惑星系形成論

太陽系には８つの惑星があり，内側の４つ（水星・金星・地球・火星）は軽い岩石惑星，中間の２つ（木星と土星）は非常に重いガス惑星，外側の２つ（天王星・海王星）は比較的重い氷惑星である．このような太陽系の基本構造は，少なくとも定性的には，「京都モデル」と呼ばれる太陽系形成標準モデルの枠組みで説明される．しかし，定量的にはいくつかの重要な課題が未解決のままである．さらに，今日までに数千個に及ぶ惑星が太陽以外の恒星のまわりに検出されている．それらは太陽系外惑星あるいは系外惑星と呼ばれる．系外惑星および系外惑星系は，例えば，惑星質量や軌道の特徴，バルク密度などに大きな多様性を見せる．そのような多様性のために，我々は太陽系形成の現在の描像を再検討せざるを得なくなった．惑星および惑星系の多様性をもたらすものは何か．京都モデルが見落としているプロセスは何か．どのような特徴が惑星系一般のもので，どのような特徴が太陽系固有のものなのか．本講義では，進化し続ける惑星系形成理論の基礎を理解することを目指す．

There are eight planets in our Solar System. The inner four are light rocky planets (Mercury, Venus, Earth and Mars), the intermediate two are quite massive gaseous planets (Jupiter and Saturn), and the outer two are relatively massive icy planets (Uranus and Neptune). Such basic structure of our Solar System can be explained, at least, qualitatively in the framework of the standard formation model called the Kyoto model. Quantitatively, however, there remain important issues to be resolved. Until today, several thousands of planets have been detected around stars other than our Sun; Such objects are called exoplanets or extra-solar planets. Those planets and planetary systems exhibit large diversity, for example, in planet mass, orbital property, bulk density, and so on. Those observed diversities oblige us to improve our present picture for the Solar System formation, which is based basically on the Kyoto model. What brought about the diversity of planets and planetary systems? What processes are missing in the Kyoto model. Which properties are common to planetary systems and which are unique to our Solar System? This course will give an introduction to evolving theories of planetary system formation.