New Frontiers of Extrasolar Planets: Exploring Terrestrial Planets

Grant-in-Aid for Scientific Research by the Ministry of Education, Culture, Sports, Science and Technology  Scientific Research on Innovative Areas (Research in a proposed research area) 2011-2015

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Introduction of adoption public research ⑤

Determination of dust size distribution in disk gaps: Construction of a theoretical model for observation and direct comparison
(Kazuhiro Kanagawa / University of Hokkaido)


It is believed that planetary systems such as our solar system are created in a protoplanetary disk around a fixed star at the time when the star is formed. Recent observational studies have discovered large numbers of exoplanetary systems, and when candidate planets are included, more than 4,000 exoplanets have been discovered. As a result, while we have discovered exoplanets with characteristics similar to planets in our solar system, we have also discovered many exoplanets with characteristics that are vastly different from solar system planets. For example, there are gas planets called “hot Jupiters” that orbit extremely close to the star, as well as giant gas planets which orbit at several tens of AU from the star. The discovery of these exoplanets with characteristics that do not exist in the solar system have shown us that planets in the universe exist in much greater variety than is predicted by the conventional standard model for planetary formation. Although the origins of this diversity of exoplanets is not clearly understood, it is believed that differences in the planetary formation and evolution process in the protoplanetary disk that is the matrix of planetary formation are related to exoplanet diversity.

The structures of these protoplanetary disks that are the places where planets form have been identified through high spatial-resolution observation by the Subaru Telescope and ALMA Telescope. The disks known as pre-transitional disks are extremely distinctive disks which include a ring-shaped gap where the disk gas density is low. A strong explanation for the structure of these disk gaps is that they are formed as a result of the disk gas being pushed out of the way along the orbit of a giant gas planet that exists in the disk. Therefore the existence of a gap structure in the protoplanetary disk suggests the presence of a giant gas planet. However because the planet itself is much smaller than the disk and is buried in disk gas, it is difficult to observe directly even with the Subaru or ALMA telescope. It is expected that future use of Subaru and ALMA will discover more disk gap structures and identify their structures in detail. However connecting the results of disk observations with the invisible planet that is buried there requires a step which compares the observation results with a theoretical model of the disk gap created by the planet.

The structure of disk gaps created by gas planets is important as described above, and is the subject of much active research. The use of numerical fluid simulations and model computations are in the process of identifying the mass of the planet and the gas structure of the disk gap. However the observations by the ALMA Telescope that are expected to provide more detailed observation of the gap structure in the future will primarily be observations not of radiation from the disk gas but rather of radiation from the dust particles contained in the gas. It is predicted that there are differences in the orbiting speeds of the disk gas and dust particles in the gap, and that there may be large differences between the distribution of disk dust and dust particles as a result of this speed difference. Consequently the results from the above studies of planetary mass and disk gas structure cannot be used directly. This study intends to provide a method for estimating the planetary mass by finding the distribution of dust particles based on the distribution of disk gas in the gap, and by constructing a model which can be compared directly with actual observations to observationally determine the detailed structure. The results of this study will be important for understanding the results from the continually advancing observations of protoplanetary disks as they relate to the planets, and will help us understand how planets in the disk form and evolve.