Numerical study of diversity in exoplanet atmospheric dynamics using a deep convection model
(Shinichi Takehiro/ University of Kyoto)

 

With advances in astronomical methods in recent years, greater numbers of gas planets have been discovered outside the solar system. Considering the diverse circulations and patterns of planetary atmospheres previously seen on Jupiter, Saturn, Uranus, and Neptune, we expect to find even greater variety in the upper atmospheres of these newly discovered extrasolar gas planets. This study aims to investigate the diversity of exoplanet upper atmospheric motion based on parametric studies using a general-purpose model for gas planet atmospheres.

Studies aimed at modeling the atmospheres of extrasolar gas planets have been started in recent years, and at present many such studies are underway. One type of the models used in these studies is the shallow model, also known as a “shallow-water model” or “primitive model,” which can express only atmospheric circulation that is broad in the lateral direction and shallow in the vertical direction. It models the upper atmosphere of a planet with stable stratification that is subjected to the effects of solar radiation. On the other hand, for the atmospheres of gas and ice planets in our solar system which have been more precisely observed and better studied than the atmospheres of exoplanets, in addition to shallow models there are also deep models which consider convective motion caused by the effects of the flow of heat from inside the planet. These shallow models and deep models have been studied independently, and are used separately to explain the motion of the planetary upper atmosphere and deep convective motion. Based on this prior research, we are carrying out a basic study that attempts to explain both the upper atmosphere and deep convection in a gas planet at the same time.

There is the possibility that deep atmospheric motion resulting from internal heat sources may affect exoplanet upper atmosphere circulation in the same way as we think it does in the gas and ice planets of our solar system. This study uses a model that can simultaneously express the fluid motion of both the upper atmosphere and the deep parts of an extrasolar gas planet. We use the conditions that are predicted for a planet such as Jupiter as the starting point, and investigate the changes in the motion of the upper atmosphere that occur when various external parameters are changed in order to study the diversity of upper atmosphere fluid motion in extrasolar gas planets.

One reason for studying exoplanets is that in addition to learning new information about planets outside the solar system, we also gain a better understanding of our own solar system. By understanding the diversity of exoplanet atmospheres determined through parametric studies, we expect to better understand the atmospheric motion of Jupiter and other solar system gas and ice planets, and identify their universal and particular characteristics.

In academic year 2014, we are carrying out a numerical experiment to investigate the diversity of atmospheric circulation in asynchronously rotating gas planets using a non-elastic rotating spherical shell model of a gas planet atmosphere that was constructed in academic years 2012 – 2013. A flow of heat from deep parts of the planet and a uniform flow of heat in the longitudinal direction towards the outer boundary caused by radiation from the central star were applied as thermal forcing, and the resulting conditions of upper atmosphere motion and deep convection were investigated.

 

Figure 1 shows the surface zonal flow distribution of atmospheric circulation computed using the planetary parameters of Jupiter when the computation area is an atmosphere thickness approximately the same as in studies using a conventional shallow model. You can see that there are approximately 80 m/s retrograde jets (in the reverse direction of planetary rotation) at the equator and prograde jets (same direction as planetary rotation) in the polar regions. Significant band structures are not seen in the mid-latitude areas. In contrast, Figure 2 shows the surface zonal flow distribution of atmospheric circulation when the atmosphere thickness is doubled (increasing the pressure at the bottom edge of the area by 5 times). Compared with Figure 1, the amplitude of the jets is larger, with approximately 200 m/s prograde and retrograde jets formed alternatively in the equatorial area and low latitudes. Band structures, although faint, can be seen in the mid- and high-latitudes.

In the future, we will observe the changes in circulation and temperature distribution which occur when factors such as the speed of planetary rotation, eddy viscosity, thermal diffusivity, and thickness of the atmosphere are changed, and will also conduct parametric studies simulating synchronously rotating gas planet with thermal forcing from the planet surface only in daytime (a “hot Jupiter”).

 

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