in the primordial atmosphere.